microstructural evolution and hardening behaviour of cast and heat-treated ru-al and ru-al-ni alloys

METALS AND MATERIALS International, Vol. 14, No. 1 (2008), pp. 123~132

Microstructural Evolution and Hardening Behaviour of Cast and

Heat-treated Ru-Al and Ru-Al-Ni alloys

Anil Borah1

, P. S. Robi1,*

, Ajit L. Mujumdar1

, and A. Srinivasan2

1

Department of Mechanical Engineering, Indian Institute of Technology Guwahati,

Guwahati-781039, India2

Department of Physics, Indian Institute of Technology Guwahati, India

Ru-Al alloys with compositions of Ru59Al41, Ru46Al35Ni19, and Ru39Al13Ni48 were prepared via an arc melting

technique and subsequent heat treatment at 1450oC for 24 h. Microstructures of the alloys in the as-cast

and heat-treated conditions were studied. After heat treatment, cast Ru59Al41 and Ru46Al35Ni19 alloys revealed

the presence of needle shaped precipitates with a simultaneous increase in hardness, exhibiting precipitation-

hardening behaviour. A morphological instability resulting in the formation of lamellar structures was evident

in the heat-treated Ru39Al13Ni48 alloy. Examination indicated that prolonged heat treatment is required to obtain

equilibrium phase compositions resulting in a single lamellar structure in this alloy.

Keywords: RuAl, Ru-Al-Ni, solidification, SEM-EDX, microstructure, microhardness, Heat-treatment, precipitation

hardening, lamellar structure

1. INTRODUCTION

Intermetallics, such as aluminides of nickel, titanium,

cobalt, and iron are characterized by high strength at ele-

vated temperatures and high resistance to oxidation and cor-

rosion [1,2]. However, these materials exhibit poor room

temperature ductility and toughness, which impedes the

application of these materials to high temperature structural

use. Initial work carried out by Fleischer and colleagues

[3,4], followed by Wolff et al. [5-8] highlighted the promis-

ing properties of ruthenium aluminide (RuAl) alloys for high

temperature structural applications. Mucklich and Ilic [9]

have recently discussed in detail the physical properties,

microstructure, and processing of this intermetallic alloy.

RuAl exhibits an unusual combination of properties, includ-

ing a very high melting point, high strength at room and ele-

vated temperatures, high resistance to environmental corrosion

in some very severe chemical media, thermal and electrical

conductivity compared to pure metals, and useful room tem-

perature ductility and toughness. The high ductility observed

in the cubic B2 (CsCl) crystal-structured RuAl phase has

been attributed to the availability of <1 0 0>, <1 1 1> and <1

1 0> slip vectors [3]. Pollock and co-workers have shown

that slip in RuAl may occur by both <110> and <100> dislo-

cations resulting in the sufficient five independent slip

systems needed for plastic deformation in polycrystalline

materials [10,11]. Study of the flow stress characteristics of

Ru-Al alloys in the temperature range between 298 K-1173

K [12,13] have indicated dynamic strain aging characterized

by serrated flow, flow stress plateau, maxima in the work

hardening rate, and minima in strain-rate sensitivity. Flow

stress as high as 1140 MPa at 1173 K was reported for Ru-

Al-Ta alloys [14]. The deformation behavior in these alloys

was attributed to the <111> type dislocations on {110} planes.

In spite of the promising properties exhibited by Ru-Al

alloy systems, the high cost of ruthenium limits the commer-

cial exploitation of these alloys in structural applications.

The present trend is a lower cost substitution of ruthenium

with elements like nickel and cobalt in RuAl [6,15]. Micro-

structural studies of Ru-Al-Ni alloys have been reported for

a limited number of compositions [6,15-17]. Chakravorty

and West [16,17] investigated the Ru-Al-Ni ternary system

and proposed an extensive three-phase field comprising β-

RuAl (designated as β2), a Ru solid solution, and disordered

γ (Ni solid solution) phases. They pointed out the presence of

a miscibility gap between the solid solution of ruthenium in

NiAl (β1-phase) and the solid solution of nickel in RuAl ((β2

-phase).

A majority of the studies on Ru-Al/Ru-Al-Ni systems

were carried out on samples processed by arc melting inside

a water-cooled copper hearth [1-7,15,18-19]. It is very likely

that these processing conditions lead to non-equilibrium

solidification, resulting in deviations from equilibrium com-

position and microstructure for the constituent phases. Heat*Corresponding author: [email protected]

doi: 10.3365/met.mat.2008.02.123 Published 26 February 2008

124 Anil Borah et al.

treatment of the cast alloys is expected to modify the compo-

sition and microstructure of the constituent phases present in

the alloy. Further studies on heat-treated RuAl/RuAlNi

alloys would provide insight into the evolution and the pos-

sibility of tailoring the microstructure by heat treatment.

Thus, the present study was carried out to investigate the

microstructure of cast Ru47Al53, Ru43Al39Ni18, and Ru38.5Al16.5

Ni45 alloys and the effect of heat treatment on the microstruc-

tural evolution in these alloys. Detailed investigations are

required on the microstructure of the binary Ru-Al system in

the Ru-rich end of the phase diagram. To the best of our

knowledge, the effect of heat treatment on the microstructure

of the cast Ru-Al system has not reported so far. Since alu-

minium vaporizes at temperatures above 1900o

C during

casting [7], a starting composition with higher aluminium con-

tent (Ru47Al53) was used.

The selection of ternary compositions of Ru43Al39Ni18 and

Ru38.5Al16.5Ni45 was based on the two-phase (β2 + Ru) and

three-phase (γ + β2 + Ru) regions, respectively, of the isother-

mal sections of the ternary Ru-Al-Ni phase diagram reported

earlier [16,17].

2. EXPERIMENTAL PROCEDURES

Elemental powders of Ru (purity of 99.4 % and average

particle size 4 μm), Al (purity of 99.9 % and average particle

size 7 μm), and Ni (purity of 99 % and average particle size

~ 10 μm) were used as the starting materials. The nominal

target compositions of the alloys investigated in this work

are listed in Table 1.

Appropriate quantities of elemental powders were mixed

homogeneously in an agate mortar and pestle and were cold

compacted into 50 mm × 12 mm × 10 mm rectangular rods.

These rods were placed in a water-cooled copper mould and

arc melted under an argon atmosphere. The samples were

melted several times to ensure proper homogeneity and sub-

sequently allowed to solidify. The solidified alloy rods were

heat treated at 1450o

C for 24 h in an argon atmosphere. The

low diffusivity of Ru at lower temperatures and the poor sin-

tering characteristics of mechanically alloyed Ru-Al com-

pacts at 1050o

C [20] prompted us to choose a heat-treatment

temperature of 1450o

C.

The as-cast samples, as well as the cast and heat treated

samples were sliced using a precision saw and were polished

for X-ray diffraction (XRD) and scanning electron micros-

copy (SEM) observations, following the standard metallo-

graphic specimen preparation procedures. Structural analyses

of the cast samples were carried out using a commercial

X-ray diffraction system (SEIFERT XRD 3003 T/T). Cu Kα

radiation (λ = 1.5405 Å) with a nickel filter was used. The

instrument was calibrated with a standard silicon reference

sample. The XRD patterns were recorded with a 40 kV

acceleration potential and a 30 mA tube current. The back-

scattered electron images of the alloy microstructure were

observed using a Leo make model 1430 VP scanning elec-

tron microscope (SEM). Composition of constituent phases

present in the alloy was analysed using an OXFORD energy

dispersive X-ray spectroscopy (EDX) system attached to the

SEM. Composition analyses were carried out after calibra-

tion of the EDX using high purity (99.999 %) cobalt as the

standard. The compositions reported are the average of at

least 5 independent determinations in each case. Imaging of

different specimens was done with 10 KeV to 20 keV of

energy and a 10 mm to 20 mm working distance. The EDX

analyses were carried out under an accelerating voltage of

20 KeV and a working distance of 15 mm. The composition

of the alloys is shown in column 2 of Table 1.

Hardness values of the alloys were determined using a

microhardness tester (Buehler make, model: Micromet 2101)

at room temperature. The overall alloy hardness and the

hardnesses of the constituent phases in the alloy (Microhard-

ness) were obtained by making indentations with a Vickers

diamond pyramid. An indentation time of 25 s was fixed for

all the hardness measurements. Microhardness measure-

ments reported here were performed with different loads,

and the indentation spread over the region within a single

phase or over large regions covering different constituent

phases. The former case is referred to as the microhardness

of the respective phase, and the latter as the overall hardness

of the alloy. The hardness values reported here are the aver-

age values of at least ten independent indentations under

identical conditions.

3. RESULTS

Microscopic observation of the cast samples revealed the

presence of gas porosities and microscopic shrinkage poros-

ities in all the samples. Micro-structural features obtained for

the samples in the as-cast as well as heat treated conditions

are discussed in the following sections.

3.1. Alloy-1 (Ru59Al41)

XRD patterns obtained for alloy-1 in as-cast as well as cast

and heat-treated conditions within the 2 θ range of 20o

to 85o

are shown in Figs. 1(a) and (b), respectively. All the XRD

peaks seen for the as-cast alloy in Fig. 1(a) could be indexed

to reflections from RuAl and Ru planes alone. This shows

that only two phases, viz., a ruthenium phase (henceforth

referred to as [Ru]) and a RuAl phase are present in this

alloy. The XRD pattern for the cast and heat-treated Ru-Al

Table 1. Nominal target and overall compositions of the alloys

Alloy Alloy composition

Alloy-1

Alloy-2

Alloy-3

Ru59Al41

Ru46Al35Ni19

Ru39Al13Ni48

Microstructural Evolution and Hardening Behaviour of Cast and Heat-treated Ru-Al and Ru-Al-Ni alloys 125

alloy (cf. Fig. 1(b)) shows reflections from (100), (102),

(110) and (103) planes of Ru, which did not appear in the

XRD pattern for the as-cast alloy. Ru or a Ru-rich phase [Ru]

might have been precipitated out during the heat treatment,

resulting in the extra peaks in the XRD pattern.

An electron micrograph of the as-cast alloy observed at

low magnification is shown in Fig. 2(a). The overall compo-

sition of the alloy as determined by EDX analysis was

Ru59Al41, indicating loss of aluminium during the melting

stage. The microstructure consisted of a pro-eutectic phase

surrounded by a network of eutectic mixture, as seen in Fig.

2(b). The composition of the pro-eutectic phase (labelled “I”

in Fig. 2(b)) was identified as Ru56Al44. The overall eutectic

composition was identified as Ru76Al24, and it consisted of a

mixture of [Ru] having a composition Ru87Al13 (labelled “H”

in Fig. 2(b)) and a metastable phase with composition

Ru63Al37 (labelled “G” in Fig. 2(b)).

An SEM micrograph of alloy-1 after heat treatment (cf.

Fig. 3), shows the presence of second phase precipitates in

the form of fine needles found distributed uniformly in the

pro-eutectic phase. EDX analysis of the pro-eutectic RuAl

phase, including the precipitated phases, revealed no change

in the overall composition of this region after heat treatment.

Heat treatment resulted in the formation of two distinct con-

stituent phases for the eutectic region, viz., a white [Ru]

phase and a grey β-RuAl phase (labelled “W” and “G”, respec-

tively, in Fig. 3) having compositions of Ru55Al45 and Ru98Al2,

respectively.

The eutectic region also exhibited a morphological change

during heat treatment. Though the structure after heat treat-

ment consisted of a mixture of two phases, the observed fea-

tures did not represent a lamellar structure. Coarsening of the

β-RuAl phase in the eutectic region was observed, and this

coarsening resulted in partial destabilization of the lamellar

structure. The driving force for the coarsening of the constit-

uent phases during annealing is the reduction in the total sur-

Fig. 1. XRD patterns corresponding to alloy-1 in (a) as-cast and (b)

heat-treated conditions.

Fig. 2. SEM micrographs of alloy-1 in as-cast conditions at (a) low

and (b) high magnification.


face area of the eutectic mixture. The EDX analysis indicated

an average composition of Ru60Al40 for the needle shaped

precipitated phase. Additionally, a Ru-rich white layer with a

composition Ru97Al3 was observed around the gas porosity

regions (shown in Fig. 4), indicating aluminium loss from

these regions after heat treatment.

Overall hardness of as-cast and heat-treated Ru-Al alloy

samples were determined as 290 VHN and 334 VHN, respec-

tively. The microhardness values of RuAl-phase in as-cast

alloy (cf. Fig. 2(b)) and with the precipitated needles in the

heat treated Ru-Al alloy (cf. Fig. 3) were determined as 368

VHN and 457 VHN, respectively.

3.2. Alloy-2 (Ru46

Al35

Ni19

)

The XRD patterns of the alloy in as-cast as well as cast

and heat treated conditions, within the 2 θ range of 20o

to 85o

are shown in Figs. 5(a) and (b), respectively. XRD reflec-

tions seen in Fig. 5(a) could be indexed to reflections from

RuAl and Ru planes. An XRD pattern similar to the case of

the as-cast sample was also observed in the heat-treated alloy

(cf. Fig. 5(b)). Two additional peaks corresponding to reflec-

tions from the (100) and (002) planes of Ru were also observed

in the XRD pattern of the alloy in the heat-treated condition.

The as-cast microstructure of alloy-2 is shown in Figs. 6(a)

and (b). EDX analysis revealed an overall composition of

Ru46Al35Ni19 for the cast alloy indicating aluminium loss as

compared to the starting composition. A high-magnification

SEM micrograph (cf. Fig. 6(b)) revealed three phases in the

as-cast structure. These phases have been identified as a

white [Ru] phase (labelled “D”) of composition Ru64Al12Ni24, a

dark grey γ-phase adjacent to the white phase (labelled “F”)

having composition Ru37Al40Ni23, and a light grey β2-phase

(labelled “E”) with composition Ru47Al44Ni9.

The micrograph of the alloy in the as-cast condition is not

indicative of eutectic solidification (lamellar structure) but

Fig. 3. SEM micrograph of the alloy-1 in heat-treated condition.

Fig. 4. SEM micrograph of cast and heat-treated alloy-1 showing the

Ru-rich white layer around a macroscopic porosity region.Fig. 5. XRD patterns corresponding to alloy-2 sample in (a) as-cast

and (b) heat-treated conditions.


rather represents a three-phase field region. Its major constit-

uents are the [Ru] and the β2-phase, with the volume fraction

for the γ-phase being marginally small. Contrast variation in

the backscattered SEM image is indicative of occurrence of

coring as evidenced by concentration gradients in the [Ru]

and β2-phase.

An SEM micrograph of the alloy after heat treatment,

shown in Fig. 7, reveals fine needle shaped second phase

precipitates uniformly distributed in the β2-phase, indicating

the occurrence of a precipitation reaction during heat treat-

ment. EDX analysis indicated a composition of Ru60Al13Ni27

for these precipitates. A thin layer around the white phase

having composition Ru35Al43Ni22was observed in the β2-

phase (labelled “G” in Fig. 7), which was depleted of the

precipitated phases. The white regions (regions labelled “A”

and “B”) seen in the SEM micrograph (cf. Fig. 7) mainly

consist of two types of lamellar structures. Region “B” con-

sists of a mixture of a white phase and a grey phase. Though

region A appears to be a single-phase region, observation

under very high magnification (micrographs not shown due

to poor quality) revealed that this region is also composed of

a mixture of two fine phases. EDX analysis revealed an

overall composition of Ru59Al9Ni32 for the two regions “A”

and “B”. Analyses also indicated the compositions of the

constituent white and grey phases in region “B” as Ru80Al4Ni16

and Ru39Al37Ni24, respectively. However, due to the very fine

size of the grey phase in region “B”, there is the possibility of

interaction of the electron beam with the neighbouring

phases. Under such circumstances, the reported composition

for the grey phase may not be accurate. Such observational

difficulties have also been reported earlier by others [15,16].

Since the overall composition of region “A” is the same as

that of region “B”, it can be presumed that region “A” is the

same as region “B” with finer constituent phases. This pre-

sumption was confirmed when region “A” was viewed under

higher magnification. The morphology of the white phase, as

seen from Figs. 6(b) and 7 indicates localized decomposition

of the [Ru] during heat treatment.

Overall hardness of the as-cast and heat treated Ru43Al39Ni18

alloys was determined to be 696 VHN and 835 VHN,

respectively. The microhardness values for the β2-phase (region

“E” in Fig. 6(b)) and [Ru] (labelled as “D” in Fig. 6(b)) were

found to be 635 VHN and 849 VHN, respectively, in the

as-cast alloy. The average microhardness values of the con-

stituent phases in the cast and heat-treated alloy were found

to be as follows:

Grey β2-phase with precipitates = 848 VHN

White region (Region “B” in Figure 7) = 896 VHN

Grey phase depleted of precipitates (Region “G” in Fig. 7)

= 796 VHN

3.3. Alloy-3 (Ru39Al13Ni48)

Figures 8(a) and (b) show the XRD patterns corresponding

Fig. 7. SEM micrograph of alloy-2 after heat treatment.

Fig. 6. SEM micrographs of as-cast alloy-2 at (a) low and (b) high

magnification.


to alloy-3 in the as-cast as well as cast and heat-treated con-

ditions, respectively. Eight peaks were observed in the XRD

pattern, corresponding to the as-cast alloy (cf. Fig. 8(a))

within a 2 θ range of 20o

to 85o

. All the XRD reflections

could be indexed to the reflections from either Ru or Ni

planes. Hence, one can infer that the as-cast alloy consisted

of two-phases, viz., [Ru] and [Ni]. Figure 8(b) shows the

XRD pattern for alloy-3 in the heat-treated condition. The

XRD pattern shows no new peaks in the heat-treated alloy

when compared to the XRD pattern of the as-cast alloy.

SEM micrographs of as-cast alloy-3 are shown Figs. 9(a)

and (b). A low magnification SEM micrograph (cf. Fig. 9(a))

revealed a dendritic growth during solidification along with

micro porosities. EDX analysis revealed an overall composi-

tion of Ru39Al13Ni48, indicating a decrease in aluminium and

increase in nickel concentrations compared to the starting

composition. High magnification observation of the micro-

structure (cf. Fig. 9(b)) revealed a white phase (labelled “A”

and “B”) and a black phase (labelled “C”). The contrast var-

iation observed in region “A” of the back-scattered SEM

micrograph indicates the occurrence of coring. Analyses of

the EDX spectra corresponding to the regions “A”, “B”, and

“C” revealed the respective phase compositions to be

Ru66Al7Ni27, Ru56Al9Ni35 and Ru28Al15Ni57.

Heat treatment of alloy-3 resulted in a structural evolution,

as evident in Fig. 10. The morphology of both the white and

black phases seen in the as-cast alloy (cf. Fig. 9(b)) has

changed to a lamellar type of structure after heat treatment.

The EDX analyses indicated the overall compositions of the

white (labelled “W” in Fig. 9(a)) and black lamella regions

(labelled “B” in Fig. 9(a)) were Ru51Al10Ni39 and Ru33Al13Ni54,

respectively. Figures 11(a) and (b) show high-magnification

SEM micrographs of black and white lamella regions,

respectively. The white and the black lamella consisted of

alternate layers of two different phases.

EDX analysis revealed the composition of the black

(labelled “B1”) and white (labelled “B2”) regions inside black

lamella as Ru17Al13Ni70 and Ru56Al10Ni34, respectively. Simi-

Fig. 8. XRD patterns corresponding to the alloy-3 in (a) as-cast and

(b) heat-treated conditions.

Fig. 9. SEM micrographs of as-cast alloy-3 at (a) low and (b) high

magnification.


lar analyses identified the white (labelled “W1”) and black

(labelled “W2”) phases inside white lamella as Ru59Al7Ni34

and Ru19Al19Ni62, respectively.

Overall hardness values of 557 VHN and 763 VHN were

obtained for the Ru38.5Al16.5Ni45 alloy in the as-cast, cast, and

heat treated conditions, respectively. The microhardness val-

ues of the white (region W) and black (region B) lamella

regions in the cast and heat-treated alloy (shown in Fig. 10

(a)) were determined as 825 VHN and 750 VHN, respectively.

4. DISCUSSIONS

4.1. Microstructure

Both XRD and SEM-EDX studies on alloy-1 in the as-cast

condition revealed that the alloy structure is composed of

three phases, viz., a metastable phase (of composition

Ru63Al37 and observed in the eutectic mixture), a [Ru] phase

(of composition Ru87Al13), and a RuAl phase (of composi-

tion Ru56Al44). The binary phase diagrams reported by differ-

ent authors for the Ru-Al system indicate a eutectic composition

of Ru70Al30 [21] and Ru74Al26 [9]. From microstructural stud-

ies conducted on vacuum arc melted Ru-Al alloy samples, a

eutectic composition of Ru76Al24 was proposed by Ilic et al.

[18]. In the present investigation, the observed composition

for the eutectic mixture is in good agreement with this pro-

posal. The microstructural investigations of these samples

were conducted in the as-cast conditions where the high

cooling rate might have induced non-equilibrium solidifica-

tion. Under this condition, the possibility of a shift in the

eutectic composition from the equilibrium composition can-

not be ruled out. The binary Ru-Al phase diagram also indi-

cates a composition of 54 at.% Ru at 1920 ºC for the RuAl

phase, which decreases to 50.3 at.% Ru at room temperature

[9]. However, based on the same reasoning mentioned above,

one can presume that the composition of Ru56Al44 observed

in the present investigation is a non-equilibrium composition

of β-RuAl phase. Hence, it can be concluded that due to the

very fast solidification process, the alloy solidifies under

non-equilibrium conditions, resulting in a shift in the eutectic

and β-RuAl phase compositions towards the ruthenium rich

end of the phase diagram [24].

Cast alloy-1 after heat treatment revealed three interesting

features. The first was the appearance of a Ru-rich white

layer around macroscopic porosity regions after heat treat-

ment. Aluminium diffusion into the free surface (porosity)

from the surrounding matrix is thought to have resulted in

the formation of the Ru-rich layer in the region surrounding

the porosity.

The second feature was the change in the composition and

morphology of the eutectic lamella. The ruthenium concen-

tration in the Ru-rich phase of the eutectic lamellae increased

from 87 at.% to 98 at.% after heat treatment. Simultaneously,

an increase in the aluminium concentration was observed in

the metastable Ru-rich phase of the eutectic lamellae. Heat

treatment resulted in the transformation of this phase (of

Fig. 10. SEM micrograph of heat-treated alloy-3.

Fig. 11. SEM micrographs of heat-treated alloy-3 showing (a) black

lamella and (b) white lamella.


composition Ru63Al37) into a RuAl phase of composition

Ru55Al45. The ruthenium enrichment of the [Ru] with a

simultaneous formation of a RuAl phase in the eutectic

lamella indicates inter-diffusion of individual species of Ru

and Al in the constituent phases of eutectic lamella during

heat treatment. The heat treatment resulted in the decompo-

sition and the coarsening of the eutectic phase, leading to the

loss of lamellar structure.

The third feature was the evidence of precipitation of nee-

dle shaped ruthenium-rich phase in the heat-treated β-RuAl

matrix. It has to be pointed out that the Ru-Al phase dia-

grams presented by Massalski [21] and Mucklich and Ilic [9]

show distinct differences. Further, the earlier studies have

concentrated more on the aluminium rich side of the Ru-Al

phase diagram with quite a few disagreements among one

another. The ruthenium rich side of the Ru-Al phase diagram

has not been explored in such detail. The non-equilibrium

solidification of the cast alloy resulted in the formation of the

primary RuAl phase (Ru56Al44) with ruthenium content

higher than the equilibrium composition. Ru56Al44 is not an

equilibrium phase of the Ru-Al system. During the heat-

treatment at 1450o

C, excess Ru in this phase segregates,

since it is super-saturated with Ru. The low diffusivity of Ru

and the proximity of Al in this phase lead to the formation of

the Ru60Al40 (Ru3Al2) in the form of needles, as seen in

region Fig. 3. Fleischer et al. [3] have also observed such

needle-like structures in Ru53Al47 alloy. However, they have

not reported the composition of this needle-like structure.

Moreover, the ruthenium-rich side of the Ru-Al phase dia-

gram has not been fully explored in the phase diagrams pre-

sented so far [9,21]. Hence, the presence of an intermetallic

phase (or compound) of composition Ru3Al2 in the binary

phase diagram cannot be ruled out, especially in samples

which have been heat-treated after casting. This brings to

light the need for a detailed study of compositions in the Ru-

rich region of the Ru-Al phase diagram.

XRD analysis of as-cast alloy-2 showed the presence of

two phases in alloy-2 (cf. Fig. 5(a)). However, SEM analysis

(cf. Fig. 6(b)) indicated the presence of three phases in the

as-cast alloy, viz., a white phase [Ru], a metastable dark grey

γ-phase, and a light grey β2 (Ru,Ni)Al phase. The formation

of the metastable dark grey γ-phase of composition Ru37Al40Ni23

around the white phase may be attributed to coring during

solidification. Since the percentage of this phase in the alloy

is very low as compared to the other two phases, it could not

be observed in the XRD patterns. It is thought that with

proper heat treatment the alloy structure could result in a

two-phase structure, as pointed out by the equilibrium phase

diagram.

XRD pattern of cast and heat-treated alloy-2 (cf. Fig. 5(b))

showed that all the reflections from Ru planes present in the

XRD pattern of the as-cast alloy were present in the heat-

treated alloy along with two additional reflections from

(100) and (002) planes of Ru. Further amounts of Ru or a

Ru-rich phase precipitated out from the β2(Ru,Ni)Al phase

during heat treatment, resulting in the XRD reflections from

all planes of Ru. The metastable dark grey γ-phase observed

in the as-cast alloy microstructure (region “F” in Fig. 6(b))

was eliminated by heat treatment and resulted in the above

two phases. The composition of Ru80Al4Ni16 for [Ru] obtained

in the present study was found to be close to the reported

composition of Ru85Al3Ni12 for the same phase in an alloy of

composition Ru50Al25Ni25 [17]. Precipitation of Ru-rich nee-

dles of composition Ru60Al13Ni27 in the grey β2-phase occurred

after heat treatment. These Ru-rich needles have precipitated

from the β2(Ru,Ni)Al phase (region “E” in Fig. 6(b)) of the

as-cast alloy during heat-treatment. While the Ru in the inte-

rior of the β2-phase region contributed to the formation of the

Ru-rich needles, the Ru at the periphery of these regions dif-

fused into the “A” and “B” regions, leaving behind a precip-

itate-free zone (region “G” in Fig. 7) adjacent to the phase

boundaries.

Both XRD and SEM analyses confirmed a two-phase

structure in the as-cast alloy-3. SEM observation of the as-

cast microstructure at high magnification [cf. Fig. 9(b)] showed

evidence of coring in the white (labelled “A” and “B”) and

black (labelled “C”) phases.

The overall composition of the alloy (Ru39Al13Ni48) falls in

the three-phase (γ + β2 + Ru) field and also in the neighbour-

hood of the two-phase (γ + Ru) field of the isothermal phase

diagrams at 1250o

C and 1000o

C [17]. The present observa-

tions on heat-treated alloy-3 indicate a shift in the two-phase

(γ + Ru) field boundary towards the three-phase (γ + β2 +

Ru) field as well as toward the γ-phase field. At 1450o

C, the

(γ + Ru) region has expanded further in comparison to the

two-phase region of the partial isothermal phase diagram at

lower temperatures (i.e., at 1250o

C or 1000o

C).

Microstructural studies on the cast and heat-treated alloys

revealed two types of lamellar structures, each with unique

overall compositions. These lamellae consist of mixtures γ

and Ru phases with distinct compositions for each constitu-

ent phase. The presence of two distinct lamellar structures

gives an indication that there are two eutectoid composi-

tions. It was also observed that the [Ru] in the two lamellae

gave slightly varying compositions, viz., Ru56Al10Ni34 and

Ru59Al7Ni34. The corresponding compositions of the γ phases

are Ru17Al13Ni70 and Ru19Al19Ni62, respectively. Karunaratne

and Reed [22] have pointed out that the inter-diffusion of

ruthenium in nickel alloys is sluggish. Since the [Ru] and γ

phase compositions in the two lamella structures are close, it

appears that homogenization requires a longer heat treatment

time in order to obtain equilibrium phase compositions.

According to Bryon and Pollock [23], heat treatment of ter-

nary bulk Ru-Al-Ni alloys for obtaining equilibrium phase

compositions require a prolonged period of exposure time.

The above arguments show that there exists the possibility of


obtaining a single lamellar structure in the alloy after pro-

longed heat treatment.

4.2. Hardness

Hardness test results revealed a 15 % increase in the over-

all hardness of alloy-1 after heat treatment. Comparison of

the micro-hardness values of the RuAl phase in the as-cast

and heat treated conditions reveals a ~24 % increase in hard-

ness after heat treatment. The presence of needle shaped

Ru3Al2 precipitates after heat treatment (cf. Fig. 3(a)) indi-

cates that the RuAl phase is precipitation hardenable. The

precipitation of Ru3Al2 needles in the RuAl phase results in

an increase in the overall hardness of the alloy. However, a

quantitative estimation of the contribution of precipitation

hardening in this alloy was not possible, since the hardness

of [Ru] could not be determined due to its very fine size.

Addition of 19 % nickel into binary Ru-Al alloy, in the as-

cast condition, resulted in a drastic increase in overall hard-

ness by ~140 %. From the SEM/EDX analysis of alloy-2, it

is evident that nickel atoms substitute for Ru in the RuAl

phase. The β2 phase containing 22 at.% Ni resulted in an

increase in microhardness by ~72 % compared to the RuAl

phase in the binary Ru-Al alloy in the as-cast condition.

From these findings, one can infer that nickel addition

results in solid solution hardening of the RuAl phase. Heat

treatment resulted in ~20 % increase in the overall hardness

of alloy-2 as compared to the as-cast alloy. The microhard-

ness values of the β2 and [Ru] phases after heat treatment

indicated an increase in hardness by 33 % and 5.5 %, respec-

tively, as compared to that in the as-cast condition. Presence

of needle shaped Ru60Al13Ni27 precipitates in the β2-phase

(cf. Fig. 7) during the heat treatment resulted in the harden-

ing of β2-phase, thereby increasing the overall hardness of

the alloy. This confirms the precipitation hardening behav-

iour in Ru46Al35Ni19 alloy. The increase in the overall hard-

ness of the heat-treated alloy is the result of combination of

precipitation hardening of β2-phase and hardening of [Ru].

The microstructure of as-cast alloy-3 reveals two ternary

solid solution phases. Nickel forms solid solution phases in

the Ru-Al alloy, thereby increasing the overall hardness of

the alloy by ~92 % compared to that of the binary Ru-Al

alloy. Heat treatment resulted in a morphological decompo-

sition of these phases into lamellar structures, resulting in an

increase in the overall hardness values by ~37 %. Wollf and

Sauthoff [8] have highlighted strengthening due to the for-

mation of a lamellar structure in the binary Ru70Al30 alloy.

Formation of a lamellar structure increases the interface den-

sity, leading to a progressive strengthening in terms of rule of

mixture. The inter-lamellar spacing of the constituent phases

also plays an important role in the hardening of the alloy.

Inter-lamellar spacing below a critical value results in a

threshold stress inversely proportional to the inter-lamellar

spacing in a Ni-Fe-Al alloy that was also highlighted by the

same authors [8]. In the heat-treated alloy-3, the inter-lamel-

lar spacing was observed to be much lower than 1 μm. This

provides a very small mean free path for the dislocation

movement inside the constituent phases, resulting in the dis-

locations being blocked at the interfaces leading to harden-

ing. The increased hardness observed for alloy-3 after heat

treatment is similar to the mechanism of grain boundary

strengthening.

5. CONCLUSIONS

• Ru-Al and Ru-Al-Ni alloys of compositions Ru59Al41,

Ru46Al35Ni19, and Ru39Al13Ni48 were prepared via an arc melt-

ing technique. The microstructures of these alloys were

investigated in the as-cast and heat treated conditions. The

important findings are summarized below.

• Macroscopic gas porosities and microscopic shrinkage

porosities were observed through out the casting. Presence

of gas porosities is attributed to the volatilisation of alumin-

ium at the high processing temperatures.

• The microstructure of the as-cast Ru59Al41 alloy revealed

a primary RuAl phase surrounded by a eutectic network. The

β-RuAl and eutectic compositions shifted towards the ruthe-

nium rich end of the binary Ru-Al phase diagram due to a

non-equilibrium cooling rate.

• Heat treatment of the cast Ru59Al41 alloy resulted in the

precipitation of fine needle shaped second phases in the pri-

mary RuAl phase in addition to a morphological change of

the constituent phases in the eutectic network.

• The as-cast Ru46Al35Ni19 alloy structure revealed three

distinct phases, viz., a β2 phase, a Ru-rich phase, and a meta-

stable γ phase. Heat treatment of the cast alloy resulted in a

precipitation of a Ru60Al13Ni27 phase in the β2-phase. Local-

ized decomposition of the [Ru] phase in the cast structure

resulted in features exhibiting morphological instability.

• As-cast Ru39Al13Ni48 alloy revealed primary [Ru] and a

γ-phase. Heat treatment resulted in the formation of two dis-

tinct lamellar structures. This is attributed to the slow inter-

diffusion of elements in the Ru-Al-Ni system. It is also

inferred that a single lamellar structure can be obtained in

this alloy by prolonged heat treatment.

• Cast Ru59Al41and Ru46Al35Ni19 alloys after heat treatment

exhibited precipitation hardening behaviour.

• Hardening of Ru39Al13Ni48 alloy after heat treatment is

due to the formation of a lamellar structure, where the hard-

ening mechanism is similar to grain boundary strengthening.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from

Indian Space Research Organization (ISRO) through project

sanction No. 9/1/(1)/99-II. The authors thank Mr. K. Sushee-

lan Nair of Vikram Sarabai Space Centre, Thiruvananthapuran,


India, for his valuable suggestions and technical support dur-

ing the course of this work.

REFERENCES

1. D. B. Miracle, Acta metall. mater. 41, 649 (1993).

2. R. L. Fleischer, Platinum Met. Rev. 36, 138 (1992).

3. R. L. Fleischer, R. D. Field, and C. L. Briant, Met. Trans. A

22, 404 (1991).

4. R. L. Fleischer, D. W. McKee, Met. Trans. A 24, 763 (1993).

5. I. M. Wolff and G. Sauthoff, Metall. Mater. Trans. A 27,

1395 (1996).

6. I. M. Wolff and G. Sauthoff, Acta mater. 45, 2949 (1997).

7. I. M. Wolff, JOM 49, 34 (1997).

8. I. M. Wolff and G. Sauthoff, Metall. Mater. Trans. A 27,

2642 (1996).

9. F. Mucklich and N. Ilic, Intermetallics 13, 5 (2005).

10. D. C. Lu and T. M. Pollock, Acta mater. 45, 1035 (1999).

11. T. M. Pollock, D. C. Lu, X. Shi, and K. Eow, Mat. Sci. Eng.

A 317, 241 (2001).

12. T. K. Nandy, Q. Feng, and T. M. Pollock, Scripta materia-

lia 48, 1087 (2003).

13. T. K. Nandy, Q. Feng, and T. M. Pollock, Intermetallics 11,

1029 (2003).

14. Q. Feng, T. K. Nandy, B. Tryon, and T. M. Pollock, Inter-

metallics 12, 755 (2004).

15. I. J. Horner, N. Hall, L. A. Cornish, N. J. Witcomb, and M.

B. Cortie, and T. D. Boniface, J. Alloy. Compds. 264, 173

(1998).

16. S. Chakravorty and D. R. F. West, Scripta metall. 19, 1355

(1985).

17. S. Chakravorty and D. R. F. West, J. Mater. Sci. 21, 2721

(1986).

18. N. Ilic, R. Rein, M. Goken, M. Kempf, F. Soldera, and F.

Mucklich, Mater. Sci. Eng. A 329-331, 38 (2002).

19. F. Soldera, N. Ilic, N. M. Conesa, I. Barrientos, and F.

Mucklich, Intermetallics 13, 101 (2005).

20. P. K. Gudla, Anil Borah, A. Srinivasan, and P. S. Robi, Proc.

Conf., National vonference on Recent Advances in Material

Processing (RAMP-2001), sep. 7-8, Annamalai University,

India (2001).

21. T. B. Massalkski, Binary Alloy Phase Diagrams, p. 102, ASM

International, Materials Park, OH (1986).

22. M. S. A. Karunaratne and R. C. Reed, Acta mater. 51, 2905

(2003).

23. B. Tryon and T. M. Pollock, Mat. Sci. Eng. A 430, 266 (2006).

24. Anil Borah, Ph.D thesis, Indian Institute of Technology

Guwahati (2006).

microstructural evolution and hardening behaviour of cast and heat-treated ru-al and ru-al-ni alloys

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