peculiarities of tensile deformation of...

METAL 2007 22.-24.5.2007, Hradec nad Moravicí

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PECULIARITIES OF TENSILE DEFORMATION OF MOLYBDENUM

AND MOLYBDENUM-RHENIUM SINGLE CRYSTALS AT ROOM

TEMPERATURE

G.S. Burkhanov, V.M. Kirillova, A.R. Kadyrbaev, V.V. Sdobyrev, and

V.A. Dement’ev

Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,

Leninskii pr. 49, Moscow, 119991 Russia

Abstract

In this work, we report data on the tensile deformation of molybdenum and molybdenum-

rhenium (by an example of the Mo-7 wt % Re alloy) single crystals at room temperature. The

rhenium alloying was shown to change both the low-temperature deformation regime, which

is typical of pure molybdenum, to the mediate-temperature regime of unidirectional slips and

dependence of properties on the crystallographic orientation. Molybdenum single crystals

having the [110] orientation are characterized by minimum strengthening and maximum

plasticity, whereas the maximum plasticity of molybdenum alloyed with rhenium is observed

for [100] single crystals. An experiment consisting in the noncontinuous tensile deformation

of the Mo-7 wt % Re single crystal is described for the first time. Each interruption and

subsequent continuation were found to cause an increase in the stress that is proportional to

the interruption time and disappears on continuation the tension.

The alloying-induced change in the deformation behavior of the single crystals agrees with

results obtained in studying the slipbands on the surface of deformed samples by an Opton

optical microscope using a Nomarskii interference-contrast method.

1. INTRODUCTION

Study of plastic deformation of refractory metals is of practical importance from the

viewpoint of their machiability and increase in the mechanical properties. In this connection,

some theoretical problems related to crystallographic and atomic mechanisms of plastic flow

of metals depending on their orientation, purity, and deformation conditions should be

clarified. Single-crystal metals are ideal subjects to solve the problems since they allow one to

avoid difficulties related to nonuniform deformation of grains in polycrystalline samples.

Most complete data on the plastic deformation and mechanical properties (determined in

tensile testing) of molybdenum single crystals produced by zone melting are given in

monography [1]. Substantial dependence of properties on the crystallographic direction is

observed, namely, the anisotropy of strength and plastic properties can reach 30-40 and 100%,

respectively. The tensile deformation exhibits a local character; in this case, the minimum

strengthening and maximum plasticity are observed for the deformation along the [110]

direction, whereas the maximum strengthening and minimum plasticity are observed in

testing along the [100] direction. The behavior of deformation curves for different

crystallographic orientations and the anisotropy of plasticity were explained assuming the

orientational dependence of intersecting slip for the bcc lattice and the probability of

formation of α<100> dislocations. To confirm the assumption, X-ray techniques were used.

The aim of the study is to study the effect of alloying on the tensile deformation of

molybdemun alloys single crystals and character of dependence of the mechanical properties

on the crystallographic directions. As the model alloy, we used the molybdenum alloy

containing 7 wt% Re. Such a choice is explained by the fact that molybdenum-rhenium alloys

occupy a highly important place owing to the rhenium effect on the physical and mechanical


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properties of molybdenum. Rhenium increases both strength and plasticity of molybdenum

(so-called “rhenium effect”) [2]. Such an effect is explained by causes related to the

neutralization of unfavourable effect of interstitial impurities and realization of the additional

mechanism of twinning deformation.

2. EXPERIMENTAL

[100] pure molybdenum single crystals and [100] and [110] Mo-7 wt% Re single crystals

were grown by electron-beam zone melting [1] in a vacuum of ~5⋅10-5 mmHg that was

produced by roughing-down and diffusion oil-vapor pumps, nitrogen trap, and sorption trap

with anodized titanium sorbent.

As the starting material, we use the alloy produced by vacuum arc melting. Molybdenum-

rhenium alloy blanks were prepared from molybdenum and rhenium powders that were

mixed, annealed and wetted with an alcoholic solution of glycerol, and pressed to rods, which

were sintered in hydrogen atmosphere and degassed in vacuum to remove gas impurities.

Subsequently, the rods were arc melted.

The single crystals were grown using threefold run of zone; the third run was realized when

melting from the seed. After each melting, the single crystal was turned over. The speed of

two first runs is 4 mm/min; the speed of the third run is 2 mm/min. The diameter of single

crystals is 15 and 22 mm; their length is 200 mm.

The concentrations of impurities in the single crystals were determined by mass spectrometry

(see Table)

Table. Concentrations of impurities in the single crystals

Impurities C O H Fe W Cr P Mn Na Ca K Co Ta

pure Mo 1.8 1.2 1.0 13 180 5.5 1.8 0.5 0.7 95 56 0.3 0.4 Mo-7 wt% Re 1.2 2.2 1.0 33 320 3.9 1.5 0.2 1.0 60 25 0.1 0.4

The study of the microstructure of Mo and Mo-7 wt % single crystals shows that the alloying

with Re increases the angular misorientation of the alloy by 1.5-2 times. Moreover, the

alloying changes the structure of subgrains (Figs. 1a and 1b), namely, subgrains do not form

closed polygons (like in the case of pure molybdenum) that are diffuse at the end.

Blanks ~40 mm in length were spark-cut from the single crystals obtained. For mechanical

tests, cylindrical samples having a gage length of 15 mm and a diameter of from 2.9 to 3.8

mm (Figs. 1c and 1d) were prepared using a grinding machine.

Before loading, to remove deformed layer and obtain adequate surface (that is necessary to

study slipbands with an optical microscope) the surface of samples was electropolyshed at

room temperature at a voltage of 10 V and a current of 1.2 A using an electrolyte containing

86% ethanol and 14% sulphuric acid.

The tensile tests of the [110] Mo and [110] Mo-7 wt% Re single crystals were realized at

room temperature at a rate of 1.11⋅10-3 s

-1 using an Instron TT machine; the [100] Mo-7 wt%

Re single crystal was tested at a rate of 9.3⋅10-3 s

-1.

The deformed surface of the samples was studied in an Opton optical microscope using the

Nomarskii interference-contrast technique that allows the surface relief characterized by slight

difference in the hill heights to be revealed.

Moreover, a sample oriented so that an unidirectional slip is realized was cut from the Mo

single crystal. The sample was subjected to tensile test at a rate of 1.1⋅10-3 s

-1. The aim of the

experiment is to study changes in the surface relief as compared to that of the aforementioned

samples.


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a b

c d

Fig. 1. Microstructure (x100) of (a) Mo and (b) Mo-7 wt% Re single crystals.

(c) Orientation of deformation axis. (d) Appearance of a tested sample

Two samples cut from the [110] Mo-7 wt% Re single crystal were subjected to tensile tests;

one of them was loaded noncontinuously and the other was tested continuously at a rate of

1.1⋅10-3 s

-1. The deformation curves and changes in the surface relief of samples were

analyzed comparatively.

3. RESULTS AND DISCUSSION

Figure 2 shows the deformation curves of the [100] Mo single crystals and [100] and [110]

Mo-7 wt% Re single crystals.

150

200

250

300

350

400

450

500

550

600

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4

Elongation, mm

Tensile stress, MPa

Mo [100]

Mo - 7 wt% Re [110]

Mo - 7 wt% Re [100]

Fig. 2. Tensile deformation curves for the [100] Mo single crystal and [100] and [110]

Mo-7 wt% Re single crystals.


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The [100] Mo single crystal loaded at room temperature exhibits a parabolic curve of

strengthening characterized by so-called low-temperature regime of deformation, at which

several equivalent slip systems are realized primarily. The ultimate strength is sufficiently

high; it is σb = 555 MPa.

This value agrees with data obtained earlier for a molybdenum single crystal produced by

electron-beam melting [3].

Figure 3 shows the micrograph of the surface of the [100] Mo single crystal subjected to

tension. A fine slightly distinguishable relief exhibiting several slip systems (Fig. 3a) is

observed; this is typical of the deformation of pure molybdenum [3].

a b`

c

Fig. 3. Relief (x400) of deformed surface of Mo single crystals subjected to tensile

deformation: (a) [100] orientation, several slip systems are observed; (b) orientation of

unidirectional slip: slipbands are more pronounced; primary slip system is dominant

(secondary slip systems are observed locally); and (c) relief at the sample neck.

Samples of Mo single crystals, which were especially oriented for the unidirectional slip,

exhibit more clear slipbands at the surface (Fig. 3b). The (101) [111] primary slip system is

mainly observed. Traces of secondary slipbands can be observed locally. The surface relief of

the sample neck that is characterized by substantial local plastic deformation is more

pronounced (Fig. 3c).

The alloying of molybdenum with rhenium changes the deformation character of the single

crystals (Fig. 2). The tension curves of the Mo-7 wt% Re single crystals with the [110] and

[100] orientations differ from not only the curves for the Mo single crystal but also from one

another.


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(i) The deformation curve of the [110] single crystal is virtually parabolic, whereas the

deformation curve of the [100] single crystal exhibits the presence of three stages that

correspond to the “mediate-temperature” deformation regime with the unidirectional slip.

(ii) The elongation of the [110] single crystal is less than that of the [100] single crystal.

(iii) The maximum dσ/dε value for the [110] Mo-Re single crystal is less than that for the

[100] Mo-Re single crystal.

(iv) The temperature dependence of the critical stress for the [110] single crystal lies above

that for the [100] single crystal, whereas its strength is lower.

The surface relief of alloyed samples is more clear as compared to that of the Mo single

crystal. Figures 4a-4d show the surface of different sections of the [100] Mo-7 wt% Re single

crystal. As is seen, the relief character depends substantially on the section under study, i.e.,

on the surface orientation with respect to the operating slip systems. The slipbands are

observed the most clearly in Fig. 4d, which was obtained in rotating the sample by 45° with

respect to the plane shown in Fig. 4. The slipbands are clearly pronounced. The unidirectional

slip is dominant (traces of the secondary slip are observed). The height of some bands is

substantial.

a b

c d

Fig. 4. Relief (x400) of the deformed surface of the Mo-7 wt% Re single crystal (σb =

387 MPa): (a) section of the side surface with slightly distinguishable slipbands

(Burgers vector of the primary system is in-plain); (b) section with irregular surface

(the rotation of sample by 90° about an axis); (c) section of the surface with clear

slipbands (the rotation of sample by 45° about an axis from the plane shown in Fig.

4a; and (d) deformed surface of the same single crystal after tensile deformation, when

one of the slip systems is dominant.


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Figure 5 shows the deformation curves for [110] Mo- 7 wt% Re single crystal subjected to

noncontinuous and continuous tensile tests. The subsequent deformation after the interruption

was found to cause the increase in the stress that is proportional to the interruption time. After

the deformation renews, the increase disappears.

160

180

200

220

240

260

280

300

320

340

0.00 0.04 0.08 0.12 0.16 0.2 0.24 0.28

Elongation, mm

Tensile stress, MPa deformation to the 1st interruption

subsequent deformation to the

2nd interruption

subsequent deformation after the

2nd interruption

continuous deformation

Fig. 5. Tensile deformation curves for the [100] Mo-7 wt% Re single crystals

subjected to noncontinuous and continuous deformation at a rate of 1.1⋅10-3 s

-1.

4. CONCLUSIONS

1. Molybdenum single crystals subjected to tensile deformation are characterized by the

parabolic strengthening curve with so-called low-temperature deformation regime. In terms

of the regime, several equivalent slip systems operate from the beginning of deformation.

This is confirmed by studies of the surface relief of deformed samples.

2. The alloying of molybdenum with rhenium changes the low-temperature deformation

regime to the mediate-temperature regime with unidirectional slip.

3. The alloying of molybdenum with rhenium changes the dependence of the properties on

the crystallographic direction. The [110] Mo single crystal is characterized by minimum

strengthening and maximum plasticity, whereas the [100] Mo-7 wt% Re single crystals

exhibit the maximum plasticity.

4. In the case noncontinuous tensile deformation of the molybdenum-rhenium alloy, each

interruption causes the increase in stresses that is proportional to the time of interruption

and disappears when the deformation renews.

BIBLIOGRAPHY

1. SAVITSKII, E.M., BURKHANOV, G.S., Single crystals of refractory and rare metals and

alloys. Moscow : Nauka, 1972. 259 p.

2. POVAROVA, K.B., BANNYKH, O.A., ZAVARZINA, E.K. Low and high-rhenium

alloys: properties, production, and treatment. In Proceedings from Internal Symposium

“Rhenium and rhenium alloys”. Orlando : TMS. Miner., Metals, Mater, 1997, pages 691-

705.


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3. SAVITSKII, E.M., BURKHANOV, G.S., BOKAREVA, N.N. Orientational dependence

of tensile deformation of molybdenum single crystals. In Single crystals of refractory and

rare metals. Moscow : Nauka, 1971, pages 171-176.

4. KOTTREL, A. Kh. Dislocations and plastic flow in crystals. Moscow : Metallurgizdat,

1958. 267 p.

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