Materials Science and Engineering A314 (2001) 186–193

Fatigue and fracture behavior of nickel–titanium shape-memoryalloy reinforced aluminum composites

G.A. Porter a,*, P.K. Liaw a, T.N. Tiegs b, K.H. Wu c

a Department of Materials Science and Engineering, The Uni�ersity of Tennessee, Knox�ille, TN 37996-2200, USAb Metals and Ceramics Di�ision, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

c Department of Mechanical Engineering, Florida International Uni�ersity, Miami, FL 33174, USA

Abstract

A shape-memory alloy, nickel–titanium (NiTi), has been distributed throughout an aluminum matrix, using powder-metallurgyprocessing, in the hope of using the shape-memory effect to achieve strengthening and improve the fatigue resistance, as comparedwith the aluminum matrix. The shape-memory effect was activated by cold rolling the samples at −30°C. Upon reheating to theaustenite phase, the NiTi was expected to return to its original shape, while embedded in the aluminum matrix. This action createdresidual, internal stresses around each particle, which strengthened the material. The yield and ultimate strengths, and fatigue livesof the NiTi reinforced aluminum composites, have been improved considerably, as compared with the unreinforced material. Thecross-sectional microstructures of the composites, as well as, the modes of crack growth, have been examined with scanningelectron microscopy (SEM) to identify fatigue and fracture mechanisms. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Scanning electron microscopy; Yield strength; Ultimate strength

www.elsevier.com/locate/msea

1. Introduction

A shape-memory alloy (SMA) has the unique prop-erty of recovering its original shape after deformationhas occurred. Once only regarded as a phenomenon, ithas been proven that the unique shape-memory behav-ior is due to a phase change in the material. This phasechange from a softer martensite to a harder austenite istemperature-dependent, and its micromechanism hasbeen theoretically explained in detail by authors suchas, Wayman [1] and Nishiyama [2]. The shape-memoryeffect is the remarkable result of this phase transforma-tion. For example, if a shape-memory alloy is cooledbelow a certain temperature labeled as the martensiticstarting temperature (Ms), a phase change to martensiteoccurs. If there is deformation of the material while inthe martensite phase, then the material will return to itsoriginal shape upon reheating to another specific tem-perature (usually higher than the Ms temperature) la-beled as the austenitic starting temperature (As). Up to6–10% deformation can be recovered through this pro-cess, thus providing enough change in shape to allowmany applications of the shape-memory properties.

The purpose of the present study was to create aNiTi/Al metal matrix composite (MMC) material,which would have mechanical properties superior tothose of the aluminum matrix. The goal was to fabri-cate a composite material by dispersing a shape-mem-ory alloy (NiTi) in the form of a powder, into analuminum matrix, using cold isostatic pressing or hotpressing, powder-metallurgy processing techniques. Theauthors wished to obtain a composite material with asatisfactory density (�97% of theoretical density),greater strength, and improved fatigue resistance, rela-tive to the aluminum matrix.

This work represents the formulation of a new, andunique idea pertaining to particulate strengtheningmechanisms. Tiegs, Alexander, and Becher [3], at theOak Ridge National Laboratory (ORNL), had the ideaof dispersing the nickel-titanium shape-memory alloy,in the form of a powder, throughout an aluminummatrix in the hope of using the shape-memory effect toachieve strengthening in the aluminum matrix. Somework in this field has already been done using NiTifibers [4], but only one source, by Wei et al. [5], hasbeen found on this subject, using NiTi powders. Unfor-tunately, Wei did not give conclusive, in-depth evidence* Corresponding author.

0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0921 -5093 (00 )01915 -8

G.A. Porter et al. / Materials Science and Engineering A314 (2001) 186–193 187

about the mechanical properties of the shape-memoryalloy particulate reinforced aluminum composite thathis group had fabricated.

When shape-memory alloy particles are embedded inan aluminum matrix, as in the present work, the shape-memory effect is utilized by deforming the compositematerial below the Ms temperature, which is around−20°C [6]. This action will also deform each NiTiparticle within the matrix (since the martensitic phaseof NiTi has a much lower yield strength than aluminumat that temperature). Upon reheating to the austenitephase, the NiTi will return to its original shape, (within8–9% of deformation), embedded within the aluminummatrix, which has a much lesser degree of thermalstrain. This action will create residual, internal stressesaround each NiTi particle, tensile stresses in the longi-tudinal and transverse directions, and compressivestresses in the through-thickness direction. This ar-rangement of stresses will increase the yield strength,and strengthen the material in a similar fashion asthermal stresses strengthen a ceramic-particle reinforcedmetal-matrix composite upon cooling from the manu-facturing temperature.

2. Procedures

2.1. General procedure for composite materialfabrication

The authors adopted pure aluminum as a matrixmaterial, in an effort to avoid complications arisingfrom the reaction of NiTi with aluminum alloyingelements, such as copper. The basic fabrication proce-dure was as follows. The NiTi powders were mechani-cally milled for 6 h to reduce the powder size and toproduce coarse surfaces on powders, using a procedurewhich was developed earlier by the authors [7].

After processing, the NiTi powders were combinedwith Al 1090 aluminum powders in a 10 vol.% propor-tion, and were mixed in a blender for 4 h. The powderwas removed from the blender, and weighed into ap-propriately sized charges of about 110 g for pressing. Asteel die was used for pressing, with the inner surfaceslined with a carbon foil. The die was then filled with the

charge of powders and placed in the press. The pressureinside the press was reduced to 1.33×10−3 Pa. Thesample was then brought up to the desired pressingtemperature of 550°C, and pressed for 10 min using ahydraulic ram.

After pressing, the heating elements were turned off,and the chamber was allowed to cool. An inert flushinggas, such as nitrogen, argon, or helium, may have beenused to induce a greater heat flow away from thesample in cooling. After the temperature dropped be-low 300°C, the atmospheric pressure was equalized, andthe chamber was opened.

The densities of the pressed billets were measuredusing the standard Archimedes immersion technique,and then the samples were sliced into pieces, 1.4 mmthick. These samples were cold-rolled, some at roomtemperature, and some at the martensitic temperatureto enact the shape-memory effect, with 10% deforma-tion, resulting in pieces, 1.27 mm thick. Rolled sampleswere cut into pieces slightly over 38 mm in length.Differential Scanning Calorimetry (DSC) can detect thechange in heat flow/conductivity of a sample during atemperature rise, thus determining the occurrence ofphase transformations. DSC tests were conducted toverify the preservation of the phase transformationassociated with the shape-memory effect. Tensile testcoupons were then formed from these samples with adie punch in a hydraulic press. These tensile test cou-pons were in accordance to the American Society forTesting and Materials (ASTM) standard E8-91 [8], andwere of the dimensions depicted in Fig. 1. Samplesmade without the NiTi reinforcement material weremanufactured in the same way as the reinforced mate-rial, which has been described earlier.

2.2. Mechanical testing

A servohydraulic Material Test SystemTM (MTSTM)test frame, with a load capability of 10 kN, was used toconduct tension and fatigue tests for the NiTi rein-forced aluminum matrix composite materials, as well assample of unreinforced matrix materials. Tension testswere conducted at a strain rate of 4.1 mm s−1. Fatiguetests were performed in order to develop the maximumapplied stress versus fatigue life cycle (S-N) curves.These tests used a load ratio, R, of 0.2, where, R=�min/�max, and �min and �max are the minimum and maxi-mum stresses experienced during the cycle, respectively.

A peak to peak compensator was used in the com-puter program to control the accuracy of the loadlimits. Fatigue tests were started at a frequency of 1 Hz,to allow the compensator to quickly adjust, and thenthe frequency was slowly increased to 50 Hz. Thisprocedure ensured that each cycle experienced accuratevalues of �min and �max, and that the test producedreliable data. For higher stress level tests (e.g., fatigueFig. 1. Dimensions of the tensile test coupons. Dimensions are in mm.

G.A. Porter et al. / Materials Science and Engineering A314 (2001) 186–193188

life �500 cycles), the frequency was only increasedto 10 Hz, in order to allow more control andaccuracy over the short length of the test. At least tenfatigue tests were conducted for each materialover a wide stress range, so that a fairly accuraterepresentation of the data could be displayed in the S-Ncurves.

Following the material testing, the microstructure ofthe materials and the fracture surfaces, were carefullyexamined using a Hitachi 8500 scanning-electron mi-croscope (SEM).

3. Results and discussion

For convenience in the following discussion, the vari-ous materials have been given an abbreviated label,indicating the composition, the powder sizes, and thedeformation temperature. The first letters signify thematrix material. The next number is the matrix materialpowder size in microns. The next series of letters repre-sent the reinforcement material (if any). The followingnumber is the reinforcement powder size in microns.The last number is the deformation temperature in °C.For example, the composite material, designated as‘Al20.NiTi5-30’, has a mixture of 20 �m aluminumpowder and 5 �m NiTi powder, cold-rolled at a temper-ature of −30°C.

3.1. Product density

It has been found that NiTi and aluminum powderparticles of different shapes and sizes affect theproduct density. Table 1 contains a few com-binations of the various particle sizes used to producedifferent samples of materials, their pressing tempera-tures, and the resulting densities. There are twocombinations of 5 and 40 �m NiTi powders, with Al1090 aluminum powders of a 20 �m diameter, and onematerial that has no NiTi powder at all. The pressingtemperature is appoximately 550°C, and thedensities ranged from 99.61 to 99.94%. German [9]states that the materials are more easily densifiedwhen combining particles of many differing sizes, asopposed to using particles that all have the samesize. The mechanical milling procedure producesa large assortment of particles with different sizes, withthe average size being around 5 �m. It can generally besaid that the materials, made with the smaller, 5 �mdiameter NiTi particles (Al20.NiTi-30), which havebeen mechanically milled, have higher densities thanthose made with the larger, unmilled NiTi(Al20.NiTi40-30), or the one with no NiTi particles(Al20-30), considering the differences in pressingtemperatures.

3.2. The influence of the particle size on mechanicalproperties

The powder sizes of the NiTi reinforcement plays animportant role in the mechanical properties of thematerials. It may be observed from Table 1 that theNiTi reinforced composite materials exhibit higherstrengths than the unreinforced materials, and that thesmaller, 5 �m NiTi powder particles provide a greaterimprovement in the strengths in comparison to the 40�m NiTi particle reinforced composites. This behaviormay be due to the larger NiTi particles that are almostperfectly spherical in shape, lending a minimum of aninterfacial bonding area. The smaller, mechanically-milled particles have a greater amount of surface areaavailable for matrix bonding. As assumed, the enhance-ment of this interface, and possibly also the presence ofintermetallic compounds, which may have been formedduring hot pressing, contributed greatly towards themechanical behavior of the composite material.

3.3. The influence of the NiTi reinforcement onmechanical properties

Fig. 2 reveals the comparison of the stress-straincurves for the composite materials and the aluminummatrix. It can be seen that the material, which wasfabricated using the powder metallurgy processing(Al20-30), exhibits a greatly improved strength, tough-ness, and elongation in comparison with the pure castsample. Furthermore, the mechanical properties of theNiTi reinforced materials, (Al20.NiTi5–30 andAl20.NiTi40-30), display a further amount of strength-ening over the unreinforced matrix. The material, madewith the mechanically-milled NiTi powders(Al20.NiTi5-30), reveals an enhancement of thestrength in comparison with the composite materialmade with the unmilled NiTi powders (Al20.NiTi40-30), although the elongation properties are reduced.However, this behavior may be commonly observed inalmost any composite reinforced material.

The numerical values for the mechanical properties,which have been mentioned earlier, are listed in Table1. The composite material, containing NiTi particleswith a 40 �m diameter (Al20.NiTi40-30), provides a 3%benefit in the yield strength, and a 25% increase in theultimate strength, over the unreinforced material (Al20-30). Whereas, the composite material, made with themechanically milled powder (Al20.NiTi5-30), which hasa NiTi particle size of about 5 �m, drastically supple-ments the yield strength 42%, and ultimate strength54%, over the unreinforced matrix. In essence, thecomposite materials show a drastic improvement in theyield and ultimate strengths, as well as the moduli.However, the elongation has decreased considerably, ascan be expected for composites.

G.A

.P

orteret

al./M

aterialsS

cienceand

Engineering

A314

(2001)186

–193

189

Table 1A comparison of the physical and mechanical characteristics of the NiTi SMA reinforced composite materials (Al20.NiTi5–30 and Al20.NiTi40-30), unreinforced matrix material (Al20-30), castmaterial (Al-1090), and wrought material (Al-1100)a

Name of sample Hot-pressingNiTi powder sizeAl powder size Density Deformation Elongation(%)�yield Young’s modulus�ultimate (MPa)(MPa)temperature (°C)or alloy (MPa)(% th.)b(�m) temperature (°C)(�m)

62 68 18 800 14.8Al-1090 � � Cast 94.9 2390 107 35 000 24−30540 99.61Al20-30 20 �

−3020 93 134 37 500 1440 550 99.49Al20.NiTi40-3099.94 −30 128 165 47 000 520Al20.NiTi5-30 5 550

a All samples were produced using 69 MPa pressure on a 5.715 cm steel die.b Percentage of the theoretical density.

G.A. Porter et al. / Materials Science and Engineering A314 (2001) 186–193190

Fig. 2. The stress vs. strain curves for the 20 �m powder aluminum matrix composites.

Fig. 3. Peak stress versus number of cycles until failure for the aluminium and NiTi reinforced aluminum matrix composites.

3.4. The influence of the particle size on the fatigueproperties

In Fig. 3, it can be seen that the NiTi reinforced, 20�m sized aluminum particle materials have a clearadvantage in the fatigue life, as compared with thematrix material, especially at higher stress levels. De-spite the fact that the composite material made with the40 �m NiTi particles (Al20.NiTi40-30) only provided a3% increase in the yield strength, relative to the matrixmaterial (Al20-30), the fatigue resistance has improvedapproximately two orders of magnitude at peak stressesabove 100 MPa. The composite material, made with themechanically-milled NiTi powders (Al20.NiTi5-30), dis-plays a remarkable amplification of both the peak-stress capability and the fatigue life, for the entire testedrange of values. Especially at the peak stress of 120MPa, where the matrix material can only withstand 50cycles until failure, the composite material, made with

the mechanically milled NiTi powders, retains its in-tegrity for 150 000 cycles.

The S-N curve (in Fig. 3) for the material made withthe 40 �m NiTi powder (Al20.NiTi40-30) convergeswith that of the matrix material (Al20-30) at just over300 000 cycles and upwards. The corresponding stressfor this value is about 95 MPa. This is just above theyield stress for the matrix material (90 MPa). The S-Ncurve for the material made with the 5 �m NiTi pow-ders (Al20.NiTi5-30) converges with that of the matrixmaterial at 8 000 000 cycles with a corresponding stresslevel of 93 MPa. Again, this stress is just above theyield stress for the matrix material. For numbers ofcycles greater than 10 000 000, it is thought that thecurves approach the yield strength value of the matrix(90 MPa) asymptotically. For higher stress levels, thecomposite materials definitely have a longer life thanthe unreinforced matrix, and the material, which con-tains the mechanically milled NiTi powders

G.A. Porter et al. / Materials Science and Engineering A314 (2001) 186–193 191

(Al20.NiTi5-30), has a greatly improved lifetime char-acteristic, which is three orders of magnitude betterthan the matrix (Al20-30) at 120 MPa.

3.5. The influence of the NiTi reinforcement on thefatigue life

The peak stress during the fatigue test was plottedagainst the number of cycles endured by each sample,producing the S-N curves displayed in Fig. 3. The S-Ncurve for the pure cast aluminum is given as a refer-ence, and it has the lowest fatigue life of any of the

Fig. 6. The fracture surface of a fatigued NiTi/Al composite sample(Al20.NiTi5-30), made with NiTi particles with a diameter of 5 �m.The maximum stress is 163 MPa. Notice that the plane of fracture islocated at a 45° angle from the direction of applied loading.

Fig. 4. The fracture surface of a fatigued NiTi/Al composite sample(Al20.NiTi5-30), made with NiTi particles with a diameter of 5 �m.The maximum stress is 99 MPa.

materials. Since most materials have a S-N curve simi-lar to the curve for the pure cast aluminum, which hasthe concave side facing upwards, it is interesting to findthat the S-N curves for the composite materials areeither flat, or have the concave side facing downward.It is thought that this behavior is due to excellentbonding characteristics between the NiTi particles andthe aluminum matrix, which may deflect crack pathsunder the loading conditions.

Figs. 4–6 are representative of the fatigue fracturemorphologies of the composite materials at differentload levels. Fig. 4 identifies the fracture surface of afatigued NiTi/Al composite sample, made with NiTiparticles with a diameter of 5 �m, and aluminumpowders with a diameter of 20 �m (Al20.NiTi5–30).This sample was tested at a low stress level (i.e., N�1 000 000 cycles). The maximum stress for this sample is99 MPa, which is the lowest stress level tested for thismaterial. This particular sample endured 2 673 129 cy-cles before failure. As can be seen, the crack propaga-tion region is flat, at a 90° angle, while the overloadregion rises sharply at 45° angle. However, the massivefailure region levels somewhat to about 30°. This exam-ple is different from most others, because the crackoriginated from the side surface of the specimen, in-stead of the edge or corner, as is usually seen.

Fig. 5 exhibits the fracture surface of a fatiguedNiTi/Al composite sample, made with NiTi particleswith a diameter of 5 �m and aluminum powders with adiameter of 20 �m (Al20.NiTi5-30). This sample wastested at an intermediate stress level (i.e. 1000�N�1 000 000 cycles). The maximum stress for this sample is116 MPa. This particular sample endured 202 870 cycles

Fig. 5. The fracture surface of a fatigued NiTi/Al composite sample(Al20.NiTi5-30), made with NiTi particles with a diameter of 5 �m.The maximum stress is 116 MPa.

G.A. Porter et al. / Materials Science and Engineering A314 (2001) 186–193192

before failure. As is seen in the earlier specimen, thecrack propagation region is flat, at a 90° angle from theloading direction, and the massive failure region isabout 30°.

Fig. 6 is another fracture surface of a fatigued NiTi/Al composite sample, made of NiTi particles with adiameter of 5 �m, and aluminum particles with adiameter of 20 �m. This sample experienced 139 cyclesbefore failure. This is a specimen, which was tested at ahigh stress level (i.e., N�1000 cycles), with a maximumstress of 163 MPa. The presence of a high stress levelhas caused an entirely different mode of failure fromthe others. The entire fracture surface is a flat, massivefailure region, located at a 45° angle from the directionof applied loading. The crack-initiation region is sominute, that it could not be located.

Fig. 7 reveals a higher magnification of the fracturesurface of a sample made with NiTi particles with adiameter of 40 �m and aluminum particles with adiameter of 20 �m. This photo reveals a crack propaga-tion region, with apparently no NiTi particles on thesurface of the crack. This trend indicates that the cracktip tends to avoid the particles and interface with thematrix. The upper left-hand area of the figure shows arough crater that was formed when the crack tip en-countered a reinforcement particle. It was expected thatthe fracture would occur along the interface, and forma smooth imprint of the particle in the matrix. How-ever, this was not the case. The crack avoided theinterface and propagated through the matrix materialin the vicinity of the particle, forming the texturedcavity shown.

It can be seen from the SEM study, that the overallcross-section of the sample is weakened from the con-tinual void formation during the extended period ofcyclic loading. As a result, the prime mode of crackpropagation is largely matrix-dominated, due to voidcoalescence through the matrix material. However, asevidenced by the SEM study of the crack faces, thestresses, which cause the crack propagation, vary fordifferent load levels. For high stress levels, the primarystresses leading to failure are shear stresses, whereas forlow stress levels, normal stresses contribute most to-wards fracture. It is, therefore, thought that much ofthe failure at high stress levels, as contrasted with theincrease in the fatigue life, is intrinsically linked tostress fields interacting with strain barriers, as describedby the Peach–Koehler relation which is given in Eq. (1)[10].

F= − [�zyby+�zxbx ]j+ [�yyby+�yxbx ]k (1)

This equation describes the retarding force, F, whichis experienced by the dislocation. � is the shear stress, bis the edge dislocation vector, and � is the tensile stress.j and k are the y and z vectors, respectively.

In the present case at this time, it is difficult toquantitatively determine the exact mechanism by whichthe shape-memory alloy affect crack growth, but it maybe possible that the increased fatigue life at higherstress levels, is due to a stress-induced phase transfor-mation in the shape-memory particles. A deeper under-standing of the relationship cannot be ascertainedwithout some additional experimentation.

4. Conclusions

The nickel– titanium shape-memory alloy reinforcedaluminum composite, with 10 vol. % NiTi, has beensuccessfully fabricated using powder-metallurgy andhot-pressing techniques, with the shape-memory effectintact.

The 5 �m diameter sized, mechanically-milled NiTipowder will produce a composite sample of a slightlygreater density than the unreinforced aluminum, pro-vided that the temperature and pressure are held con-stant, among other variables. Regardless, densities havesurpassed 99% for most materials.

Monotonic tensile tests have proven that the com-posite material has achieved an elevation in the yieldand ultimate strengths, as compared with the aluminummatrix material. This material has exhibited a 43% gainin the yield strength, and 54% gain in the ultimatestrength. However, the elongation is sharply reduced.Mechanical testing has revealed that a better-fatigue lifeperformance has been achieved in the NiTi SMA rein-forced aluminum composite material.

Fig. 7. A closer view of the surface of the crack propagation regionof Al20.NiTi20-30 sample. The formation of the crater at the top leftcorner (circled by a dotted line) was due to the presence of a NiTiparticle.

G.A. Porter et al. / Materials Science and Engineering A314 (2001) 186–193 193

At higher stress levels, especially those above theyield strength of the material, the improvement on thefatigue life of the composite material is outstanding,averaging between two to three orders of magnitude,and up to four orders of magnitude in some materials.However, at lower stress levels, the fatigue life remainsvirtually unchanged. All the tested materials show astrong convergence in the S-N curves near the yieldstrength. Usually, the effects of the microstructure onthe fatigue life tend to be stronger at lower stresses, butthis is the opposite of what has been found in thepresent case. Tests conducted at higher stress levelsshowed the greatest increase in mechanical properties,such as the maximum strengths and the fatigue lives.The enhanced performance of the material at highstress conditions may indicate a phase change in theNiTi particles at those stresses. Further research ispresently being conducted to verify these interestingspeculations behind the causes of the observedbehavior.

Acknowledgements

The present work was supported by the Oak RidgeNational Laboratory, under the subcontract 11X-SY356V, to the University of Tennessee, and by theDivision of Advanced Energy Projects, Office of BasicEnergy Science, US Department of Energy, under con-tract DE-AC05-840R21400 with UT-Battelle, LLC. We

are also grateful to the National Science Foundation,under contract numbers of DMI-9724476, EEC-9527527, and DGE-9987548, with Dr D.R. Durham,M.F. Poats, Dr W. Jennings, and Dr L. Goldberg asprogram monitors, respectfully, and also the Southeast-ern Universities Research Association with Professor T.Hutchinson as the contract monitor.

References

[1] C.M. Wayman, MRS Bulletin, pp. 49–56, April 1993.[2] Z. Nishiyama, Martensitic Transformation, Academic Press,

New York, 1978.[3] T. Tiegs, K. Alexander, P. Becher, Shape-Memory Allow Rein-

forcement of Metals, Oak Ridge National Laboratory, Metalsand Ceramics Division, Oak Ridge, TN 1996.

[4] W.D. Armstrong, T. Lorentzen, P. Brondsted, P.H. Larsen, ActaMaterialia 46 (10) (1998) 3455–3466.

[5] Z.G. Wei, C.Y. Tang, W.B. Lee, L.S. Cui, D.Z. Yang, Mater.Lett. 32 (1997) 313–317.

[6] G.A. Porter, P.K. Liaw, T.N. Tiegs, K.H. Wu, Nickel-TitaniumShape-Memory Alloy Reinforced Aluminum Composites, Jour-nal of Metals 52 (10) (2000) 52–56.

[7] G.A. Porter, P.K. Liaw, T.N. Tiegs, K.H. Wu, Scripta Metallur-gica et Materialia 46 (12) (2000) 1111–1117.

[8] ASTM (American Society for Testing and Materials) Standard(E8M-91), Standard Test Methods of Tension Testing of Metal-lic Materials, 1996.

[9] R.M. German, Powder Metallurgy Science, Metal Powder In-dustries Federation, Princeton, NJ, 1989.

[10] J. Verhoeven, Fundamentals of Physical Metallurgy, Wiley,1975.

.