Temperature hysteresis of martensite transformation in agingCu–Mn–Al alloy

V.V. Kokorin, L.E. Kozlova *, A.N. Titenko

Institute of Magnetism, 36-b Vernadsky Blvd., 03142 Kiev-142, Ukraine

Received 27 February 2002; accepted 21 March 2002

Abstract

The coherent precipitates inherited by martensite crystals can change the transformation parameters. Non-monot-

onous change of temperature hysteresis width of b1 $ c0 transformation versus aging time is revealed.� 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Ferromagnetic precipitates; Martensite; Aging alloys; Cu–Mn–Al alloy

1. Introduction

Aged alloys Cu–Mn–Al undergo a martensitetransformation. The system of disperse ferromag-netic inclusions situated in a non-ferromagneticmatrix can be formed by means of a heat treat-ment. It was shown before [1], that the small co-herent inclusions of Cu2MnAl phase precipitatedduring austenite aging of Cu–Mn–Al alloy in (Cu–Mn)3Al matrix. Their mean size is much less thana critical martensite nucleus. The changes of theinclusion magnetic properties connected withmartensite transformations of a surrounding ma-trix can be observed. It is known that the shapedeformation induced by martensite transformationis mainly a shear one. It leads to the shear defor-mation of the inclusions located inside the mar-tensite crystals. The elastic deformation of the

precipitates in martensite gives rise to a change oftheir magnetic properties. This paper is devoted tothe study of martensite transformation in non-uniform Cu–Mn–Al alloy. The characteristictemperatures and hysteresis of martensite trans-formation mainly depend on the composition ofalloy, the size and number of the precipitates.

2. Experimental results and discussion

The investigated alloy Cu–Mn–Al with com-position (wt.%) Cu-4, 8Mn-13, 1Al was melted inan induction furnace in an argon atmosphere.After homogenization annealing at 1123 K during10 h the samples were quenched into water andthen aged at 498 K. The temperature dependenceof electrical resistance q and low-field magneticsusceptibility v have been measured. The characterof qðT Þ and vðT Þ changes is conditioned by themartensitic transformation. An existence of tem-perature hysteresis of the dependence qðT Þ andvðT Þ (Figs. 1 and 2) is evidence for this conclusion.

Scripta Materialia 47 (2002) 499–502

www.actamat-journals.com

*Corresponding author.

E-mail address: kozlova@im.imag.kiev.ua (L.E. Kozlova).

1359-6462/02/$ - see front matter � 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462 (02 )00136-7

Growth of electrical resistance (Fig. 1) duringcooling the sample can be explained by the re-placement of a part of the sample volume by themartensite phase having higher q in comparisonwith high-temperature phase. This type of resis-tance change versus temperature is inherent forCu-based alloys with thermoelastic martensite [2].The temperature dependence of magnetic sus-

ceptibility and electrical resistance for previouslyaged samples at 498 K during 2 h is shown on Fig.2.The sharp decrease of v in temperature interval

of q growth can be seen. According to Figs. 1 and2 the transformation of low-temperature c0-phaseinto b1 is completely finished during heating thesamples to the room temperature.The martensite transformation affects the

properties depending on the structural state of thematrix and also on the magnetic properties whichare completely determined in this case by the stateof the disperse precipitates. A matrix is paramag-

netic and its contribution to the v is negligible. Themagnetic properties changed visibly at tempera-ture near Ms. The changes of properties are con-ditioned by the appearance of internal stresses inthe precipitates inherited by the martensite crystalsfrom high-temperature phase [1]. Magnetic stressanisotropy is responsible for a decrease of v valuein martensitic phase.Characteristic temperatures of martensite trans-

formation and treatment conditions are shown inTable 1.Following definitions are used H ¼ Af �Ms––

hysteresis of martensitic transformation, As andAf––respectively start and finish temperature ofreverse c0 ! b1 transformation, Ms and Mf––startand finish temperature of direct b1 ! c0 transfor-mation.The hysteresis of transformation (H) as well as

temperatures Ms, Mf , As and Af were determinedfrom the graphs of the temperature dependence ofthe electrical resistance q=qm (qm––max value of q)and low-field magnetic susceptibility. In case of

Table 1

Alloy heat

treatment

Ms (K) Mf (K) As (K) Af (K) H (K)

Quenching

1123 K

176 165 197 213 37

498 K, 1 h 197 193 222 232 35

498 K, 2 h 203 193 222 233 30

498 K, 3 h 222 210 223 234 12

498 K, 5 h 227 213 225 245 18

Fig. 1. Temperature dependence of electrical resistance q of samples of Cu–Mn–Al alloy. (a) Quenching, (b) aging 1, (c) 3 h, (d) 5 h.

Fig. 2. Temperature dependence of magnetic susceptibility v (a)and electrical resistance q (b) of the sample aged at 498 K, 2 h

500 V.V. Kokorin et al. / Scripta Materialia 47 (2002) 499–502

Cu–Mn–Al alloy one can see (Fig. 1) the increaseof Ms with increasing annealing time. The tem-perature hysteresis has a minimum value H ¼ 12K for aging 3 h and further increasing of agingtime gives rise to the growth of H.The disperse precipitates (size of 5–10 nm) were

formed as a result of aging in the high-temperatureb1-phase [1]. There were the characteristic distri-butions of the diffuse scattering intensity of X-raysin the vicinity of the main matrix reflexes. Suchdiffuse scattering is typical for the decomposedsolid solutions with nearly spherical shape pre-cipitates (dilatation centers) which are uniformlydistributed in the sample volume [3]. Narrowing ofthe martensite transformation hysteresis was alsoobserved for aging Fe–Ni–Co–Ti [4] with the samekind of structure of the decomposed solid solutionas Cu–Mn–Al. Let us discuss the origin of ob-served change ofH using an analogy between thesealloys. It was shown for Fe–Ni–Co–Ti alloy sys-tem [3] that thermoelastic c $ a transformation inthis alloy was realized in case of high-enough de-gree of tetragonality of martensite crystal lattice.The tetragonal lattice distortion is initiated bycoherent c0-phase precipitates inherited by mar-tensite from the aged austenite. The martensitetetragonality reaches the value c=a � 1:15 for thecorresponding aging time and c $ a transforma-tion shows narrow hysteresis with a minimumwidth of temperature hysteresis equal to 20 K [4,6].The increase of c=a leads to a decrease of macro-scopic shear in case of martensite transformationfcc austenite into bcc martensite. Such decreasepromotes the lowering of coherent stresses on in-terphase martensite–austenite boundary. It meansthat the elastic energy of the martensite crystal willbe lowered. As it was shown for Cu–Mn–Al alloy[1] the inclusions with cubic lattice are also co-herent with orthorombic martensite lattice so someanalogy with alloy type Fe–Ni–Co–Ti can be con-sidered. Let us discuss the hysteresis nature in moredetail. The condition of thermo-elastic growth ofthe martensite crystal, according to Kurdymov [5],may be written as follows:

DF � Ee ¼ 0; ð1Þ

where DF ¼ F c � F a––difference of free energyfor high-temperature and low-temperature phases,

Ee––elastic energy of coherent martensite crystal inan austenite matrix. Coherent stresses related to anindividual crystal of martensite decrease due to aninevitable relaxation during the temperaturechanges. The decrease of the martensite crystal sizestarts not just after the beginning of the heatingbut at a higher temperature in the course of atransition from cooling to heating. The conditionsof thermo-elastic equilibrium will be different forcooling and heating:

DF ðt1Þ � Ee ¼ 0; ð2Þ

DF ðt2Þ � aEe ¼ 0; ð3Þ

where Eq. (2) corresponds to the growth and Eq.(3) to the decrease at the same size of martensitecrystal, t ¼ T=T0. T0 is the equilibrium temperaturebetween austenite and martensite phases, at whichDF ¼ 0 in case Ee ¼ 0. t2 � t1 ¼ H=T0 is the tem-perature hysteresis of the transformation; a6 1––acoefficient depending on strength of the materialand determining the degree of relaxation of co-herent stresses. By taking into account that H=T0 1 (see Figs. 1 and 2) we can write:

DF ðt2Þ ¼ DF ðt1Þ þ ðoðDF Þ=otÞt¼t1ðt2 � t1Þ þ � � �

ð4ÞUsing Eqs. (2)–(4) we can obtain the followingexpression:

H=T0 ¼ Eeð1� aÞ=L; ð5Þ

where L ¼ �T0oðF c � F aÞ=oT ¼ T0ðSc � SaÞ, Sc, Sa

are the entropy of austenite and martensite phasescorrespondingly, in this case oF =t ¼ �T0S, L––heat of martensite transformation. An expressionsimilar to the Eq. (5) was first obtained concerningto the discussion of narrow hysteresis martensitetransformation in Fe–Ni–Co–Ti alloy [6]. As can beseen from (5), hysteresis will decrease if the mar-tensite crystal growth takes place at less level ofelastic energy and at less degree of coherent stressrelaxation. In case of Cu–Mn–Al alloy the volumefraction of Cu2MnAl phase precipitates increasesduring aging in the high-temperature phase. Theseinclusions become the shear centers being inheritedby martensite that in turn promote the decreaseof shape deformation for the transformed region.

V.V. Kokorin et al. / Scripta Materialia 47 (2002) 499–502 501

It leads to a decrease of elastic energy Ee of themartensite crystal coherently connected with thesurroundings. In accordance with Eq. (5) it leadsto a decrease of temperature hysteresis of mar-tensite transformation as well. The age hardeningis also an essential factor affecting on the stressrelaxation and hysteresis width. The precipitatesgrow during aging of the austenite phase and reacha critical value giving rise to the coherency loss ofthe inclusion lattice and martensite. Further pre-cipitate coarsening results an elastic energy in-crease and a hysteresis widening.

3. Conclusions

The influence of aging on the meaning of tem-perature hysteresis for b1 ! c0 transformation inCu–Mn–Al alloy has been considered. The hys-teresis reaches a minimum (H � 12 K) at aging

for 3 h at 498 K. It is concluded that narrowing ofH is conditioned by the increase of volume frac-tion of precipitates inherited by the martensitecrystals from the austenite. The inclusions (dila-tation centers at T > Ms) transform to the shearcenters in martensite. It gives rise to a reduction ofthe elastic energy of coherent martensite crystal.

References

[1] Kokorin VV, Osipenko IA, Shirina TV. Fiz Metal Metal-

loved 1982;53:732.

[2] Warlimont H, Delaey L. In: Martensitic transformations in

Cu, Ag and Au-based alloys. New York: Pergamon Press;

1974. p. 204.

[3] Kokorin VV. In: Martensite transformations in inhomo-

geneous solid solutions. Kiev: Nauk Dumka; 1987. p. 166.

[4] Kokorin VV, Gunko LP, Shevchenko OM. Scripta Met et

Mater 1993;28:35.

[5] Kurdymov GV. J tekhnicheskoy fiziky 1948;XVIII:999.

[6] Kokorin VV, Gunko LP, Shevchenko OM. The Physics of

Metals and Metallography 1992;74(5):502.

502 V.V. Kokorin et al. / Scripta Materialia 47 (2002) 499–502