Study of kinetics of phase transitions in the CuMnAl alloys

Abstract

The alloys of the system Cu-Mn-A1 have been extensively investigated. The results of systeniatic studies of phase equilibria in those alloys have been recapitulated by WEST, THOMAS, and by KOSTER, GODECKE. However, the alloys of the composition approximating Cu,MnAl have been drawing special attention.At high temperatures the alloys of such composition are comprised in a range of eyuili-I)riritti of the @-phase which has siniple crystalline structure and is often called the Heusler ~ihase. The ~iroperties of the above alloys a.re very interesting due to the siiiiultaneous occicrrence of two kinds of structural phase transitions: the "order-disorder" ttansforiiiations in tlic range of /3-phase and the decotnposition of the @-phase in the teinperature range of its riletastability. It was indicated that ahove 1050 K the @-phase has cubic disordered structure of the type A2 while, when cooled, it transforms a t 1050 K into ptwtially ordered B2-type structure and at ahout 950 K into completely ordered L2,-type one, DUBOIS (LISSE).On the other hand, the @-phase is inetastable a t teniperatures below 970 K and decom-1)oses into three phases of the types BRlx, y-Crt9Al, and T-Cu,Mn,AI (WEST, THOMAS; KGSTER, GODECKE).The siinriltaneous occurrence of the ordering and decomposition processes may be an interesting subject of theoretical considerations (NISHIZAWA et al.). The processes tnay be corisitlered either on microscopic level requiring various simplifying assiiiiiptions or through the stridy of processes of precipitation in real structicre. Thc aiin of tho presented work was the detailed research of the process of deconi1)osition of @-phase in the stoichiotnetric alloy Cu,MnAl (KOZUBSKI et 81. 1982, 1983a, 11, c). During t h e investigations the methods of X-ray diffraction and the measureinent of electrical resistivity as well as electron microprobe were applied. The X-ray diffraction and riiicroprobe tiieasurements were performed a t room tempmature antl the specimens quenched i n water after the desired annealing were examined. All samples of the alloy were a t first Iiotilogenizetl a t 1123 K and then annealed a t various temperatures between 423 antl 973 K . Figure 1 shows the results of quantitative X-ray diffraction analysis of the samples annealed isothermally at fi33, 733 and 843 K (Figs la--c) and aged isochronally for 24 h between 423 and 973 K (Fig. Id). It is visible that the kinetics of the reaction a t 733 K arid 833 K differed strongly from those taking place at 633 K. While a t 633 K single process was observed, a t 733 and 833 K the decomposition of @ phase proceeded in two stages. At first rapid precipitation of B-Mn was accompanied by the decay of B-phase and the simultaneous formation of a new phase called @' isostructriral with the @-phase, however, with the reduced lattice parameter. This process was followed by slow decomposition of the @'-phase into y-Cu,Al, and T-Cu,Mn,Al. The rate of the latter process was strongly decreased when the annealing temperature was increased.The investigation of t...