Search

Transformation strain associated with martensites and bainites has been determined by surface relief measurements with an atomic force microscope. To this end, morphological data of transformation products have been combined with data on their crystallographic orientations, which have been determined by the electron back-scatter diffraction technique. The results have shown that the transformation strain of bainites has a comparable value to that of martensites in the same alloy. The orientation relationship between the transformation products and the parent crystal has also been determined. The relationship for bainites as well as martensites was consistent with the prediction of the phenomenological theory for the transformation. It was concluded that the transformation mechanism of bainites involves lattice shearing in a manner similar to that of the martensitic transformation.

The relation between atomic ordering and martensitic transformation temperature, M s , in Ag-Zn-Al alloys was examined mainly by means of electrical resistivity measurements. Disordered bcc phase was frozen-in by quenching from a temperature above the critical temperature for ordering, T c . In a Ag-22.3at%Zn-8.9at%Al alloy, the M s temperature has been found to decrease by aging in the parent phase at temperatures between 253 and 293K. The resistivity also decreased in accord with the M s temperature. This indicates that atomic ordering proceeds by aging. The relation between the decrease in the reverse transformation temperature, A f . and the degree of long range order was obtained. In a Ag-11.0at%Zn-15.5at%Al alloy, in which the M s temperature in the as-quenched state is higher and the T c temperature is lower than that of the former alloy, aging in the martensite phase was performed. In this case, the aging brought about the increase in the A f temperature. This is in contrast to the results of aging in the parent phase. Furthermore, the effect of aging in the parent phase at temperatures higher than T c was examined. Both the transformation temperature and the resistivity were found to become higher. These changes are due to lowering in the degree of short range order.

In a previous paper, it has been pointed out that the rubber effect and stabilization of the martensite phase are caused by short range reordering during aging [K. Marukawa, K. Tsuchiya, Scripta Metall. Mater. 32 (1995) 77]. This view was further examined by experimental and theoretical studies. It has been found that the change in electrical resistivity produced by aging is well correlated with magnitudes of these effects. The relation between the short range order parameters and the representative quantities of these effects was formulated on the basis of thermodynamics. Quantitative evaluation was performed by numerical calculations utilizing the Monte Carlo method. It was found that the rubber effect is prominent when the aging temperature is in the vicinity of the order–disorder transition temperature. It was also shown that in most cases disordering or lowering in the long range order causes the stabilization.

The relation between the degree of long-range order and the martensitic transformation temperature was examined by means of electrical resistivity measurements and electron microscopic observations in Ag–Zn–Al alloys having a composition near Ag2(Zn, Al). The disordered state of the β phase (bcc structure) can be frozen by quenching from a temperature above the critical temperature for ordering. By aging at or around the room temperature, atomic ordering is developed. The ordered structure after aging in the parent phase was found to be the C11b structure. Specimens in the disordered β phase transform to the martensitic phase on cooling below room temperature. It has been found that the martensitic transformation temperature is lowered by aging at temperatures between 253 and 293 K. This indicates that ordering brings about the decrease in the transformation temperature. The resistivity of the β phase measured at the aging temperature and that of the martensitic phase at liquid nitrogen temperature also decrease by aging. The resistivity change was related to the degree of long range order and the decrease in the transformation temperature was expressed as a function of the degree of order.

The critical temperature for the order-disorder transition, T c , and the ordered domain structure of the B2 phase have been investigated by electrical resistivity measurements and electron microscopic observations in Ag-Zn alloys containing 38, 40 and 42 at%Zn. Specimens quenched from the disordered β phase were all found to be ordered in the B2 structure. Resistivity change measured during aging of the B2 phase shows a plateau at the initial stage of aging. This indicates the degree of order reaches the equilibrium value immediately after the start of aging. The B2 domain size as measured after aging and quenching increases as the aging temperature increases. However, it takes a smaller value when the aging temperature is beyond T c . Values of T c were determined by monitoring the domain size as 441, 405 and 367 K for the 42, 40 and 38Zn alloys, respectively. The ragging or fluctuating of anti-phase domain boundaries was recognized in the specimens aged at a temperature just below T c . The activation energy for the domain growth was obtained by measuring growth rates as 49.3 kJ/mol, which was judged to correspond to the migration energy of excess vacancies.

Martensitic and bainitic transformations in β phase Ag-Zn alloys have been studied mainly by optical and electron microscopy. The martensitic transformation with thermo-elastic nature is found to appear in Ag-38.0 at%Zn. The transformation temperature determined by resistivity measurements is M s =136 K. The crystal structure of the martensite is of the ordered 9R type. The martensite is also induced by the stress, through stretching or bending deformation. When the transformation is induced by stretching, it is immediately followed by the second transformation to the ordered 3R structure

The morphology of bainite structure is examined by optical and electron microscopy for Cu-Zn-Al alloys with various compositions. It depends not only on the heat treatment for the bainite formation but also strongly on the electron atom ratio, e/a, and the M s temperature of the alloy. In alloys with e/a smaller than 1.46, the 9R bainite plates are formed with a pair of the variants having (551) and (551) habit planes in a form of long hinge

The phase change from metastable β to stable α upon heating Cu-Zn and Cu-Zn-Al alloys at moderate temperature is studied by optical and electron microscopy. At higher temperature, the rod-shaped α having {321} β side surfaces is observed with dislocations building up suberain boundaries in adjacent β matrix. On the other hand, when the temperature is lower than B s , the bainite plate, α b , with a {551} β habit plane is formed at the site of α nucleus

Analyse de nombreuses courbes contrainte-deformation pour des monocristaux de Cu−Zn−Al avec un rapport electron atome de 1,40. Evaluation de la cission reduite critique pour les deux transformations successives β 1 ⇄β' 1 ⇄α' 1 . Les courbes sont analyses avec une procedure systematique de calcul de deformation de la forme du a la transformation

Etude cristallographique detaillee de la transformation martensite a martensite β' 1 →α 1 ' induite secondement par contrainte. La transformation se produit par un cisaillement simple sur le plan de base de la martensite β' 1 avec une structure d'empilement 3R quadratique a faces centrees

Determination de la deformation d'allongement dans les alliages a memoire de forme Cu-31,6 Zn-4,2 Al et Cu-28,9 Zn-6,1 Al lors d'un chauffage sous contrainte a temperature moderee. La variation de forme depend de la temperature de chauffage, de la direction de traction et de la contrainte appliquee. cette variation intervient apres une periode d'incubation et suit une courbe decrite par l'equation de Austin-Ricketts representant la cinetique d'une transformation controlee par diffusion. Determination de l'energie d'activation