Pseudoelastic Deformation of CuAlMn Single Crystals

Martensitic transformation associated with a tensile deformation of CuAlMn single crystals in [loo] direction was studied using in-situ transmission electron microscopy. Stress/strain curves obtained during pseudoelastic deformation of microscopic samples were slightly different fiom bulk ones due to martensite formation in the stage of stress increase and a permanent deformation during the first cycle. Individual needles of 2H martensite were formed already in the early deformation stage. They were growing during stress increase, joining together into large plates. At later deformation stages 18R martensite was formed usualy in stacks of single plates. Following crystallographic relationship between parent and martensitic phases was observed: [001]P, 11 [010]y',P' and [110]P, 11 [001] y',fY. At a high stress concentration near crack tips, a' martensite of 3R lattice was observed. Identification of martensite structure was very often not possible there, due to an extremely high random stacking faults density. 1.INTRODUCTION Martensitic transformation associated with deformation was most intensively studied in CuAlNi alloys [I-31, where multistage stress strain curves corresponding to a successive martensite to martensite transformation has been reported [2,3]. Similar phenomena were found during pseudoelastic deformation in CuAlFe [4] and CuAlZn [5] alloys. Structure of martensite was identified using X-ray diffraction. Application of in-situ high voltage electron microscope (HVEM) tensile experiments of CuAlFe single crystals [6] allowed to distinguish the morphology of 18R and 2H martensites, their crystallographic relationship with a matrix and a ledge mechanism of martensite growth. Shape memory properties of CuAlMn alloys were demonstrated several years ago [7] being sensitive to a pretraining treatment [8]. Recent investigations of stress strain behaviour associated with the thermoelastic martensitic transformation in CuAlMn single crystals have shown similarities to that of CuAlNi with multistage stress-strain curves [9]. In the present study single crystals of (001)[100] orientation of CuAlMn alloys were studied using in-situ HVEM tensile deformation. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995242 C2-276 JOURNAL DE PHYSIQUE IV 2. EXPERIMENTAL PROCEDURE Two alloys Cu23at% A1 7at% Mn (1) and Cu20.8at% Al 9.5at% Mn (2) were cast in an induction hrnace fi-om 4N purity Cu and Al and 3N purity of Mn. Single crystals were prepaired using the Bridgman method. The single crystals were quenched from 850°C into water at 50°C. Martensitic transformation temperatures were determined using DSC Du Pont 9000 thermal analyser. Specimens for in-situ straining experiments were cut in a form of thin sheets of size 0 . 1 ~ 3 ~ 8 rnm of (001)[100] orientation using a diamond saw. Then, a two stage electrolytic polishing was applied; first using platinium mask of size 1x2 mm in CrO, saturated &PO, to create a flat dimple in the center, then a final electropolishing in113 HNO, 213 CH,OH solution at -30°C until perforation. In-situ tensile experiments were performed using JEOL lOOOkV electron microscope equipped with a tensile stage designed by Messerschmidt and Appel [lo]. 3. RESULTS AND DISCUSSION Characteristic transformation temperatures determined for alloy 1 were: Ms = 14"C, Pq= 40°C and for alloy 2, M,= -30°C and A,= -34"C.Since A, of alloy 1 lies above RT it does not show superelastic behaviour at RT. Fig.1 shows typical stress strain curves obtained during tensile deformation at various temperatures up to 6% strain. The stresslstrain curve (a) at 17OC i.e. below A, is smooth, contrary to (b) which shows large serrations. They result from the formation and propagation of y'(2H) martensite. Increasing the tensile temperature to 25°C (d) one observes two plateaus due to formation of two types of ZOOr (c) p, -p; 2 0 0 r (dl 3S°C Strain (%I Fig. 1 Stresslstrain curves of alloy 1 single crystals in [loo] direction at various temperatures martensite i.e. P1(18R) and y'(2H) during stress increase. Such a curve was not observed by Oishi and Brown [2] during tensile deformation of CuAlNi single crystals. At 71°C (e) a typical pseudoelastic tensile curve can be seen. Deformation proceeds by increasing density of fine plates as observed by optical microscopy [12]. Fig.2 shows a sequence of HVEM in-situ loadlelongation curves obtained fkom a single crystal of alloy 2 at (001)[100] (sample surfaceltensile direction) orientation. The first curve shows only a small shape recovery due to inhomogeneous deformation in thin areas where plastic deformation of martensite takes place. Thus, it was difficult to observe shrinking of martensitic plates during stress release in the first deformation cycle. As compared to bulk samples [9], martensite formed much earlier in the present study, already in the stage of stress increase.