EBSD and TEM study of self-accommodating martensites in Cu 75.7Al 15.4Mn 8.9 shape memory alloy

Abstract

Morphologies and orientation relationships of martensites in Cu 75.7Al 15.4Mn 8.9 shape memory alloy were studied for the first time by means of electron backscatter diffraction (EBSD) technique, transmission electron microscopy (TEM) and phenomenological calculation. The martensite was identified to be an 18R 1 type monoclinic one with its β angle very close to 90°. Each plate group of martensite variants consists of four variants A, B, C and D showing characteristic spear- and fork-like morphologies. Variants A and C (and equivalently B and D) are twin-related with respect to the ( 1 ̄ 2 ̄ 8) 18 R plane of the 18R 1 martensite, which is transformed from, and nearly parallel to (with an error of only 0.60°), a (0 1 1) P plane of the parent phase. Variant-pair A–D (and equivalently B–C) is twin related with respect nearly to the (2 0 20) 18 R plane, which is transformed from a ( 1 ̄ 0 0) P parent plane. However, the discrepancy between these two planes, (2 0 20) 18 R and ( 1 ̄ 0 0) P , is 2.01°, larger than that (0.60°) between ( 1 ̄ 2 ̄ 8) 18 R and (0 1 1) P . Therefore, this twin boundary prefers to consist microscopically of several low index planes, forming a zigzag morphology, as observed by TEM. Phenomenological calculation of the cubic D0 3 parent phase to monoclinic M18R 1 martensite transformation revealed that although the magnitude of the shape strain produced by the martensitic transformation is calculated to be rather large ( m=0.2005), the average shape deformation of each self-accommodating plate group is rather small. And the total average shape deformation for six plate groups becomes nearly zero, except for a small volume contraction. The angles between the basal plane of the 18R 1 martensite and the twin planes of different variant-pairs in a plate group, determined from EBSD and TEM experiments, are consistent with phenomenological calculations.