Twin Relation Between Martensite Variants In Cu-Al-Ni Shape Memory Alloy

The twinning relations of A/C, A/B and A/D types in the 18R thermoelastic martensite of Cu-25.46Al-3.41Ni(at.%) are investigated by XRD and TEM methods. The steps of rationally determining selected-area electron diffraction patterns of the twinning structure and twin elements are studied and given in this paper.It is difficult to determine selected-area electron diffraction patterns of the twinning structure because the c axis is more longer than the a axis and the b axis in 18R thermoelastic martensite and the angle ofβis close to 90°. We used the classical twinning theoretics to induct determining of selected-area electron diffraction patterns and investigate the twin elements and twin relations. Electron diffraction patterns of A:C, A:B and A:D types are indexed respectively. Results show that A:C is type I twinning, which has twinning elements of K1=(129), K2=(8.1295 11.1280 26.9865),η1=[33.4499 25.3311 1.9125],η2=[991]. Type A:C has orientation relationship of rotation ofπabout the normal to K1. The boundry of A:C twinning is straight and identical with K1. Type II twinning A:B has twinning elements of K1=(8.1295 11.1280 26.9865), K2=(129),η1=[991],η2=[33.4499 25.3311 1.9125], which has orientation relationship of rotation ofπaboutη1. The A:B boundry is mostly straight, which is composed of (129)A and (109)A, (129)B and (109)B periodically. Compound twinning A:D has twinning elements of K1=(109), K2=(109),η1=[901],η2=[901]. Type A:D has orientation relationship of rotation of p about the normal to K1. The straight boundry of A:D is identical with K1. The curved A:D boundry is composed of (100)A and (001)A, (001)D and (100)D microscopically.The boundary structure is reconstructed according to the characteristics of boundary and twin elements and relation of A:C, A:B and A:D types respectively. The arranging of atoms in the structure succeed the order in the parent phase well. The orientation relationship between the lattices of the twining variants is agree with the twinning orientation relationship, so as the atoms. The boundary model we reconstruct can perfectly explain why the twin boundary can move easily and reversibly.