Effects of thermal cycling on microstructure and properties in Nitinol

The effects of thermal cycling through the martensite–austenite transformation were investigated in NiTi shape memory alloys with DSC and TEM. Thermal cycling caused a ∼25 K decrease in Ms with a concomitant increase in dislocation density from ∼1012 m−2 to 5 × 1014 m−2 after 100 thermal cycles. Thermodynamic analysis is consistent with increasing elastic strain energy and irreversible frictional energy with cycling. The transformation-induced dislocations were determined to be shear loops with 〈0 1 0〉A{1 0 1}A slip system, corresponding to the twinning direction and plane in martensite. It is speculated that the loops form during the movement of the martensite interface and that repeated interfacial movement tends to create bands that consist of highly tangled sessile dislocations. These dislocation bands form along ( 0.756 ¯ ,   0.383 ¯ ,   0.192 ) A , which is 2.3° from the accepted ( 0.889 ¯ ,   0.404 ¯ ,   0.215 ) A lattice invariant plane. Furthermore, plastically deformed austenitic Nitinol exhibits slip on 〈0 1 0〉A{1 0 1}A slip system and forms {1 1 0} shear bands with several variants of the dislocations within a given region.