Intrinsic pre-strain effect in Ti-Ni based shape memory alloys

Shape memory effect (SME) in shape memory alloys (SMAs) results from deformation below A_f, accompanying pre-strain effect (PSE) which was usually implicated by dislocations. We systematically investigate the effects of small pre-strain (<3.8 %, average martensitic transformation strain) in Ti₄₉Ni_45.7Cu_4.9Cr_0.4 SMA with strong dislocation resistance against (negligible dislocations introduced during) martensitic transformations. It is demonstrated by the disappearance of functional fatigue upon thermal cycling, and unchanged transformation characteristics after removing PSE by reverse transformation. With small pre-strain, martensites remaining twinned (multi-domain) by TEM, however exhibit increased reverse transformation characteristic temperatures. It is thus defined as intrinsic PSE (IPSE), which is contrast to the common belief that PSE is solely attributed to elastic energy releasing of detwinning (single-domain). The IPSE is explained by local stress field raised by surrounding constraints on the pre-detwinned and reoriented martensites. Our findings shed lights on the understandings of PSE and thus associated SME in SMAs.