Effect of Cold Working on Transformation and Deformation Behavior after Pre-Deforming in Ti–50 at%Ni Shape Memory Alloy

Abstract

It is reported that the transformation characteristics of Ti-Ni shape memory alloys (SMAs) are influenced by manufacturing conditions, such as composition, heat-treatment temperature, cold working, and so on. To understand correctly the effects of these manufacturing conditions on transformation characteristics of Ti-Ni SMAs make it possible to control the transformation temperature and recovery stress. The purpose of this work is to clarify the effect of cold working ratio on transformation and deformation behavior after pre-deforming SMA. The specimens were Ti-50 at%Ni annealed at 673 K for 3.6 ks. The variation of the recoverable strain, recovery stress and transformation temperature with cold working ratio was investigated experimentally. The solution treated material was also used as a reference material. The effect of cold working ratio on the transformation and deformation behavior is discussed in relation to the volume fraction of the residual martensite subjected to slip deformation.

Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading

“…After 5.9% strain is reached, the wire has become fully martensitic and the applied stress starts to increase again with increasing strain, which is associated with a macroscopically homogeneous deformation of the tensile specimen. It is difficult to determine the elastic modulus of martensite due to the fact that the martensite has already accumulated some small amount of strain before the phase transformation is complete; however, it is still possible to find a range of 25 GPa (determined from a linear fit of the data between a load of 430 MPa up to 1060 MPa) to 28 GPa (determined from a linear fit of the data between a load of 430 MPa up to 530 MPa) for the apparent slope, which is within the range of macroscopic moduli values reported in the literature (E martensite = 20-70 GPa [27,62,63,65,66]). However, it should be noted that this apparent elastic modulus results from various deformation mechanisms which act simultaneously, such as elasto-plastic deformation, variant reorientation and detwinning of martensite [27,62,63,65,66].…”

“…It is difficult to determine the elastic modulus of martensite due to the fact that the martensite has already accumulated some small amount of strain before the phase transformation is complete; however, it is still possible to find a range of 25 GPa (determined from a linear fit of the data between a load of 430 MPa up to 1060 MPa) to 28 GPa (determined from a linear fit of the data between a load of 430 MPa up to 530 MPa) for the apparent slope, which is within the range of macroscopic moduli values reported in the literature (E martensite = 20-70 GPa [27,62,63,65,66]). However, it should be noted that this apparent elastic modulus results from various deformation mechanisms which act simultaneously, such as elasto-plastic deformation, variant reorientation and detwinning of martensite [27,62,63,65,66]. Understanding the complex interactions between these deformation mechanisms and the stress-induced martensitic transformation is a key objective of current research activities [63,67,68].…”

Influence of Dynamic Compression on Phase Transformation of Martensitic NiTi Shape Memory Alloys

“…Within Stage I, samples deform linear-elastically as strain increases up~0.5 pct strain for both quasi-static and dynamic compression. While both samples exhibited an elastic modulus of about 32 GPa which is within the range of elastic moduli values reported in the literature, [15,52,55] there does appear to be some slight deviation from linearity in the dynamic compression curve in Stage I. After about 0.8 pct, the onset of the stress plateau was observed (Stage II), where the stress in the specimen increases less prominently with increasing strain.…”