Final report DOE project, ''Origins of asymmetric stress-strain response in phase transformations,'' DEFG02-93ER143993

Abstract

For the first time, experiments on NiTi under pressure loadings were conducted in Ref. (1). This work showed that the stress-strain response of NiTi is highly pressure sensitive and there was an asymmetry of tension and compression results. The results were obtained based on the special rig developed in (Ref. 2) by Sehitoglu and his students. Several experiments under pressure were also conducted on CuZnAl alloys with also pressure dependent response. accounted for variant-variant interaction and texture effects in the case of NiTi alloys (Ref. 3). It was found that the polycrystalline version of these materials has a strong texture due to the cold rolling process (Figure 4). Consequently, they almost behave as single crystals oriented in the [111] direction (Figure 3). We showed that if the texture effects are not accounted for the models give the incorrect trends when compared with experiments (Figure 5). Our work also showed that the evolution of the variants in tension is much more rapid compared to the compression case (Ref. 3). In the second year of the work, our attention focused exclusively on the deformation behavior of single crystals. Several key results were achieved with single crystals. Initially, we studied the role of aging treatment on tension compression asymmetry and crystal orientation dependence. It was shown that the orientation dependence of critical resolved shear stress is significant in the case of peak aged crystals while the orientation dependence decreases with overaging. A micro-mechanical model was developed to explain these trends based on the determination of the local shear stresses due to the precipitate on the 24 possible martensite variants (Figure 6). It was found that those variants that have high resolved shear stress due to external loading experience low local stresses due to the precipitate weakening the orientation dependence (Refs. 4-6). Overall the results and the model showed that the introduction of precipitates reduce the critical transformation stress in these materials and reduce the orientation dependence (Figure 7). It was also noted that overaging results in loss of Ni in the matrix resulting in decrease in strength levels. as a function of orientation and stress direction (Refs. 8-9). We made preliminary attempts at explaining the orientation dependence of the recoverable strains based on the C W formation and detwinning models (Table 2) but pointed out that further work is necessary in this area. The precipitates curtail the detwinning phenomenon and reduce the overall transformation strain levels in these alloys (Table 2). Finally, we, note the unusual amount of hardening under cyclic loading (Ref.9) with combined effects of twinning and slip. The strengthening in compression was found to be remarkably high (Figure 10). Further study of cyclic loading was put aside for the proposed research. The stress-strain results for NiTi were predicted with a micro-mechanical model, which in the third year of the research, our attention focused on cyclic loading and recoverable strains.