In situ X-ray diffraction strain-controlled study of Ti-Nb-Zr and Ti-Nb-Ta shape memory alloys: crystal lattice and transformation features

Phase and structure transformations in biomedical Ti-21.8Nb-6.0Zr (TNZ) and Ti-19.7Nb-5.8Ta (TNT) shape memory alloys (at.%) under and without load in the -150 to 100 degrees C temperature range are studied in situ using an original tensile module for a low-temperature chamber of an X-ray diffractometer. Alpha- and beta-phase lattice parameters, the crystallographic resource of recovery strain, phase and structure transformation sequences, and microstress appearance and disappearance are examined, compared and discussed. For both alloys, the crystallographic resource of recovery strain decreases with temperature increase to become 4.5% for TNZ and 2.5% for TNT alloy (at RT). Loading at low temperatures leads to additional alpha ''-phase formation and reorientation. Heating under load, as compared to strain-free heating, affects the reverse transformation sequence of both alloys in different ways. For TNZ alloy, strain-free heating results in simultaneous omega ->beta and alpha ''->beta transformations, whereas during heating under stress, they are sequential: beta + omega ->alpha '' precedes alpha ''->beta, For TNT alloy, strain-free heating results in reverse alpha ''->beta transformation, whereas during heating under stress, alpha ''->beta transformation is preceded by alpha ''-phase reorientation. (C) 2013 Elsevier Inc. All rights reserved.