Anisotropic Stress-Strain-Behavior and Elastic-Constant-Tensor of Monoclinic B19' NiTi

NiTi shape memory alloys (SMAs) are used for medical and engineering applications with increasing commercial success. Despite their technological importance, the elastic constants and thus the orientation-specific Young’s moduli of the monoclinic B19’ martensite phase are still unknown, as martensite single crystals are not available for mechanical testing. DFT calculations have provided elastic constants for zero K involving a monoclinic angle of γ≈107° instead of γ≈97°, as typically observed in experiments. Hence the elastic behavior of B19’ at ambient temperatures is still a matter of debate.

In the present work we determined the elastic constants by in-situ neutron diffraction experiments using a load frame where the sample and load axis can be rotated in an Eulerian cradle. This technique allows to separate the effects of elastic and inelastic (detwinning) deformation processes. We recorded neutron diffraction patterns of a commercial, textured NiTi alloy (Mf≈44°C) at constant uniaxial loads.

Evaluation of previous neutron measurements showed the onset of the pseudoplastic plateau at a macroscopic strain of ~1% and indicated complex behavior with changes from constant-strain to constant-stress microstructures during loading to 4% strain. In our recent experiment diffractograms were recorded in the elastic regime at ~0.2, ~0.4 and ~0.6% strain. Evaluating this data we separated the orientation-dependent elastic strain of B19' from texture changes (caused by variant reorientation). Finally, an elastic-constant-tensor was calculated which now can be used for micromechanical modeling of NiTi B19’ shape memory behavior.