In situ strain, texture, and phase fraction measurements during thermal-mechanical testing of NiTi and NiTiPd alloys

This paper relates results from conventional macroscopic mechanical and thermal testing methodologies to direct atomic-scale observations from in situ neutron diffraction measurements performed at Los Alamos National Laboratory. Results from NiTi and NiTiPd shape-memory alloys intended for aerospace applications are presented.  The experiments conducted include selected combinations of loading and heating/cooling while simultaneously acquiring neutron spectra and recording macroscopic strains. Both Rietveld and single-peak analyses are used to quantitatively follow the strain, texture and phase fraction evolution linking macroscopic and microscopic behaviors. Atomic-scale measurements of elastic modulus, Poisson’s ratio and the coefficient of thermal expansion tensor are presented. Additionally, the dimensional and thermal stability of these alloys are correlated with the volume fraction and texture of retained martensite, and the internal strain evolution in these alloys.  The role of symmetry, i.e., B19' monoclinic martensite vs. B19 orthorhombic martensite in these alloy systems is also assessed.