Due to these favorable properties, they have potential for various industrial and medical applications. The shape memory property in these materials is possible due to thermally- or stress-induced transformation from austenite phase to martensite phase; however microstructural elements in nickel-titanium (NiTi) SMAs – such as precipitates, inclusions and grain boundaries – can be sources of constraints that negatively influence response to their deformation. Prof. Aaron Stebner group from Colorado School of Mines characterized the deformation and the evolution of microstructure during a tension test in a superelastic NiTi specimen. This work was recently reported in Acta Materialia [1].
To determine crystal orientations of the specimen under various deformed states, Dr. Harshad Paranjape, a postdoc in Prof. Stebner group and his colleagues performed in situ high-energy X-ray diffraction microscopy (HEDM) at CHESS F2 station, with the specimen (1 mm3 gage section) mounted on the RMAS2 load frame. The experiment involved carrying out a series of tension tests, from zero load (austenite phase) to near the peak load (martensite phase) (Figure 1a). At each load step, the sample was fully rotated about the loading axis and one diffraction pattern was recorded every 0.1-degree rotation. Also obtained during each load step were digital image correlation images to measure surface strain fields in the center region of one of the faces of the specimen. A microstructural model (Figure 1b) was generated by information obtained with the experimental data from HEDM, complemented by electron microscopy. This model then was used to simulate the local stress state in the specimen. The simulations showed that the stress state around inclusions can activate martensite microstructure (Figure 1c, right) that produces less-than-optimal transformation strain (3%, compared to maximum of 6% – see Figure 1c, left). In addition, it was determined that deformation gradients exist in martensite under load. This study is an excellent example of how combined 3D experimental-modelling approach can be used to study the effect of constraint on the deformation in metals and alloys with hierarchical microstructure.
Reference:
[1] Harshad M. Paranjape, Partha P. Paul, Behnam Amin-Ahmadi, Hemant Sharma, Darren Dale, J.Y. Peter Ko, Yury I. Chumlyakov, L. Catherine Brinson, Aaron P. Stebner, In situ, 3D characterization of the deformation mechanics of a superelastic NiTi shape memory alloy single crystal under multiscale constraint, Acta Materialia 144 (2018) 748-757.