Jorge Malarría

The martensitic transformation of sputtered Ni_48.4Ti_31.3Hf_20.3 (at. %) thin films was investigated. Amorphous samples were crystallized at 773 K, 873 K, 973 K and 1073 K to establish a correlation between microstructure and thermodynamic properties. Regardless of the crystallization temperature, all the samples exhibited similar grain size dispersions with average values around 100 nm. Grains with similar sizes revealed a widening of martensite plates with increasing crystallization temperature. Calorimetry measurements showed that both transformation temperatures and transformation heats increase as the crystallization temperature increases. Experimental data allowed for frictional work and stored elastic energy calculations. It was found that the reduction in stored elastic energy and the increase in transformation heat with rising crystallization temperature are interrelated phenomena. The stored elastic energy reduction with rising crystallization temperature is caused by the larger size of precipitates and their higher concentration at grain boundaries. The frictional work showed approximately the same value regardless of the crystallization temperature. Both parameters are higher in NiTiHf thin films compared to those calculated for a NiTiHf bulk sample with the same Hf content, mainly attributed to the reduced grain size in the thin films.

The characterization of the mechanical behavior of shape memory thin film alloys is essential for their application as microactuators. Depending on the particular application, different elements have been considered for the fabrication of ternary NiTi-based alloys in order to shift the martensitic transformation toward a lower or a higher temperature range. This work presents the design and implementation of a specific device to perform thermal cycles at constant stress on film-like samples over a wide temperature range, between −130 and 300 °C. The uniaxial stress application, the elongations and contractions measurements associated with the martensitic transformations, and the cooling and heating methods are detailed in the present work. The fabrication and operation features of this relatively low cost device facilitate and promote the study of shape memory thin films.

The Fe_43.5Mn₃₄Al₁₅Ni_7.5 (at.%) alloy exhibits outstanding pseudoelastic behaviour over a wide range of temperatures. This alloy undergoes an unusual martensitic transformation from a disordered body-centred cubic (α) parent phase to a face-centred cubic (γ′) product phase. In the present work, the orientations of the parent and product phases for quenched samples were analysed by electron backscatter diffraction. Bain, Kurdjumow–Sachs, Pitsch, Nishiyama–Wassermann and Greninger–Troiano orientation relationships between the parent and product phases were compared with experimental results. The Pitsch relationship appears to be the most suitable to describe the α→γ′ martensitic transformation. This result provides experimental support to the dislocation-based heterogeneous Bogers–Burgers type transformation mechanism. No indications of variant selection were detected in the thermally activated transformations.