Stress-assisted aging results two-way shape memory effect in NiTiHfPd alloys

Shape memory alloys are defined as materials that tend to resume their original shape once the deforming force is removed. These alloys can be fabricated from copper-aluminum-nickel, zinc, gold and even iron. Among the most prevalent shape-memory alloys, those derived from nickel-titanium stand out. As a consequence, many potential applications for this compound have been put forward, specifically, in the areas of construction, medicine and electrical appliances among others. However, nickel-titanium based shape memory alloys suffer a critical drawback in terms of low strength and limited transformation temperatures. To this effect, many studies have been undertaken with the core purpose of tailoring the stoichiometry, formation of precipitates or alloying additions so as to improve on the inherent limiting drawbacks. So far, ternary NiTiHf alloys have been found to be very promising for high temperature and strength applications.

Recently, Dr. Emre Acar at Erciyes University in collaboration with Dr. H. Tobe at Institute of Space and Astronautical Science in Japan, Dr. Haluk Karaca at University of Kentucky and Dr. Ivanovic Chumlyakov a Siberian Physical-Technical Institute at Tomsk State University studied effects of aging under a compressive stress of 300 MPa on the shape memory properties and microstructure of [1 17]-oriented Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals. They mainly focused on studying the shape memory behavior of the aforementioned compound and determine their suitability for various applications. Their work is currently published in the research journal, Materials Science & Engineering A.

In brief, the research method used by the authors commenced with the homogenization of the [-1 17] oriented Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals in sealed quartz tubes followed by a water quench. They were then aged at 550 °C and 600 °C for a given period and then quenched in water again. The researchers then proceeded to characterize the single crystal samples after which strain was measured. Lastly, transmission electron microscope observations were taken from top and side of each sample to analyze the precipitate variants in more detail.

The authors observed that the stress-assisted aging resulted in 4 variants of the H-precipitates in contrast to 6 variants observed after stress-free aging. In addition, it was revealed that precipitate properties and martensite morphologies strongly depended on aging conditions. Most importantly, as a consequence of the aligned precipitates and generated internal stress, two-way shape memory effect and higher recoverable strains were observed in the tested Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals after stress-assisted aging.

In conclusion, the international research collaboration study by study Emre Acar and colleagues presented a thorough assessment of the paraphernalia of aging under induced stress on the two-way shape memory effect behavior of Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals along the [-117] orientation. In general, they observed that the aligned precipitates resulted in an aligned stress field where 0.5% two-way shape memory strain was possible for the Ni_45.3Ti_29.7Hf₂₀Pd₅ alloys. Altogether, the difference in recoverable strains of 550 °C and 600 °C samples were attributed to the difference in martensite morphologies.