Directions for High-Temperature Shape Memory Alloys’ Improvement: Straight Way to High-Entropy Materials?

Abstract

Nowadays, all thermo-mechanical effects, associated with the martensitic structural phase transitions, are still in the focus of scientists and engineers, especially once these phenomena are taking place at elevated temperatures. The list of the materials, undergoing high-temperature martensitic transformation, is constantly widening. Still, industrial application of these materials, called hightemperature shape memory alloys, is far enough due to the lack of understanding of the peculiarities of the high-temperature martensitic transformation and shape memory effect. The present work attempts to show how the development of the proper directions for high-temperature shape memory alloys' improvement might lead to the creation of essentially new functional materials.Keywords Martensitic phase transformation Á Crystal and electronic structure Á Shape memory behaviour Á High temperature shape memory alloys Á High entropy alloys

Microstructure Formation in Cast TiZrHfCoNiCu and CoNiCuAlGaIn High Entropy Shape Memory Alloys: A Comparison

“…However, the equiatomic Ti16.67Zr16.67Hf16.67Co16.67Ni16.67Cu16.67 intermetallic compound [14] and the quasi-equiatomic (Co22.33Ni22.33Cu22.33Al11Ga11In11) (inset in Figure 1b) did not show any sign of martensite. In the case of the TiZrHfCoNiCu system, a variation of the Co, Ni, and Cu content was needed to introduce a martensitic transformation associated with shape memory behavior [15]. In particular, the composition Ti16.67Zr16.67Hf16.67Co10Ni25Cu15 has shown a martensitic transformation and shape memory at elevated temperatures.…”

“…Exclusion of Co from the equiatomic composition resulted in a martensitic transformation accompanied by perfect shape memory effect at elevated temperatures due to the significant two-fold increase in strength compared with binary TiNi [14]. It was shown that HEA properties were those that high temperature shape memory alloys needed for their improvement, and a number of six component high entropy shape memory alloys (HESMA) from the TiZrHfCoNiCu system were developed with enhanced yield strength of 1200 MPa and reversible strains up to 2% in the as-cast state [15]. The peculiar high temperature crystal structure-a triclinically distorted one of the B2 type that carries these distortions into the B19' martensite phase (modeled with the help of ab-initio calculations and confirmed by X-ray diffraction measurements) [16]-was proposed to be the origin of the HESMA yield strength and shape memory behavior enhancement.…”

“…The peculiar high temperature crystal structure-a triclinically distorted one of the B2 type that carries these distortions into the B19' martensite phase (modeled with the help of ab-initio calculations and confirmed by X-ray diffraction measurements) [16]-was proposed to be the origin of the HESMA yield strength and shape memory behavior enhancement. In this context, it was shown that for the TiZrHfCoNiCu system a B2↔B19' type of martensite transformation takes place [14][15][16]. Recently, an extra six component system CoNiCuAlGaIn, which emulates the B2 NiAl intermetallic compound was introduced [17].…”

Martens-ite

“…Hysteresis is normally measured by the difference between the temperature at which 50% austenite (T 50A ) occurs on heating and the temperature of 50% martensite formation on cooling (T 50M ). As these temperatures may be difficult to determine, the As, Af, Ms, and Mf temperatures can be used to estimate the hysteresis [48]. Transformation hysteresis, however small, indicates that frictional resistance opposes the reverse motion of the interface, usually because of defects generated by the formation of the martensite.…”

“…Cu-based shape memory alloys are particularly susceptible to ageing effects and thermal cycling can result in drifting of transformation temperatures and even the elimination of martensitic transformation. Decomposition processes are particularly relevant to "high temperature" shape memory alloys based on Ti-Ni which include elements such as Zr, Hf, and Pd that serve to raise transformation temperatures [48]. Ti-Ni-based alloys designed to operate over lower and even sub-zero temperature are typically more stable, and pseudo-elastic alloys can sustain hundreds of thousands of stress cycles without fatigue failure, provided the maximum stress and strain values are restricted (typically, 1% recoverable strain and 70 MPa maximum stress [72]).…”