The Ni₃Ta intermetallic compound is quite known as a strengthening component for high temperature structural eutectic or directionally solidified materials (superalloys, composites etc.) [1]. The alloy of Ni₃Ta stoichiometry was arc-melted from electrolytic Ni and Ta of the highest purity in a pre-gettered argon atmosphere, re-melted 6 times and zone-melted for purification. Zone-melted specimens were homogenised by heat treatment in vacuum at 1673 K for 4 hours. Thin foils for TEM were prepared by Ar⁺ ion milling. The bright field image presented in Fig.1 shows variants of martensite present in the specimen. The variants have different widths and can be found throughout the complete sample. By comparing simulated and experimental diffraction patterns, the monoclinic structure (a=0.45319 nm, b=0.51253 nm, c=0.8632 nm, β=100.79°) as proposed by Zhao et al. [2] has been confirmed. Furthermore, an (001) twinning plane was determined from the diffraction pattern (Fig.2) taken along the [210] zone axis orientation.

It was found that Ni₃Ta undergoes a martensitic transformation (MT) from high (tetragonal) to low (monoclinic) phase with a 50 K temperature hysteresis at elevated temperatures: M_s=530 K, M_f=500 K, A_s=550 K, A_f=580 K (Fig.3). Thermal cycling over the temperature interval of the MT does not affect the MT temperatures except for a slight shift of the reverse transformation temperatures during the first cycle. The volume change during forward MT deduced from the austenite and martensite lattice parameters is -0.45%. The shape recovery after 10% compression at room temperature is almost complete although it takes place in a wide temperature interval – about 300 K. This alloy might open new perspectives for the application of high temperature shape memory alloys (HTSMA).

References:

[1] Mollard F. et al., 1974, Zeitschrift Für Metallkunde 65, 461-466

[2] Zhao J-T. et al., 1991, Acta Cryst. CA7, 479-483

Fig.1

Fig.3

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