Influence of thermal treatments on Ni-Ti-Zr and Ni-Mn-Ga-X High Temperature Shape Memory Alloys

Abstract

Shape memory alloys (SMA) are well-known for their unique properties, which have been widely used in many industries. A variety of different alloy systems are briefly categorized by their transformation temperatures, and those ones working at high temperatures are focused in the present work, including Ni-Ti-X and Ni-Mn-Ga-X alloy systems. The binary Ni-Ti alloys are the most successful material for applications, but can only serve below 370 K. Hence, the Ni-Ti-X ternary alloys have often been suggested and explored with tuned high transformation temperatures. Among all the developed possibilities, Ni-rich Ni-Ti-Hf/Zr alloys have drawn a lot of attention for not only increasing transformation temperature, but also demonstrating promising shape memory properties as the result of nanosized H-phase precipitates. Therefore, in the present work we investigate the thermal treatment effect on shape memory properties and the corresponding microstructure of Ni50.3Ti24.7Zr25 polycrystalline alloys which had not been investigated yet. The Ni50.3Ti24.7Zr25 alloy demonstrates single step B2-B19’ transformation with high transformation temperature and relatively good thermomechanical properties after proper thermal treatment (ageing 820 K 3 h), due to precipitation of H phase. However, small irrecoverable strains can still be observed in thermomechanical cycles of Zr25 alloy in comparison with reported Zr15/20 or Hf20 alloys with basically no irrecoverable strain, which could be ascribed to an excessive amount of H-phase precipitates. A novel state consisting of locally disordered lattice strains, denoted as strain glass (SG), was found in some alloys with suppressed ferroelastic martensitic transition during cooling. For the present work, Ni50.3Ti29.7Zr20, Ni50.3Ti24.7Zr25 and Ni50.9Ti24.1Zr25 alloys have also been found to present suppressed martensitic transformation after different thermal treatments. Hence, several experimental methods have been performed to verify if they are at SG state or not. It is considered that the early stage formation of precursor H-phase precipitates, accompanied by Ti, Zr atoms redistribution, disrupt the long-range ordering of matrix and normal martensitic transformation after thermal treatments. Only nanometric martensitc-like domains can be formed upon cooling, while the overall matrix remains, in average, in the cubic parent phase structure. Ni50.3Ti29.7Zr20 and Ni50.9Ti24.1Zr25 alloys show frequency dependent E modulus and they are confirmed to develop the SG state, while the results of Ni50.3Ti24.7Zr25 suggest an “incubation” state previous to the fully developed SG state. Moreover, special stress-strain smooth loops have been found on Ni50.3Ti29.7Zr20 prolonged aged specimen with little temperature dependence, and a qualitative model has been suggested to explain this property. Furthermore, the “elinvar” effect has been found on the Ni50.9Ti24.1Zr25 SHT alloy before the SG transition during cooling by DMA measurement, and confirmed through a high accuracy resonant ultrasonic technique. The effect is ascribed to the formation of finite martensite domains with distributed wide temperature range of local Ms. Alternatively, the Ni-Mn-Ga alloys were found to intrinsically possess some potential as HTSMAs, with Ms over 400 K and high thermal cycling stability in single crystalline alloys. Then researchers have turned their focus on the polycrystalline alloys since they are easier to produce. However, polycrystalline alloys show a high level of brittleness at grain boundaries. Hence, a variety of quaternary elements (Cr, Cu, rare earth elements ...) have been alloyed into ternary Ni-Mn-Ga to induce precipitates, which should improve the ductility of the matrix. In the present study, we are focusing on the quaternary Ni-Mn-Ga-X (X = Cu, Sn, Hf and Zr) HTSMAs with low amount of additions, which have not been thoroughly investigated yet. Ni-rich Ni-Mn-Ga-Cu (1 at.% of Cu) alloys exhibit high thermal stability after prolonged ageing treatments, although no precipitates can be induced. The single phase Ni-Mn-Ga-Cu polycrystalline alloys show relatively low matrix strength, and the irrecoverable strains increase sharply with over 100 MPa applied stress under thermomechanical cycling. Alloying Sn into Ni-Mn-Ga polycrystalline alloys leads to the reduction of the transformation temperatures, and a narrow hysteresis is observed with over 2 at.% addition due to relatively good compatibility between parent phase and martensite. However, 4 at.% addition of Sn causes an extremely brittleness of material due to the appearance of small holes in the matrix after induction melting. In comparison, Ni-Mn-Ga-Hf/Zr polycrystalline alloys exhibit high potential of shape memory properties. Although the transformation temperatures are decreased with the increase of Hf/Zr addition, they still fit for HTSMA. The ductile second phase precipitates start to appear at 1 at.% addition of Hf/Zr and increase in amount and size with more addition, although the transformable matrix volume fraction is inevitably reduced. The hysteresis surprisingly decreases to about 8 K for Hf4/Zr4 alloys, which is attributed to the good compatibility between the austenite and martensite lattices. The b.c.t. martensite is formed in all the Ni-Mn-Ga-Hf/Zr alloys, and a small fraction of modulated 14M martensite also coexists in Hf4/Zr4 alloys additionally. The precipitates structure has been investigated and identified to possess double lattice parameter than the normal f.c.c. γ phase that forms in other Ni-Mn-Ga-X alloys. Equivalent structural models have been built up, using a double lattice parameter f.c.c. based unit cell (consisted of 32 atoms) or a smaller A6 face-centered tetragonal unit cell corresponding to the I4/mmm space group. The network of precipitates (in the alloys with over 2 at.% Hf/Zr addition) can enhance the strength of the specimens and reduce the plastic deformation under thermomechanical cycling but at the expense of a dramatic drop of the transformation strain, which also inevitably impedes the superelasticity effect.