Role of nano-precipitation on the microstructure and shape memory characteristics of a new Ni50.3Ti34.7Zr15 shape memory alloy

Abstract

The microstructure and shape memory characteristics of the Ni 50.3 Ti 34.7 Zr 15 shape memory alloy were investigated as a function of aging heat treatments that result in nanometer to submicron size precipitates. Microstructure-property relationships were developed by characterizing samples using transmission electron microscopy, differential scanning calorimetry, and load-biased thermal cycling experiments. The precipitate size was found to strongly influence the martensitic transformationprecipitate interactions and ultimately the shape memory characteristics of the alloy. Aging treatments resulting in relatively fine precipitates, which are not an obstacle to twin boundaries and easily bypassed by martensite variants, exhibited higher transformation strain, lower transformation thermal hysteresis, and better thermal and dimensional stability compared to samples with relatively large precipitates. When precipitate dimensions approached several hundred nanometers in size they acted as obstacles to martensite growth, limiting martensite variant and twin size resulting in reduced functional and structural properties. Aging heat treatments were also shown to result in a wide range of transformation temperatures, increasing them above 100 °C in some cases, and affected the stress dependence of the transformation hysteresis and the stress versus transformation temperature relationships for the Ni 50.3 Ti 34.7 Zr 15 alloy.

Effect of Ni-Content on the Transformation Temperatures in NiTi-20 at. % Zr High Temperature Shape Memory Alloys

“…Solutionized samples were then sectioned into 7 × 7 × 1 mm 3 slices, so that aging at 550, 600 and 800 • C for various times could be examined under the same starting metallurgical conditions. Aging at 550 • C was chosen since it is the generally accepted peak aging temperature for Ni-rich NiTiHf/Zr HTSMA [7,18,26]. Aging at 600 • C was chosen since this will increase the precipitation rate.…”

“…In Figure 5c, the Ni-rich 1-D diffraction data, a different broad peak is observed, centered at 2.21 Å, in the diffraction data when the alloy is treated at 550 °C or 600 °C. This peak has been indexed as the nano-strengthening Hphase seen in other NiTiZr and NiTiHf HTSMAs compositions [7,17,18,23,26,28] using the basis positions from Yang et al [24]. Once the temperature increases to 800 °C, the presence of the broad H-phase peak is no longer observed, but rather the sharp residual austenite peak at 2.19 Å is observed instead.…”

“…The reason for this temperature increase can be attributed to the formation of the Ni-rich nano-scale H-phase observed in the SR-XRD data collected during both ex situ and in situ experiments (Figures 5c and 9c respectively) and the HAADF-STEM micrograph (Figure 6c) which cause a Ni depletion in the B19' martensitic matrix. In addition, the homogenous distribution of the nano-scale H-phase, like Ni4Ti3 in binary NiTi, creates strain fields in the martensitic matrix that also aid in increasing transformation temperatures [2,7,18,19]; however, this amount of change in TTs is already achieved between 2-3 h at 600 °C, so it is likely that further heat treatments are causing coarsening of the precipitates rather an increase in phase amount. Aging at 550 °C has the same effect, but an obviously slower precipitation rate as the TTs observed at 550 °C for 3 h never fully reach the maximum observed when aging at 600 °C.…”

Mechanical and functional behavior of a Ni-rich Ni50.3Ti29.7Hf20 high temperature shape memory alloy

“…The majority of these applications demand large actuation capability, higher transformation temperatures (above 100 C) and stable responses, which restricts existing possibilities to a small group of alloys. Of these alloys, NiTi-based HTSMAs such as NiTiPd [1,2], NiTiPt [3,4], NiTiZr [5,6] and NiTiHf [6e12] have shown significant scientific and technological merit in the last few years. A comprehensive review of select HTSMAs can be found in Ref.…”

Effect of Thermal Treatments on Ni–Mn–Ga and Ni-Rich Ni–Ti–Hf/Zr High-Temperature Shape Memory Alloys

“…However, certain thermal treatments of Ni-rich Ni-TiHf/Zr HTSMAs have been recently revealed as an effective method to improve the shape memory response of these alloys [28][29][30][31][32][33][34][35][36][37] and to overcome the limitations of the (Ti ? Hf/Zr)-rich compositions.…”