Effect of Aging on the Microstructure and Shape Memory Effect of a Hot-Rolled NiTiHf Alloy

Abstract

In this article, the effect of aging on the microstructure and shape memory effect (SME) of a hot-rolled Ni 49 Ti 36 Hf 15 alloy is studied. The alloy was prepared by vacuum induction melting (VIM) and homogenized at 1000°C for 2 h. The homogenized alloy was then undergone 45% hot rolling at 850°C and aging at temperatures of 500 and 600°C for 2 h. Optical microscopy (OM) and scanning electron microscopy (SEM) conducted on the hot-rolled samples revealed that hot rolling improves microstructure, deformation, and recoverable strains such that the alloy recovers 3.10 of 3.23 and 5.61 of 6.25 strain in the homogenized and hot-rolled state, respectively. Aging, however, adversely affects the formability and SME of the alloy, which stems from the embrittling effect of the newly formed precipitates during the aging process.

Processing, Preaging, and Aging of NiTi-20 at.% Hf High-Temperature Shape Memory Alloy from Laboratory to Industrial Scale

“…Inversely related, the Ti-rich oxide decreases with Ni content; however, Ni50.5Ti34.5Hf15 50% HR sample does not follow this trend. Hot mounting the hot rolled samples, oxide levels are still comparable to previous reports on Ni50.5Ti34.5Hf15 [22]. The hot rolled condition's area fractions are similar to other reports because mechanically working the material, even in elevated temperatures, will breakdown oxides previously introduce into the system (i.e.…”

“…The second study was focused on one of the final selected compositions to characterize and determine the purity, thermal response and microstructural equivalence of a large-scale melt have with laboratory melts. The study was designed to provide an array of conditions comparable to previous pre-aging and aging characterization studies [2], [9]- [11]. Additional information on the materials response to conventional processing via hot rolling is investigated.…”

Transformation behavior and shape memory properties of Ti 50 Ni 15 Pd 25 Cu 10 high temperature shape memory alloy at various aging temperatures

“…Thermal hysteresis of the 600 1C aged material was increased by increasing the compression stress, however this behavior was found to be opposite in case of 550 1C aged material. Javadi et al [29] studied the effects of aging in NiTiHf alloy at 500 1C and 600 1C for 2 h. Due to the formation of Hf-rich precipitates, the alloy was embrittled and no strain recovery was found. Sasaki et al [30] investigated the effects of aging in Ni-rich NiTiPd for different time duration at 400 1C and 550 1C.…”

Shape memory strains and temperatures in the extreme

“…Because of elastic recovery and tension-compression symmetry it is difficult to interpret bending results. Shape memory behavior of bent samples displayed transformation strains up to 6.25% for the Ni 49 Ti 36 Hf 15 composition [69,70], even though the low Ni content of this alloy (<50%) precludes its usage as superelastic material. In the last years the interest on the mechanical behavior of NiTiHf is growing, and several studies have shown good SME, ISME and SE behaviors at temperatures above 250°C.…”