Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

Abstract

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (Ͻ600 K) rarely used for practical applications and at a high temperature (Ͼ600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.

Traditional Metallurgy, Nanotechnologies, and Structural Materials: A Sorby Award Lecture

“…A glance at the recent materials engineering and science literature demonstrates the wide spread industrial application of nanotechnologies by the traditional metallurgists. By controlling the formation of nanoparticles in various alloys, researchers have increased material toughness (15,16), strength (16,17) and creep (18) and fatigue (16) resistance, improved age hardening kinetics (17,19), increased the quality of hot-isostatically pressed super alloys (20), improved the mechanical properties of cast alloys (21), developed superior shape memory characteristics (22), increased the work hardening rates (23) and induced superplasticity (24).…”

“…Nanosized (15 nm) dispersoids of Al 7 Cr played a major role in pinning subboundaries thus allowing superplasticity to occur in an Al-Mg-Mn alloy (24) while boron additions to Type 347 stainless steels caused enhanced the precipitation of carbonitrides and nanosized borides which increased the creep strength (18) by inhibiting grain boundary sliding. The precipitation of "dense and fine Ti 3 Ni 4 precipitates with a size below 10 nm caused superior shape memory characteristics to appear" in a titanium-nickel alloy (22) because of the precipitates impact on the martensite transformation.…”

Mechanical properties of Ti–(∼49at%) Ni shape memory alloy, part II: Effect of ageing treatment

“…These treatments have focused mainly on the equiatomic Ti-Ni [16][17][18][19] or Ni-rich alloys [20][21][22][23][24]. Very few studies concern similar attempts in the case of Ti-rich Ti-Ni alloys [25][26][27][28].…”

“…The potential of the ageing treatment is attributed to the formation of fine precipitates, which act as effective obstacles for the movement of dislocations, thereby increasing the critical resolved shear stress (CRSS) for slip [23,[29][30][31][32][33]. In the case of Tirich Ti-Ni alloy, the Ti 2 Ni precipitate phase, formed under the favorable process schedules, acts as the nucleation site of the martensite (B19 0 ) [34].…”

Effect of heat treatment on transformation behavior of Ti–Ni–V shape memory alloy

“…Among these phase transformations, A ↔ M and R ↔ M transformations are characterized by large lattice distortions and large transformation hysteresis, while A ↔ R is characterized by small lattice distortion and a small temperature hysteresis [4,5]. The transformation behaviors are quite sensitive to annealing following cold working [6,7], aging treatment in Nirich Ti-Ni alloys [8,9] and the addition of third element [10,11]. The addition of V can decrease the transformation temperature of Ti-Ni alloy, and Ti-50.8Ni-0.5V alloy is a typical Ti-Ni-V SMA [12].…”