Behaviour and effect of Ti2Ni phase during processing of NiTi shape memory alloy wire from cast ingot

Binary NiTi alloy is one of the commercially successful shape memory alloys (SMAs). Generally, the NiTi alloy composition used for thermal actuator application is slightly Ti-rich. In the present study, vacuum arc melted alloy of 50.2Ti–Ni (at.%) composition was prepared and characterized using optical, scanning and transmission electron microcopy. Formation of second phase particles (SPPs) in the cast alloy and their influence on development of microstructure during processing of the alloy into wire form has been investigated. Results showed that the present alloy contained Ti2Ni type SPPs in the matrix. In the cast alloy, the Ti2Ni particles form in varying sizes (1–10 lm) and shapes. During subsequent thermo- mechanical processing, these SPPs get sheared/fragmented into smaller particles with low aspect ratio. The presence of SPPs plays a significant role in refinement of the microstructure during processing of the alloy. During deformation of the alloy, the matrix phase around the SPPs experiences conditions sim- ilar to that observed in severe plastic deformation of metallic materials, leading to localized amorphisa- tion of the matrix phase

Meta-Analysis of 28,141 Individuals Identifies Common Variants within Five New Loci That Influence Uric Acid Concentrations

Elevated serum uric acid levels cause gout and are a risk factor for cardiovascular disease and diabetes. To investigate the polygenetic basis of serum uric acid levels, we conducted a meta-analysis of genome-wide association scans from 14 studies totalling 28,141 participants of European descent, resulting in identification of 954 SNPs distributed across nine loci that exceeded the threshold of genome-wide significance, five of which are novel. Overall, the common variants associated with serum uric acid levels fall in the following nine regions: SLC2A9 (p = 5.2×10−201), ABCG2 (p = 3.1×10−26), SLC17A1 (p = 3.0×10−14), SLC22A11 (p = 6.7×10−14), SLC22A12 (p = 2.0×10−9), SLC16A9 (p = 1.1×10−8), GCKR (p = 1.4×10−9), LRRC16A (p = 8.5×10−9), and near PDZK1 (p = 2.7×10−9). Identified variants were analyzed for gender differences. We found that the minor allele for rs734553 in SLC2A9 has greater influence in lowering uric acid levels in women and the minor allele of rs2231142 in ABCG2 elevates uric acid levels more strongly in men compared to women. To further characterize the identified variants, we analyzed their association with a panel of metabolites. rs12356193 within SLC16A9 was associated with DL-carnitine (p = 4.0×10−26) and propionyl-L-carnitine (p = 5.0×10−8) concentrations, which in turn were associated with serum UA levels (p = 1.4×10−57 and p = 8.1×10−54, respectively), forming a triangle between SNP, metabolites, and UA levels. Taken together, these associations highlight additional pathways that are important in the regulation of serum uric acid levels and point toward novel potential targets for pharmacological intervention to prevent or treat hyperuricemia. In addition, these findings strongly support the hypothesis that transport proteins are key in regulating serum uric acid levels

On stability of NiTi wire during thermo-mechanical cycling

The use of NiTi wire as thermal actuator involves repeated thermal cycling through the transformation range under a constant or fluctuating load. The stability of the material under such conditions has been a concern for the past many years. Experimental results show that for a given alloy composition, the repetitive functional behaviour of NiTi wire is largely dependent on the processing schedule/parameters and the stress–strain regime of thermo-mechanical cycling (TMC). Among the various processing parameters,retained cold work in the material and the shape memory annealing temperature/time have significant influence. It has been shown in the present study that for a stable functional behaviour, the material needs to be tailored through judicious selection of these parameters. Study also shows that, after processing, the material requires an additional stabilization treatment for ensuring minimal variation in the repetitive functional response upon TMC

Functional fatigue in NiTi shape memory alloy wires - A comparative study

The functional fatigue behaviour of two near equi-atomic NiTi shape memory alloy wires obtained from different sources were evaluated. Results showed that though the wires had similar transformation temperatures and mechanical properties, their functional fatigue behaviour upon thermo-mechanical cycling was at variance. Under a variable stress in the range 150-450 MPa and 4% recovery strain, one of the wires showed better stability, and significantly higher fatigue life (~30,000 cycles) than the other (~3,500 cycles). The reasons for such wide variation in thermo-mechanical fatigue behaviour have been discussed in this paper

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 deg C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni24.7Ti50.3Pd25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 deg C and ~8.5 deg C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 deg C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ~82 GPa and hardness of ~4.7 GPa in martensite phase

Influence of stored elastic strain energy on fatigue behaviour of NiTi shape memory alloy thermal actuator wire

Influence of stored elastic strain energy, Eelse, on thermo-mechanical fatigue behaviour of NiTi shape memory alloy (SMA)thermal actuator wire was investigated. Two near equi-atomic NiTi SMA wires obtained from different sources were evaluated for quasi-static and functional fatigue properties. Study showed that the wires had similar chemical composition, transformation temperatures and static mechanical properties. However, the functional fatigue behaviour of the wires upon thermo-mechanical cycling (TMC) was found to be significantly different. Under avariable TMC stress in the range 150–450 MPa and 4% recovery strain, one of the wires showed better stability, and substantially higher fatigue life(~30000cycles)than the other (~3500 cycles). Thermodynamic and microstructural analyses indicated that the wide variation in fatigue response of the wires was due to difference in magnitude of Eelse in the material. It is observed that at a given temperature above austenite start temperature (As), the wire with higher stored Eelse, generated about 70–100MP a higher recovery stress than that of the wire with lower stored Eelse. As a consequence,the maximum temperature, Tmax, necessary for generation of preset peak stress during reverse(martensite-austenite)transformation, was always less in the former wire than that of the latter. This in turn was responsible for wide variations in thermo-mechanical fatigue behaviour of the two wires upon TMC

Preliminary Studies on the Thermo-Mechanical Characteristics of Indigenously developed Shape Memory Alloy wires

Smart materials have received increasing attention in recent years for their great potential to revolutionize engineering applications and design. The technological advantages of using smart materials over traditional materials arise from special capabilities due to unique micro structure or molecular properties. Among different smart materials like piezoelectric, electro-strictive, Magneto-strictive, magneto-rheological and electro-rheological, shape memory alloy seems to be probable candidate for semi-active control of different structural members in view of the fact that it generates a relatively large deformation and then recover upon heating

Effects of thermo-mechanical cycling on the strain response of Ni-Ti-Cu shape memory alloy wire actuator13;

The present paper deals with the strain response of Ni-Ti-Cu shape memory alloy (SMA) wire actuators on thermo-mechanical cycling (TMC). The characteristics of the actuators such as austenite (hot shape) remnant deformation and recovery strain undergo changes upon TMC. These changes are significant in the initial few cycles and the properties of SMA tend to reach a steady state on further cycling. It is believed that TMC induces defects in the microstructure and stabilizes the martensite/austenite phase. These in turn result in continuous change in strain response with the progress of TMC. It has been shown that for a stable strain response, the wire actuators need to be subjected to TMC at a higher stress than the working stress prior to application. Experiments were also conducted in order to minimize the number of TMC required for achieving stable strain response. [All rights reserved Elsevier

Strain response of Ni-Ti-Cu shape memory alloy wires on thermo-mechanical cycling

Ni-Ti base shape memory alloys (SMAs) are unique functional materials attractive for aerospace, medical and various engineering applications. The stability of the functional properties such as transformation temperatures, recovery stress and recovery strain of SMAs with thermo-mechanical cycling (TMC) is an important design13; criterion, and needs to be established before fabrication of any actuator. This paper deals with the results of an experimental study wherein the strain response of the Ni-Ti-Cu alloy wires on TMC have been reported

Strain response of Ni-Ti-Cu shape memory alloy wires on thermo-mechanical cycling

Ni-Ti base shape memory alloys (SMAs) are unique functional materials attractive for aerospace, medical and various engineering applications. The stability of the functional properties such as transformation temperatures, recovery stress and recovery strain of SMAs with thermo-mechanical cycling (TMC) is an important design criterion, and needs to be established before fabrication of any actuator. This paper deals with the results of an experimental study wherein the strain response of the Ni-Ti-Cu alloy wires on TMC have been reported