A simplified analytical model to simulate martensite reorientation and plasticity in shape memory alloy ring couplers

Recent studies on Shape Memory Alloy rings have been undertaken at the European Organization for Nuclear Research (CERN) to develop smart and leak-tight couplers for Ultra High Vacuum systems of particle accelerators. A special thermo-mechanical process (training) is needed to provide SMA rings with proper functional properties, that is to allow thermal mounting, dismounting, and leak tight coupling within a given service temperature window. Low temperature ring expansion is a crucial part of the training process as it gives suitable size, shape recovery properties, and thermal stability range to the SMA element. An analytical model, based on simplified elastic-plastic axisymmetric concepts, has been developed and implemented in a commercial software to simulate isothermal SMA rings expansions. It is particularly useful to predict the final size of a martensitic SMA coupler as a function of the initial dimensions and of the pre-deformation parameters. The effectiveness of the model has been demonstrated by analyzing the stress/deformation field occurring in a wide range of ring geometries for different load cases including martensite reorientation and plasticity. The predictions of the analytical model have been systematically compared with those obtained by axisymmetric finite element (FE) analyses based on elastic-plastic constitutive models and experimental measurements.

Physical and Structural Characterization of Monocrystalline Cu-13.7% Al-4.2% Ni Alloy Submitted to Thermo-Cyclical Treatments under Applied Loads

Monocrystalline alloy with a nominal composition of Cu-13.7% Al-4.2% Ni (wt.%) that shows reversible martensitic transformations (RMTs) was studied. The alloy, manufactured by the “Memory Crystals Group” in Russia, was subjected to thermo-cyclical treatment (TCT) under tension within a range that included critical RMT temperatures. A special device was developed to perform TCTs (up to 500 cycles) and three different loads were applied: 0.11, 0.26, and 0.53 MPa. X-ray diffraction analysis, optical microscopy, differential calorimetry, and Vickers microhardness were involved in the alloy’s characterization. Under TCTs, the alloy displayed complex structural transformation, revealing the sequence of RMT, β1 ↔ R ↔ β′1 + γ′1; the involved phases were coherently precipitated but very sensitive to the experimental conditions. It was found that during TCTs (from 300 cycles) performed under optimum load (0.26 MPa), the processes of martensite reorientation, hardening, and stabilization of the structure were the most intensive thus leading to a reduction of RMT critical intervals and increased microhardness.

Effect of ambient temperature on compressibility and recovery of NiTi shape memory alloys as static seals

The objective of this study was to investigate the effect of ambient temperature on compressibility and recovery of NiTi shape memory alloys as static seals. Experimental results indicated that compressibility and recovery of NiTi alloys were dependent on ambient temperature. At T < Af (the austenite finish temperature), the compressibility and recovery coefficients were almost unchanged when the compression stress was higher than a certain level. The residual strain of NiTi alloys increased with a decrease in temperature at T < Af. The residual strain of NiTi alloys was remarkably high at the temperature below Mf (the martensite finish temperature). The recovery coefficient of NiTi alloys at T > Af gradually increased with increasing compression loading. The compressibility and recovery coefficients of NiTi alloys were insignificantly fluctuated at the temperatures between 60°C and 150°C upon the compression loading. The features of strong deformation and martensite reorientation in the compressed NiTi alloys confirmed the temperature effect.