Number Of Thermomechanical Cycles Research Articles

Recreating Ductility-Dip Cracking via Gleeble®-Based Welding Simulation

Face-centered cubic alloys, such as nickel-based alloys and austenitic stainless steels, are important to many industries, notably nuclear power generation and petrochemical. These alloys are prone to ductility-dip cracking (DDC), an inter-mediate-temperature, solid-state cracking phenomenon. They experience an abnormal elevated-temperature ductility loss, which leads to cracking upon applying sufficient restraint. A unified mechanism for DDC has been elusive. To learn more about DDC, an experimental procedure has been designed and evaluated for use in future studies. It is a thermomechanical test that replicates welding conditions via simulated strain ratcheting (SSR) using the Gleeble thermomechanical simulator. This study evaluates SSR and aims to establish the procedure is reproducible and adequately optimized for producing DDC. A design of experiments was created with four alloys tested at varying preloads, elevated temperature strains, and a number of thermomechanical cycles. Mechanical energy imposed within the DDC temperature range was used for quantification of the effect of thermomechanical cycling on the DDC response. The materials tested were 310 stainless steel and Nickel 201 base metals as well as nickel-based filler metals 52M and 52MSS. The SSR successfully recreated DDC while maintaining higher fidelity to actual production conditions than past laboratory tests and offered a more controlled environment than large-scale weld tests. Therefore, the SSR will provide a viable experimental procedure for learning more about the DDC mechanism.

A Comparative Study of Ni49.9Ti50.1 and Ni50.3Ti29.7Hf20 Tube Actuators

A shape memory alloy (SMA) actuator typically has to operate for a large number of thermomechanical cycles due to its application requirements. Therefore, it is necessary to understand the cyclic behavioral response of the SMA actuation material and the devices into which they are incorporated under extended cycling conditions. The present work is focused on the nature of the cyclic, evolutionary behavior of two widely used SMA actuator material systems: (1) a commercially available Ni49.9Ti50.1, and (2) a developmental high-temperature Ni50.3Ti29.7Hf20 alloy. Using a recently developed general SMA modeling framework that utilizes multiple inelastic mechanisms, differences and similarities between the two classes of materials are studied, accounting for extended number of thermal cycles under a constant applied tensile/compressive force and under constant applied torque loading. From the detailed results of the simulations, there were significant qualitative differences in the evolution of deformation responses for the two different materials. In particular, the Ni49.9Ti50.1 tube showed significant evolution of the deformation response, whereas the Ni50.3Ti29.7Hf20 tube stabilized quickly. Moreover, there were significant differences in the tension-compression-shear asymmetry properties in the two materials. More specifically, the Ni50.3Ti29.7Hf20 tube exhibited much higher asymmetry effects, especially at low stress levels, compared to the Ni49.9Ti50.1. For both SMA tubes, the evolution of the deformation response under thermal cycling typically exhibited regions of initial transients, and subsequent evolution.

The effects of multi-cyclic thermo-mechanical treatment on the grain refinement and tensile properties of a metastable austenitic steel

Abstract The effects of the multi-cyclic cold rolling and annealing process on the grain refinement and tensile properties of a metastable austenitic steel were investigated. The multi-cyclic thermo-mechanical process yielded the bimodal ultrafine-grained structure through both the reverse transformation and recrystallization during annealing. The kinetics and temperatures of the reverse transformation were related to the average size of coarse grains. Not only yield and tensile strengths but also ductility of the bimodal-grained structure increased with the number of thermo-mechanical cycles.

Calibration of a three-dimensional multimechanism shape memory alloy material model for the prediction of the cyclic “attraction” character in binary NiTi alloys

As typically utilized in applications, a particular shape memory alloy device or component operates under a large number of thermomechanical cycles, hence, the importance of accounting for the cyclic behavior characteristics in modeling and characterization of these systems. To this end, the present work is focused on the characterization of the evolutionary, cyclic behavior of binary 55NiTi (having a moderately-high transformation temperature range). In this study, an extensive set of test data from recent cyclic, isobaric, tension tests was used. Furthermore, for the calibration and characterization of this material, a newly developed, multiaxial, material-modeling framework was implemented. In this framework, multiple, inelastic mechanisms are used to regulate the partitioning of energy dissipation and storage governing the evolutionary thermomechanical response.

Effect of thermomechanical cycling in an FeMnSiCrNi shape memory alloy

Previous studies on the effect of cycling in Fe-Mn-Si alloys have shown an increase in reversibility with increasing number of thermal cycles. In Fe-Mn-Si-Cr-Ni and Fe-Mn-Si-Co-Ni alloys, the shape recovery strain during thermomechanical cycling increased when the strain amplitude for each successive cycle is kept constant. Where the total strain (i.e., the sum of the transformation and permanent strain) is held constant, a small decrease in strain recovery with increasing number of cycles occurs. Nonetheless, an increase in reversibility with increasing number of thermomechanical cycles is a common feature for most studies. While previous studies have shown that training or thermomechanical transformation cycling may improve the shape recovery in these alloys, the mechanism is not still clear. Characterization of the transformation behavior during cycling is an important prerequisite for practical use of such alloys. In this paper the authors report microstructural changes during thermomechanical cycling of an Fe-Mn-Si-Cr-Ni shape memory alloy.