The effect of high-pressure torsion on the microstructure and outstanding pseudoelasticity of a ternary Fe–Ni–Mn shape memory alloy

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2021-01, Vol.802, p.140647, Article 140647
• Main Authors: Koohdar, Hamidreza, Nili-Ahmadabadi, Mahmoud, Javadzadeh Kalahroudi, Faezeh, Jafarian, Hamid Reza, Langdon, Terence G.
• Format: Article
• Language: English
• Subjects: Annealing; Austenite; Cyclic loads; Diffraction patterns; Dislocation density; Electron backscatter diffraction; Grain refinement; Heat treating; High-pressure torsion; Manganese; Martensite; Martensitic transformations; Microhardness; Microstructural evolution; Microstructure; Nickel; Pressure effects; Pseudoelastic behavior; Pseudoelasticity; Room temperature; Shape memory alloys; Strain hysteresis; Tensile tests; Ternary Fe–Ni–Mn shape memory alloy
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2020.140647

Experiments were conducted to examine the effect of high-pressure torsion (HPT) processing on the microstructure and pseudoelastic behavior of a ternary Fe–10Ni–7Mn (wt.%) shape memory alloy in both the solution-annealed and intercritically-annealed conditions. X-ray diffraction (XRD) patterns and electron backscatter diffraction (EBSD) analyses showed that the initial microstructure of the alloy in the solution-annealed condition was a fully lath α′-martensite which partially transformed to a strain-induced austenite (α′→γ) by HPT processing. Also, the austenite formed in the dual phase (α′+γ) specimens after intercritical annealing treatment at 600 °C for 7.2 ks underwent a γ→ε→α′ transformation during subsequent HPT processing such that a multi-phase microstructure was formed consisting of α′-martensite, austenite and ε-martensite. The HPT processing led to a significant increase in the microhardness value to ~690 Hv due to a high density of dislocations and the associated grain refinement of the microstructure. Cyclic loading-unloading tensile tests at room temperature revealed a strain hysteresis and pseudoelastic behavior in the HPT-processed specimens with different initial microstructures. Outstanding pseudoelasticity values of about 67% and 75% were obtained at the fourteenth loading-unloading cycle after 20 HPT turns in the solution-annealed and intercritically-annealed specimens, respectively.