Formation and toughening of metastable phases in TiZrHfAlNb medium entropy alloys

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2019-03, Vol.748, p.441-452
• Main Authors: Wang, Lu, Fu, Chao, Wu, Yidong, Wang, Qinjia, Hui, Xidong, Wang, Yandong
• Format: Article
• Language: English
• Subjects: Beta phase; Constitution; Deformation effects; Deformation mechanism; Deformation mechanisms; Dislocations; Elongation; Heat treating; Martensite; Martensitic transformations; Medium entropy alloy; Medium entropy alloys; Metastable; Metastable phases; Microstructure; Phase composition; Phase transitions; Stress induced martensitic transformation; Tensile properties; Twinning
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2018.12.033

In this work, the microstructure, phase transformation, and tensile properties were investigated for Ti55-xZr20Hf15Al10Nbx (x = 1, 2, 3…, 9, at%) medium entropy alloys (MEAs). It has been found that the phase composition of the as-cast MEAs changes from hexagonal α′ to orthorhombic α″, then to metastable β, finally to stable β phase at x = 2, 4 and 9, respectively. The phase transformation of Nb4–Nb8 MEAs evolved in the sequence of β + α″→ω + α″ + β→α + β′→β during heating process. The phase constitution and microstructure of these MEAs strongly influence their tensile properties. The formation of metastable α″ and β phase in certain compositional and temperature range results in “double yielding” phenomenon and remarkable work-hardening effect. Extraordinary static toughness represented by the product of strength and elongation has achieved for MEA with 8% Nb. The deformation mechanisms for these MEAs changes with the phase composition. For the alloy with most α″ phase, the deformation is dominated by de-twinning of martensite variants; for the alloys with most β phase, stress induced martensitic transformation from metastable β to α″ phase and the refinement of the newborn martensite by the dislocation have been verified. These integrated effects are considered to endow the MEAs excellent deformation ductility.