Exceptionally high strain-hardening and ductility due to transformation induced plasticity effect in Ti-rich high-entropy alloys

Abstract

Ti-rich body-centered cubic (BCC, β) high-entropy alloys having compositions Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅, Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂, and Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅ (in at%) were designed using bond order (Bo)-mean d-orbital energy level (Md) approach. Deformation mechanisms of these alloys were studied using tensile deformation. The alloys showed exceptionally high strain-hardening and ductility. For instance, the alloys showed at least twofold increment of tensile strength compared to the yield strength, due to strain-hardening. Post-deformation microstructural observations confirmed the transformation of β to hexagonal close packed (HCP, α') martensite. Based on microstructural investigation, stress-strain behaviors were explained using transformation induced plasticity effect. Crystallographic analysis indicated transformation of β to α' showed strong variant selection (1 1 0)_β//(0 0 0 1)_α', and [1 - 1 1]_β//[1 1 - 2 0]_α'.

Figure 1

Bo–Md diagram and microstructures of various alloys. (a) Schematic illustration of the standard Bo–Md diagram. Broken line indicates the guided reference for alloy design. Red color dots show the selected locations of alloys composed of chemical compositions, 1. Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅, 2. Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂, 3. Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅. Microstructures of various alloys in the as-cast condition. (b–d) Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅, Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂, Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅, respectively. (e) TEM image of Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅ alloy and the corresponding SAED patterns of matrix, BCC (f) and second phase, HCP (g).

Figure 2

Microstructures and chemical compositions of selected alloys. TEM images of (a) Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅, (b) Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂, and (c) Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅ alloys in the as-cast condition. In the insets of TEM images, ‘β’ indicates BCC structure, and ‘α′’ indicates hexagonal close packed (HCP) structure. Chemical composition of the alloy’s constituents evaluated using TEM are shown in table on the right-hand side of the respective TEM image.

Figure 3

Tensile stress–strain curves and strain-hardening rate behavior of various alloys deformed at room temperature for the strain rate 10^–3 s⁻¹. (a) Engineering stress–strain curves. (b) Strain-hardening rate behavior as a function of true strain.

Figure 4

Microstructures of various alloys after tensile fracture. (a–c) EBSD IPF maps of Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅, Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂, and Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅, respectively. Colors of the microstructures indicate crystallographic orientations parallel to the tensile axis (T.A.) according to the key stereographic triangle. (d–f) Phase maps of Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅, Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂, and Ti₃₈Zr₂₅Hf₂₅Ta₇Sn₅, respectively. HCP and BCC phases were shown in red and black colors, respectively. Tensile axis is parallel to the vertical axis of microstructures.

Figure 5

Microstructures of Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂ alloy after tensile fracture. (a) TEM image. SAED pattern is shown in the inset. (b) EBSD-IPF map of HCP phase. BCC phase is highlighted in black. Colors of the microstructure indicate crystallographic orientations parallel to the tensile axis (T.A.) according to the key stereographic triangle. Tensile axis is parallel to the vertical axis of microstructures. Pole figure (PF) plots of Ti₃₈Zr₂₅Hf₂₅Ta₁₀Sn₂ alloy corresponding to the microstructure shown in (b) after tensile fracture. (c) 110, (d) 0002, (e) 111, and (f) 11–20.