Toughening High Strength Titanium Alloys Through Fine Tuning Phase Composition and Refining Microstructure
YANG Rui(), MA Yingjie, LEI Jiafeng, HU Qingmiao, HUANG Sensen
Shi -Changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
YANG Rui, MA Yingjie, LEI Jiafeng, HU Qingmiao, HUANG Sensen. Toughening High Strength Titanium Alloys Through Fine Tuning Phase Composition and Refining Microstructure. Acta Metall Sin, 2021, 57(11): 1455-1470.
Titanium alloys are key materials for applications in major engineering areas, such as aerospace and marine equipment. Studies on structural titanium alloys focus on strengthening and toughening the alloys, especially the latter. The mainstream structural titanium alloys comprise both α and β phases. The optimization of the strength and toughness balance relies on the control of the compositions, volume fractions, and morphologies of both phases. In this study, some recent advances along the above line are reviewed, focusing on studies on the composition design, plastic-deformation mechanism, and microstructure tuning. Rational design of the compositions of both phases improved the deformation coordination within the α phase and across the α/β interface, suppressed the precipitation of brittle ω and α2 phases, and resulted in improved plasticity and toughness through the α-deformation twin and β-deformation-triggered phase transformation. The multiscale microstructure enhanced the strength and toughness of the titanium alloy. Using the abovementioned approaches, a series of titanium alloys with an improved strength-toughness combination were developed and fabricated. Finally, an attempt was made to predict the prospect of technology development in the field of high-strength and high-toughness titanium alloys for various applications.
Fig.1 Peierls stress (single dislocation critical resolved shear stress, CRSS) τ of Ti-xAl alloys plotted as functions of composition for the relaxed VASP-SQS calculations (a) and mechanical properties of Ti-xAl alloy as functions of Al concentration from experimental measurements (b) (VASP—vienna ab initio simulation package, SQS—special quasirandom structure, —CRSS of the prismatic <a> slip, —CRSS of the basal <a> slip)[7]
Alloy
Modulus / GPa
Modulus ratio
Eα
Gα
Eβ
Gβ
Eβ / Eα
Gβ / Gα
Ti64
159.99
63.70
71.06
25.55
0.44
0.40
Alloy-1
159.60
64.11
83.57
30.47
0.52
0.48
Alloy-2
155.81
61.67
88.55
32.37
0.57
0.52
Table 1 Young's moduli (E) and shear moduli (G) of α and β phases of Ti64 alloy and two novel titanium alloys developed in the authors' group, calculated by using first principles exact Muffin-Tin orbital method
Fig.2 Transmission electron microscope (TEM) images of the deformed 3 μm micropillar of Ti-10V-2Fe-3Al alloy[18]
Fig.3 Selected area electron diffraction (SAED) patterns (a, c) and DF TEM images (b, d) of Ti-3Al-5Mo-4.5V alloy of unaged (a, b) and 300oC aged (c, d) samples
Fig.4 True strain-stress curves (a) and work hardening rate curves (b) of Ti-3Al-5Mo-4.5V specimens under different heat-treatment conditions (750 representing 750oC, 2 h, water quenching; 750 + 300 representing 750oC, 2 h, water quenching + 300oC, 6 h, air cooling; 750 + 500 representing 750oC, 2 h, water quenching + 500oC, 6 h, air cooling)
Fig.5 Energy difference (ΔEω-β) between ω and β phases of binary Ti-xM (M = Zr, V, Nb, Mo, Cr) alloys vs content of M (x)[21,22]
Fig.6 3D reconstruction maps and Al cluster morphologies of Ti-8Al (a) and Ti-6Al (c) samples at 16% and 14% (atomic fraction) isosurface, respectively, and the proxigrams across the boundary between the aged matrix and several α2 precipitates in Ti-8Al (b) and Ti-6Al (d) samples (The dotted vertical lines corresponding to the concentration isosurfaces)[39]
Fig.7 In-situ tensile load-displacement curves of the Ti-8Al alloy under different heat-treatment conditions (550/24 representing 550oC, 24 h, air cooling; 550/100 representing 550oC, 100 h, air cooling; 550/week representing 550oC, 168 h, air cooling)[40]
Fig.8 SEM images (left), inverse pole figure (IPF) (middle), and Kernel average misorientation (KAM) (right) with 5° threshold angle of the region near the notch of 168 h aged Ti-8Al specimen at different displacements in in-situ tensile test (The red arrows in Figs.8b and c point to the dislocation slipping, the color bar below the KAM map represents the misorientation with 5° threshold angle)[40]
Fig.9 Comparison of compositions in the center of primary α (αp) and β transformation (βt) between experiments (symbols) and calculations (lines) as a function of α + β solution temperature, where the measurements were made on samples after solution for 30 min and water quenching (The element contents after solution at 970oC for 16 h are also indicated in the gray rectangle)[44]
Fig.10 Variation of chemical composition from αp to βt after 5 min thermal treatment at 920oC[44]
Fig.11 Phase field simulations of alloying element partitioning showing the Al (a) and V (b) element diffusing process and verifying the experimental results, with the arrows pointing the diffusion direction (The red and blue color refers to high and low element contents, respectively)[44]
Fig.12 Microstructures and Al, V distributions under 920oC for 2 min (a), 5 min (b), 8 h (c), and 32 h (d) (The αp grains or βt is delineated on BSE images and the element distribution pointed by arrows indicate the occurrence of αp/βt phase transformation before the elements reach their equilibrium concentration)[44]
Fig.13 Characterization of deformation twinning in crack tip plastic zone (CTPZ) of Ti64 alloy with Widmannst?tten microstructure[51]
Fig.14 Tensile true strain-true stress curves and work hardening rate curves of Ti-3Al-5Mo-4.5V alloy at different heat-treatment temperatures (β transus temperature is (860 ± 5)oC)[42]
Fig.15 Primary deformation mechanisms of β phase in Ti-3Al-5Mo-4.5V alloy as a function of β phase stability (Points 1, 2, 3, and 4 represent sample heat-treated at 880oC, 800oC, 750oC, and 700oC, respectively. SIM—stress induced martensite, [Mo]eq.—Mo equivalent)[42]
Fig.16 Schematic representation of CTPZ in titanium alloy with Widmannst?tten microstructure, showing expansion of CTPZ range by large-scale slip and deformation twinning (LEFM—linear elastic fracture mechanics)[71]
Fig.17 Schematic representation of research and development (R&D) of highly alloyed α + β titanium alloys with hierarchical microstructures based on micro-zone optimization for strength-toughness improvement[39,51,72]
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