Abstract
The impact toughness of a high-strength metastable β titanium alloy (Ti-5Cr-4Al-4Zr-3Mo-2W-0.8Fe) with two typical microstructures is studied by Charpy impact tests. The bimodal microstructure (BM) and the lamellar microstructure (LM) are obtained by the solution and aging treatments and the β annealing, slow cooling and aging treatments, respectively. In the impact crack initiation process, the deformation capacities of the primary α (αp) phase, secondary α (αs) phase and transformed β (βt) matrix in the BM are very different, and the stress gradient at the interface of the three causes the crack initiation. The lamellar α (αl) phase and βt in the LM satisfy the BOR relationship, and the effective slip transfer between α and β phases slows down the crack initiation. Meanwhile, the appearance of deformation twins in the LM improves the crack initiation energy. In the crack propagation process, the lack of coordinated deformation between the α and β phases in the BM leads to rapid crack propagation. In the LM, the deformation of αl and βt is relatively more coordinated, so the severe plastic deformation is only concentrated near the crack and at the interface. The secondary crack initiation and the crack propagation along the twin boundary reduce the stress concentration at the crack tip. The deformation twins and zigzag propagation path can improve the crack propagation energy. To summarize, the alloy with LM exhibits better impact toughness than the alloy with BM.
Highlights
Titanium alloys have been widely developed in the aerospace field due to the advantages of high-specific strength, low density, excellent corrosion resistance and satisfactory crack resistance [1,2,3]
The forged billet was cut into two pieces, one of which was solid solution treated at 880 ◦ C for 1 h and furnace cooled (FC) to 600 ◦ C for aging of 6 h to obtain the lamellar microstructure (LM); the other piece was solid solution treated at 830 ◦ C for 1 h and air-cooled (AC) to room temperature and heated to 600 ◦ C for aging of 6 h to obtain the bimodal microstructure (BM)
For the alloy with BM, due to the solid solution treatment in the α + β phase region, the equiaxed and rod-like primary α phases form, while the grain boundary α phases precipitate on the original β grain boundary in a discontinuous chain shape
Summary
Titanium alloys have been widely developed in the aerospace field due to the advantages of high-specific strength, low density, excellent corrosion resistance and satisfactory crack resistance [1,2,3]. Metastable β titanium alloys have achieved many successful applications in the manufacturing of aircraft structural parts due to their quality hardenability, strong cold working ability, optimum matching of high strength and strong plasticity. Metastable β titanium alloys can be applied in a wide range of strengths. Material scientists have focused on increasing the strength of titanium alloys to meet the more demanding application requirements on structural materials for the long term [8,9]. Huang et al [12] studied the effect of oxygen content on the impact toughness of α titanium alloy and found that the reduction of oxygen content increases the dislocation activity and the number of deformation twins and improves the resistances of crack initiation and propagation. Lei et al [15] analyzed the impact toughness of the two-phase alloy
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