Explosive welding is widely used in the manufacture of dissimilar metal composite plates. In this work, TC4/TA1/Ti6321 titanium alloy laminated composites were successfully prepared by explosive welding. The interfacial characteristics of the laminated composites were studied by energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission electron microscope (TEM), and other characterization methods. The dynamic mechanical properties of the laminated composites were investigated by using the split Hopkinson pressure bar (SHPB). The results show that the interface of laminated composites was well bonded. There was a high density of crystal defects including dislocations and twins near the interface. The composite plates showed a hard/soft/hard hardness distribution as a whole, and the microhardness increased near the interface. Due to the existence of the interface, the dynamic mechanical properties of laminated composites show different results. The dynamic compressive stress-strain curve of laminated composites showed an equilibrium type, and the strain strengthening effect was lower than that of homogeneous materials. The failure mode of laminated composites was adiabatic shear failure mode. 

Investigation of interfacial structure and dynamic mechanical behavior of titanium alloy laminated composites

Tuning microstructure and improving the corrosion resistance of Ti-6Al-3Nb-2Zr-1Mo alloy using the electron beam freeform fabrication

The impact toughness of titanium alloys is one of the key factors determining their safe service in cryogenic temperature environments. In this work, the temperature-related impact properties of grade 2 pure titanium and Ti-2.5Al–3Zr–1Mo alloys are investigated. It is found that the impact properties and deformation mechanisms of the above two materials are different with decreasing temperature (20 °C, 0 °C, −50 °C, −100 °C and −196 °C). Speaking of pure titanium: the impact toughness doesn't change obviously, and crack initiation energy (W i ) gradually increases while crack propagation energy (W p ) decreases (W p of −196 °C is 40% lower than that of 20 °C while W i is 90% higher). The slip-dominated deformation mechanism at 20 °C gradually transforms into twinning-dominated deformation mechanism under cryogenic temperature. High-density twins formed at −100 °C and −196 °C can effectively relieve stress concentration and lead to better cryogenic impact toughness. By contrast, W i and W p of Ti-2.5Al–3Zr–1Mo alloy decrease with decreasing temperature. The impact toughness of Ti-2.5Al–3Zr–1Mo alloy is higher than that of pure titanium when the temperature is above −100 °C. At 20 °C, the synergistic effect of the tortuous crack path and deformation twins of Ti-2.5Al–3Zr–1Mo alloy promotes its impact toughness higher than that of pure titanium. However, the twinning density of Ti-2.5Al–3Zr–1Mo at −196 °C is much lower than that of pure titanium, which cannot release stress effectively and cause a brittle fracture. The research provides theoretical and experimental basis for the application of titanium alloys in cryogenic environment, and provides guidance for the development of titanium alloys with better cryogenic impact toughness. 

Twinning-induced high impact toughness of titanium alloy at cryogenic temperature

Effects of the cooling rate on mechanical properties and microstructure of near-α Ti-6Al-3Zr-2Nb-Mo alloy

The microstructure of wire arc additive manufactured (WAAMed) titanium alloy was characterized by brittle martensitic, which was significantly different from that of wrought alloy. It was generally believed that brittle martensitic was detrimental to the toughness of titanium alloys. In order to obtain WAAMed titanium alloy with excellent combination of strength and toughness, Ti5111 alloy, which is a newly developed deformation twinning induced plasticity(TWIP) alloy with an excellent combination of impact toughness and strength, was prepared by arc additive manufacturing(WAAM) technology and then annealed at 900 °C + air cooling(AC) and 950 °C+ air cooling. Microstructure, tensile properties and impact toughness of WAAMed Ti5111 alloy were investigated. Electron backscatter diffraction (EBSD) observation was adopted to identify the deformation twins activated in impact samples. Results showed that as-built WAAMed Ti5111 with excellent toughness was obtained, which was 42.8% higher than that of WAAMed Ti–6Al–4V alloy. After annealing, more deformation twins and secondary twins were activated during impact loading, which could effectively improve the toughness of WAAMed Ti5111. 

Influence of annealing on the microstructure and Charpy impact toughness of wire arc additive manufactured Ti5111 alloy

The Charpy impact absorbed energy of a hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy plate with different sampling and notching directions was investigated. Strong anisotropic variations occurred in impact absorption energies due to difference of the α phase crystal orientation and microstructural characteristics along different directions. The higher crack propagation energy of the sampling-notching direction sample (RD-ND) resulted in an impact absorption energy of approximately 2.5 times larger than that of the TD-ND, TD-RD and RD-TD samples. Based on the analysis of the α crystal orientation and the TEM microstructure after impact deformation, it was found that the higher absorption energy of the RD-ND sample was attributed to the activities of a large number of { 10 1 ‾ 0 } ⟨ 1 2 ‾ 10 ⟩ prismatic slips and the resistance created by grain boundaries against crack propagation, which also resulted in the delamination of the impact fracture surface. Finally, the coupling effect of crystallographic texture and microstructural characteristics on the impact deformation mechanisms of the hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy was described.

Charpy impact anisotropy and the associated mechanisms in a hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy plate

The Charpy impact absorbed energy of a hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy plate with different sampling and notching directions was investigated. Strong anisotropic variations occurred in impact absorption energies due to difference of the α phase crystal orientation and microstructural characteristics along different directions. The higher crack propagation energy of the sampling-notching direction sample (RD-ND) resulted in an impact absorption energy of approximately 2.5 times larger than that of the TD-ND, TD-RD and RD-TD samples. Based on the analysis of the α crystal orientation and the TEM microstructure after impact deformation, it was found that the higher absorption energy of the RD-ND sample was attributed to the activities of a large number of {101‾0}⟨12‾10⟩ prismatic slips and the resistance created by grain boundaries against crack propagation, which also resulted in the delamination of the impact fracture surface. Finally, the coupling effect of crystallographic texture and microstructural characteristics on the impact deformation mechanisms of the hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy was described. 

The plastic deformation modes of commercially pure titanium (CP-Ti) were studied using an in situ tensile test monitored by electron-backscatter-diffraction (EBSD) assisted slip trace analysis. The plastic strain was primarily accommodated by prismatic slip, followed by deformation twins and pyramidal slip. The slip transmission between two adjacent grains was predicted using the geometric compatibility factor m′, which influenced not only the degree of stress concentration but also the activity of dislocation slip systems. Stress concentration mainly occurred at GBs with an m′ less than 0.5 and could be released by the activities of pyramidal slip or deformation twins with high critical shear stress (CRSS). 

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David Feldmann

Papers

Charpy impact anisotropy and the associated mechanisms in a hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy plate

Charpy impact anisotropy and the associated mechanisms in a hot-rolled Ti–6Al–3Nb–2Zr–1Mo alloy plate

Plastic Deformation Mechanism and Slip Transmission Behavior of Commercially Pure Ti during In Situ Tensile Deformation