W.D. Zeng

Fracture criterion for predicting surface cracking of Ti40 alloy in hot forming processes

Abstract Hot compression tests were conducted on Ti40 burn resistant titanium alloy in the temperature range of 900–1 100 °C and strain rate range of 0.01–10 s −1 to investigate its fracture behavior and critical fracture conditions in hot forming. It was observed that the failure of Ti40 alloy is attributed to longitudinal surface cracking due to severe oxidation of element V and the secondary tensile stresses. The critical fracture strain increases with increasing temperature and decreasing strain rate. From these observations and parallel FEM simulations, it was concluded that the critical fracture strain is a function of a single argument Zener-Hollomon parameter, and there is a linear relationship between them. An Oyane criterion successfully predicted the location of crack initiation. The critical fracture values also exhibit a liner relationship with ln Z . Based on these results, a new fracture criterion of Ti40 alloy based on Zener-Hollomon parameter was established.

Hot workability and microstructure evolution of highly β stabilised Ti–25V–15Cr–0·3Si alloy

Abstract The hot workability and microstructural evolution of a highly β stabilised Ti–25V–15Cr–0·3Si alloy have been studied using constant strain rate isothermal compression tests in the temperature range 900–1100°C and strain rate range 0·01–10 s−1. It was found that all the flow stress curves were characterised by a sharp initial discontinuous yielding followed by either a steady state or continuous flow softening with strain. This alloy showed dynamic recovery at temperatures less than 950°C but dynamic recrystallisation at temperatures higher than 1000°C. At higher strain rates (>1 s−1), necklace recrystallisation, in which grain boundaries were decorated by finely recrystallised grains, was operative. At lower strain rates (<0·1 s−1), however, a typical continuous recrystallisation with serrated grain boundaries occurred. Necklace recrystallisation was associated with Ti5Si3 particles precipitating along beta grain boundaries, whereas continuous recrystallisation was attributed to substructure formation. This alloy showed poor workability at a strain rate greater than 1 s−1 due to cracking or flow localisation. At lower temperatures (<950°C), the poor workability was associated with shear cracking along 45° orientation with respect to the compression axis due to intensive slip band formation. At higher temperatures (>1000°C), it was attributed to free surface cracking due to severe oxidation of element V and the secondary tensile stresses caused by bulging of the cylindrical specimen during upsetting. The cracking behaviour of Ti–25V–15Cr–0·3Si alloy can be evaluated from the critical strain to fracture ϵf. This strain increased with increasing temperature and decreasing strain rate. It is demonstrated that the critical strain to fracture ϵf can be expressed by a single function, namely, the Zener–Hollomon parameter Z, which combines the effects of both temperature and strain rate. ϵf and InZ obeyed a linear relationship. Based on the experimental results, a processing window, which consisted of a temperature range 950–1050°C and strain rate range 0·01–0·1 s−1, was established for optimising the process parameters and achieving microstructural control during hot working. The processing window has been validated through scaled up cylinder upsetting experiments using 140 mm diameter alloy specimens.

Flow behaviour and microstructural evolution of Ti-17 alloy with lamellar microstructure during hot deformation in α+β phase field

Abstract The effect of process variables on flow response and microstructure evolution during hot working of Ti-17 (Ti–5Al–2Sn–2Zr–4Mo–4Cr) alloy with lamellar microstructure was established using isothermal hot compression tests at strain rates of 0·001–10 s−1, test temperature between 780 and 860°C, and height reductions of 15–75%. All of the flow curves exhibited a peak stress followed by noticeable flow softening, and tended to exhibit a noticeably lower rate of flow softening at the strain of the order of 0·7. The peak flow stress decreased with increasing temperature and decreasing strain rate. Flow softening of the Ti-17 alloy was caused by kinking, break-up and globularisation of lamellas during deformation and, to a small extent, by deformation heating. At high strain rate, adiabatic shear bands and flow localisation were observed to play a role in flow softening. In α+β phase region discontinuous yielding was discovered at higher temperature (⩾820°C) and 10 s−1. According to the deformation microstructure, lamellar α phase gradually kinked, broke up and globularised of lamellas resulting from the development of shear bands and the penetration of β phase during deformation in the α+β phase field. The degree of dynamic globularisation of α phase increased with increasing strains, temperature and decreasing strain rate.

Effect of hydrogen on microstructure evolution and tensile properties of TC21 alloy

Abstract Hydrogen can be used as a temporary element to refine microstructure and improve workability of titanium alloys. In this article, the influence of hydrogen on the microstructure evolution and tensile properties of TC21 alloy is investigated. The microstructure observation reveals that the FCC hydrides δ precipitate firstly at the grainboundaries of primary α phases in TC21 alloy with hydrogen contents above 0·319 wt-%. These hydrides which block the grains deformation reduce both the strength and ductility at room temperature. With increasing hydrogen contents, the elevated temperature strength decreases first and then increases, while the ductility behaves the contrary. These are resulted from the interaction of hydrogen induced softening and hardening.

Effect of aging temperature on microstructure and mechanical properties of different rolling orientations in novel Al–Li alloy

Abstract Microstructure and properties of different rolling orientation with aging temperature varying in a newly developed Al–Li alloy was investigated by transmission electron microscopy (TEM) analysis. Sheets of experimental Al–Li alloy were solution treated at 510°C for 2 h. Predeformation of ∼3% was stretched in the 0 and 90° angle with respect to the original rolling orientation before aging treatment. All specimens were aged for 35 h at 138, 143, 148, 153, 158 and 163°C respectively. Tensile specimens were stretched along the previous orientation of predeformation. The results showed that the main strengthening phases in the alloy were δ′ (Al3Li), θ′ (Al2Cu)/θ″ (Guinier–Preston II zone), δ′/β′ (Al3Zr) and T1 (Al2CuLi) phase. Among the phases above, the strengthening effect of T1 phase surpassed the others. Moreover, very little of cubic σ (Al5Cu6Mg2) phase was observed in the alloy. The microstructure evolution was similar via comparing sheets in different stretched directions. At initial stage of aging, a large amount of fine T1, δ′, δ′/β′ and θ′/θ″ phases uniformly distributed in Al matrix with the aging temperature increasing. Subsequently, the quantity and size of T1 phase increased gradually. In the middle and later period, T1 phase was the main phase that grew at the cost of other phases. The peak aged temperature was 158°C with yield strength of 540 MPa when stretched at 0° angle to the original rolling orientation and 501 MPa when at 90° angle to the rolling direction. Meanwhile, the elongations were 9·5 and 10·9% respectively, which indicated that the anisotropy of the alloy was not obvious.