Hui-zhong Li

Dynamic property evaluation of aluminum alloy 2519A by split Hopkinson pressure bar

Abstract Impact behavior of aluminum alloy 2519A was investigated at strain rates of 600–7 000 s −1 and temperatures of 20–450 °C by a split Hopkinson pressure bar. The results show that the flow stress is dominated by temperature, and it increases with strain rate and decreases with deformation temperature. The serrated flow curves show the dynamic recrystallization occurs. The strain rate sensitivity exponents m determined are 0.066, 0.059 4, 0.059 0 and 0.057 3 at 20, 150, 300 and 450 °C, respectively. Cowper-Symonds constitutive equation expressing the plastic flow behavior was calculated by analysis and regression of the experimental results. The fracture characteristics under the experimental conditions were observed by optical microscopy(OM) and scanning electron microscopy(SEM). It is determined that the tested material fails as a result of adiabatic shearing.

Microstructure of cast γ-TiAl based alloy solidified from β phase region

Abstract The microstructures and phase transformation of Ti-43Al-4Nb alloy in as-cast and heat-treated states were investigated by using optical microscopy, scanning and transmission electron microscopy as well as differential scanning calorimetry. The results show that a fine microstructure of the as-cast alloy can be obtained by solidifying through the β phase. γ grains can nucleate directly from the β phase. The coexistence of β phase and γ phase along primary α grain boundaries contributes to the decrease in the grain size of the as-cast alloy. The phase transformation sequence during solidification of the Ti-43Al-4Nb alloy is suggested as L → L +β→β→α+β→α+β r →α+γ+β r →lamellae(α 2 +γ)+γ+β r . The microstructure of the alloy after heat treatment at 1 250 °C for 16 h exhibits a certain coarsening compared with that of the as-cast state. The remnant β phase can be removed by the heat treatment process due to the diffusion of Nb and the non-equilibrium state of β phase.

Effect of Y content on microstructure and mechanical properties of 2519 aluminum alloy

The effects of Ag on the microstructure and mechanical properties of 2519 aluminum alloy were investigated by means of tensile test, micro-hardness test, transmission electron microscope and scanning electron microscope. The results show that the addition of 0.3% (mass fraction) Ag accelerates 2519 aluminum alloy’s age-hardening, increases its peak hardness and reduces 4 h of peak aged time at 180 °C. The addition of 0.3% (mass fraction) Ag increses the tensile strength at room temperature and elevated temperature. This increment at room temperature and 200 °C is 24 MPa and 78 MPa, respectively. In contrast, the elongation of 2519 aluminum alloy is decreased with Ag addition. The increase of tensile strength of 2519 aluminum alloy with Ag addition is attributed to the high volume fraction of Ω phase.

Hot deformation behavior of extruded AZ80 magnesium alloy

The hot deformation behavior of extruded AZ80 magnesium alloy was studied through hot compression tests performed at temperatures ranging from 250 to 450 °C with strain rates varying from 0.001 to 10 s−1. The flow stress was corrected due to the deformation heating. The Zener-Hollomon parameter (Z parameter) and processing map were established to describe the hot deformation behavior. The results indicate that the applicable deformation should be conducted at the strain rate of 0.1 s−1 and the temperature range of 350–400 °C. Besides, the relationship between the microstructure evolution and Z parameter was also discussed. High temperature and low strain rate result in a low Z parameter, which leads to full dynamic recrystallization (DRX) and large DRX grain size in the microstructure. Considering processing map and microstructure, the hot deformation should be carried out at the temperature of 400 °C and the strain rate of 0.1 s−1.

Flow behavior and processing map of PM Ti−47Al−2Cr−0.2Mo alloy

Abstract The flow stress features of PM Ti–47Al–2Cr–0.2Mo alloy were studied by isothermal compression in the temperature range from 1000 to 1150 °C with strain rates of 0.001–1 s −1 on Gleeble-1500 thermo-simulation machine. The results show that the deformation temperature and strain rate have obvious effects on the flow characteristic, and the flow stress increases with increasing strain rate and decreasing temperature. The processing maps under different deformation conditions were established. The processing maps of this alloy are sensitive to strains. The processing map at the strain of 0.5 exhibits two suitable deformation domains of 1000–1050 °C at 0.001–0.05 s −1 and 1050–1125 °C at 0.01–0.1 s −1 . The optimum parameters for hot working of the alloy are deformation temperature of 1000 °C and strain rate of 0.001 s −1 according to the processing map and microstructure at true strain of 0.5.

Microstructural evolution of 2519-T87 aluminum alloy obliquely impacted by projectile with velocity of 816 m/s

Target made of 2519-T87 aluminum alloy was obliquely impacted by a projectile. Microstructural evolution around the crater was investigated by optical microscopy (OM), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The micro-hardness distribution near the crater after impact was studied. The results indicate that at the entering stage, the amount of adiabatic shear band (ASB) is the most, and the precipitates are as fine as those of the target material; the micro-hardness is higher than that at the other stages. At the stable-running stage, the amount of ASB reduces as the micro-bands increase; the precipitates tend to coarsen, which leads to the decrease of the micro-hardness. At the leaving stage, there is a large amount of micro-bands; the precipitates are refined, and the micro-hardness is higher than that at the stable-running stage. The difference in the micro-hardness of the impact stages is due to work hardening and precipitate coarsening, which is caused by adiabatic temperature rise in the alloy.

Deformability and microstructure transformation of PM TiAl alloy prepared by pseudo-HIP technology

Abstract Microstructures and deformation properties of Ti-46Al-(Cr, Nb, W, B) alloy consolidated by pseudo-HIP technology were investigated. The results show that the pseudo-HIP temperature has a significant effect on microstructures. When the sintering temperature is 1 100 °C, the microstructure of as-pseudo-HIPped alloy is similar to that of the prealloyed powder and the interfaces of these powder particles are still discernible, but a near γ microstructure appears in particles. Increasing the pressing temperature to 1 200 °C develops successfully a homogeneous and fine-grained duplex microstructure. A typically fully lamellar microstructure with residual β phase is developed at 1 300 °C. The compact exhibits excellent deformation properties at elevated temperatures. When the compression temperature is higher than 1 100 °C, high quality products without cracks can be obtained even if the engineering compression strain is up to 0.8 at strain rates of 10 −2 –10 −3 s −1 . It can be established that the mechanical twinning and matrix deformation due to ordinary dislocation slip/climb contribute to the whole hot deformation.

Characterization of hot deformation behavior of AA2014 forging aluminum alloy using processing map

Abstract The hot deformation behavior of AA2014 forging aluminum alloy was investigated by isothermal compression tests at temperatures of 350–480 °C and strain rates of 0.001–1 s −1 on a Gleeble–3180 simulator. The corresponding microstructures of the alloys under different deformation conditions were studied using optical microscopy (OM), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The processing maps were constructed with strains of 0.1, 0.3, 0.5 and 0.7. The results showed that the instability domain was more inclined to occur at strain rates higher than 0.1 s −1 and manifested in the form of local non-uniform deformation. At the strain of 0.7, the processing map showed two stability domains: domain I (350–430°C, 0.005–0.1 s −1 ) and domain II (450–480 °C, 0.001–0.05 s −1 ). The predominant softening mechanisms in both of the two domains were dynamic recovery. Uniform microstructures were obtained in domain I, and an extended recovery occurred in domain II, which would lead to the potential sub-grain boundaries progressively transforming into new high-angle grain boundaries. The optimum hot working parameters for the AA2014 forging aluminum alloy were determined to be 370–420 °C and 0.008–0.08 s −1 .

Evolution of lamellar structure in Ti–47Al–2Nb–2Cr–0.2W alloy sheet

Abstract The Ti–47Al–2Nb–2Cr–0.2W alloy sheets were obtained by hot pack rolling. The as-rolled sheet has an inhomogeneous duplex microstructure composed of elongated gamma grains and lamellar colonies. Heat treatments were conducted on the as-rolled sheets. The results show that the microstructures with different sizes and grain boundary morphologies were developed after different heat treatments. A coarse fully lamellar structure can be refined if the heating time, together with the cooling rate, is appropriately controlled. The grain growth exponent is found to be approximately 0.2, and the activation energy of grain boundary migration of the alloy is around 225 kJ/mol.

Effect of pre-deformation on microstructures and mechanical properties of high purity Al–Cu–Mg alloy

Abstract The effects of pre-deformation following solution treatment on the microstructure and mechanical properties of aged high purity Al–Cu–Mg alloy were studied by tensile test, micro-hardness measurements, transmission electron microscopy and scanning electron microscopy. The micro-hardness measurements indicate that compared with un-deformed samples, the peak hardness is increased and the time to reach peak hardness is reduced with increasing pre-strain. Additionally, a double-peak hardness evolution behavior of cold-rolled (CR) samples was observed during aging. The results of TEM observation show that the number density of S′(Al2CuMg) phase is increased and the size is decreased in CR alloy with increase of pre-strain. The peak hardness and peak strength of the CR alloy are increased because of quantity increasing and refinement of S′ phase and high density dislocation.