Biaxial deformation of Ti-6Al-4V and Ti-6Al-4V/TiC composites by transformation-mismatch superplasticity

Gas-pressure bulge forming of unreinforced Ti-6Al-4V and TiC-reinforced Ti-6Al-4V was performed while cycling the temperature around the allotropic transformation range of the alloy (880–1020 °C). The resulting domes exhibited very large strains to fracture without cavitation, demonstrating for the first time the use of transformation-mismatch superplasticity under a biaxial state of stress for both an alloy and a composite. Furthermore, much faster deformation rates were observed upon thermal cycling than for control experiments performed under the same gas pressure at a constant temperature of 1000°C, indicating that efficient superplastic forming of complex shapes can be achieved by transformation-mismatch superplasticity, especially for composites which are difficult to shape with other techniques. However, the deformation rate of the cycled composite was lower than for the alloy, most probably because the composite exhibits lower primary and secondary isothermal creep rates. For both cycled materials, the spatial distribution of principal strains is similar to that observed in domes deformed by isothermal microstructural superplasticity and the forming times can be predicted with existing models for materials with uniaxial strain rate sensitivity of unity. Thus, biaxial transformation-mismatch superplasticity can be modeled within the well-known frame of biaxial microstructural superplasticity, which allows accurate predictions of forming time and strain spatial distribution once the uniaxial constitutive equation of the material is known.     

Adjustment of Aging Temperature for Reaching Superelasticity in Highly Ni-Rich Ti-51.5Ni NiTi Shape Memory Alloy

In this research, the effects of aging temperature on the phase transformation, mechanical properties, and superelasticity of Ti–51.5 at.% Ni alloy were investigated. For this purpose, samples were solution annealed and then aged at different temperatures ranging from 300 to 700°C. The results showed that the change of aging temperature has a great influence on the behavior of the alloy. Aging at 300 and 400°C led to the occurrence of the austenite to R phase (A ? R) transformation. By aging at these temperatures, the ultimate tensile strength and elongation did not change significantly compared to the solution annealed sample. Aging at 500 and 600°C led to the appearance of B19′ phase during cooling cycle of differential scanning calorimetry tests. Samples aged at 500 and 600°C showed the best mechanical properties, in comparison with the other aging temperatures. Maximum tensile strength of 1250 MPa and the elongation of 25% were obtained by aging at 500 and 600°C, respectively. Solution-annealed sample showed no superelastic property. Complete superelasticity was observed by aging at 400 and 500°C. The sample aged at 700°C showed the transformation behavior and mechanical properties similar to the solution annealed sample.     

Deformability enhancement in ultra-fine grained, Ar-contained W compacts by TiC additions up to 1.1%

Nano-sized Ar bubbles give negative influence on the fracture resistance and occurrence of superplasticity in ultra-fine grained (UFG) W–TiC compacts. In order to enhance deformability in UFG, Ar-contained W–TiC compacts, effects of TiC addition on the high-temperature deformation behavior were examined. W–TiC compacts with TiC additions of 0, 0.25, 0.5, 0.8 and 1.1 wt% were fabricated by mechanical alloying in a purified Ar atmosphere and hot isostatic pressing. Tensile tests were conducted at 1673–1973 K (0.45–0.54 T_m, T_m: melting point of W) at initial strain rates from 5 × 10⁻⁵ to 5 × 10⁻³ s⁻¹. It is found that as TiC addition increases, the elongation to fracture significantly increases, e.g., from 3 to 7% for W–0 and 0.25TiC/Ar to above 160% for W–1.1TiC/Ar when tested at 1873 and 1973 K at 5 × 10⁻⁴ s⁻¹. The flow stress takes a peak at 0.25%TiC and decreases to a nearly constant level at 0.5–1.1%TiC. The ranges of the strain rate sensitivity of flow stress, m, and the activation energy for deformation, Q, with TiC additions are 0.17–0.30 and 310–600 kJ/mol, respectively. The observed effects of the TiC additions on the tensile properties are discussed.     

Beneficial effects of nitrogen on austenite antibacterial stainless steels

Nitrogen is a significant alloying element in austenite stainless steels. The aim of this paper is to evaluate the effects of nitrogen on the microstructure and properties of austenite antibacterial stainless steels. Two austenite antibacterial stainless steels containing copper and different nitrogen concentration (0.02 and 0.08 wt%, respectively) were fabricated. The microstructures and composition analysis were carried out using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and Auger electron spectroscopy (AES). The epsilon copper-rich precipitates are spherical and less than 20 nm in size, with a cube-on-cube orientation relationship with the matrix. They are dispersed on the steel surface with a mean space of about 200 nm. Nitrogen cannot only improve the antibacterial property but enhance significantly the corrosion resistance in chloride media. Nitrogen compensates the harmful effect of epsilon copper precipitates on the corrosion resistance. The nitrogen concentration in the surface of N-2 steel is four times as much as in the surface of N-1 steel. Nitrogen enrichment in the steel surface improves the corrosion resistance. The presence of higher nitrogen increases the strength and decreases the ductility of austenite antibacterial stainless steel, which could be related to the variation of stacking-fault energy associated with nitrogen concentration.     

A study on hot deformation behavior of Ni-42.5Ti-7.5Cu alloy

To investigate the hot deformation behavior of the Ni-42.5Ti-7.5Cu (wt%) alloy, hot compression tests were carried out at the temperatures from 800 °C to 1000 °C and at the strain rates of 0.001 s⁻¹ to 1 s⁻¹. The results show that the occurrence of dynamic recrystallization (DRX) is the dominate restoration mechanism during the hot deformation of this alloy. There is an increase in peak and steady state stresses with decreasing the deformation temperature and increasing the strain rate. The experimental results were then used to determine the constants of developed constitutive equations. There is a good agreement between the measured and predicted results indicating a high accuracy of developed model. Zener–Hollomon (Z) parameter, calculated based on the developed model, indicates that DRX was postponed when the logarithm of the Zener–Hollomon parameter fell around 33 at strain rate of 0.001 s⁻¹ and temperature of 900 °C. This phenomenon can be regarded as the interactions between solute atoms and mobile dislocations. The established constitutive equations can be used to predict and analyze the hot deformation behavior of Ni-42.5Ti-7.5Cu alloy.     

Ab initio study of stacking faults and deformation mechanism in C15 Laves phases Cr2X (X = Nb,Zr, Hf)

Based on the synchroshear model, the formation of stacking fault and twinning fault in C15 Laves phases is modeled, then the generalized stacking fault energy curves and deformation mechanism in C15 Laves phases Cr₂X (X = Nb, Zr, Hf) alloys are investigated by ab initio calculations based on the density functional theory. The results demonstrate that the unstable stacking fault and twinning fault energies of C15 Laves phases Cr₂X (X = Nb, Zr, Hf) by the synchroshear are still large while the stable stacking fault and twinning fault energies are low, and the deformation modes by extended partial dislocation and twining are feasible in C15 Laves phases Cr₂X (X = Nb, Zr, Hf). Moreover, the Cr₂Nb has the largest deformation twinning tendency, followed by Cr₂Zr and Cr₂Hf. The evolution of electronic structure during the synchroshear process is further studied to unveil the intrinsic mechanism for the formation of stacking fault and twinning fault in C15 Laves phases Cr₂X (X = Nb, Zr, Hf).