Papers by Author: Guo Liang Xie

Abstract: In this paper, the flow stresses, the constitutive equation, processing map and the critical conditions of dynamic recrystallization (DRX) of the hot forged QCr0.8 alloy are studied by hot compressive test in the 750-900°C temperature and 0.01-10s^-1 strain rate ranges using Gleeble-1500D thermo-mechanical simulator. The compression reduction of thermal compression deformation is 50%. The results show that the thermal deformation temperatures and strain rates have a significant effect on the high temperature deformation behavior of the alloy. The higher the temperature, the smaller the strain rate and the easier the DRX of the alloy is found.The peak stresses of the alloy decreases with the increase of temperature and increases with the increase of the strain rates.The flow stresses during hot deformation can be described by a hyperbolic sine function. The activation energy Q of the thermal compression deformation is determined to be 370.8KJ/mol. The constitutive equation and processing map of the alloy are established. Critical strains of DRX ε_c are studied by the inflection point characteristic of the lnθ-ε curve of the alloy and the corresponding minimum value of the ∂θ (∂θ)/∂ε-ε curve.

Abstract: In this paper, the effect of heat treatment on the microstructure and mechanical properties of hot forging Cu-Ni alloy was studied. Specimens of hot forged Cu-Ni alloy were subjected to first solution treated at 900^oC for 2hrs and then aged at different temperatures for 2hrs. The mechanical properties including tensile performance and impact energy, and the microstructure were measured for specimens before and after heat treatment. The results show that both solution and aging treatment have an influence on the grain growth. After heat treatment, the tensile strength decreases very slightly but the yield strength decreases seriously from 235.96MPa to 136.12MPa, while the elongation increases sharply from 36% to 48%. It was also observed that hardness values of the heat-treated alloys are all lower than that of the hot forged alloy. The measurement of Charpy impact energy with V-type notch was performed at 298K and 77K for different specimens. At both temperatures, the impact energies of the specimens are higher than 200J. The microstructure results show that at both temperatures, the alloys are fractured in a ductile mode.

Abstract: The tensile strength and plastic behavior of the Cr16Ni6 steel were investigated at 298K (298K) and 76K. The yield strength and tensile strength of the alloy increased significantly with lower temperatures, from 990 MPa to 1350 MPa and from 1313 MPa to over 1800 MPa, respectively. Young’s modulus and impact behavior were found when testing temperature decreased. It was revealed from the microstructure observation conducted by scanning electron microscopy (SEM) and x-ray diffraction technique (XRD) analysis that the face-centered-cubic (FCC) structure of the matrix remained stable with a very small amount of tangent phase transformation to martensitie during the tensile tests at 76 K. The matrix grains deformed at 298K were cut apart into many small sub-grains with similar crystallographic orientation in the size of only 60-200 nm wide. Very small amount of dislocation cells or bands observed in the specimens deformed at 76 K, although many sub-grains and areas containing dislocation network were found. The formation of twin-structures along the 1/3(420) planes was found in the deformation areas at 76 K. The improvement of strength was mainly attributed to the fact that it was more difficult for the dislocation slipping along lattice planes at 76 K than at 298K, due to the reduction of the movement and diffusion abilities of atoms. The friction coefficient was found to be a little higher at 93 K than at 298 K due to the bad lubricity of the graphite material at cryogenic temperature.

Abstract: Sheets of Cu-24wt.%Ag alloy were prepared through the process of forging, cold rolling and heat treatment to reveal the evolution of microstructures, mechanical properties and electrical conductivity. The experimental results showed that nanomultilayered structure of Cu and Ag phases arranged alternatively was obtained, with numerous nanoscale Ag precipitate-fibers embedded in Cu matrix. The lamellas in longitudinal section became curved gradually and shear bands appeared when the deformation exceeded 90.79%. With the increase of rolling strain, the average layer thickness and spacing decreased progressively and reached to less than 200 nm as the strain surpassed 96%, resulting in rapid enhancement of the hardness. The heat treatment at 250°C markedly improved electrical conductivity of the alloy, with little decline of the hardness. The anisotropy of the alloy reduced with rising temperature. Local spheroidization occurred when the alloy was heat treated at 300°C. Hardening of this Cu-Ag alloy is predominated by Cu/Ag interface in strain stage of 80%~99%, leaning mainly upon layer thickness and spacing.

Abstract: The influence of solution treatment on microstructure evolution and mechanical behavior of Cu-20Ni-20Mn alloy was investigated by optical microscopy (OM), X-ray diffraction (XRD) and hardness test. The results revealed that both solution temperature and holding time had effect on the grain growth behavior. The grain growth activation energy was determined by grain size of Cu-20Ni-20Mn alloy for different heat treatment temperatures and periods. With increasing temperature of solution treatment, the second phase is gradually dissolved into the Cu-rich matrix, and the lattice parameter of the matrix solution treated at 1173K for 0.5 h was about 3.668 Å. The hardness of the solution-treated alloy was lower than that of hot forging, and the hardness value decreased with the increase of solution temperature, which may be attributed to grain size. The hardening ability, corresponding to the Hall-Petch relationship, decreased linearly with D^-1/2.

Abstract: Inconel 625 superalloy tubes were hot extruded using a horizontal extruder. Optical microscope, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) technique were employed to investigate the extrusion defects mainly caused by the primary MC carbides. The results showed that a lot of surface cracks, internal cracks, grooves and shrinkage cavity defects existed in the outer surface and internal of the Inconel 625 superalloy tube after hot extrusion. However, the inner surface of the tube was smooth and no cracks were observed. In addition to the external factors during hot extrusion, the cracks are mainly caused by the primary MC carbides or large dimension of C particles formed due to a relatively short smelting time and large dimension of C particles during smelting with the decreasing plasticity of the grain boundary. This kind of cracks can be avoided through improving the uniformity of the chemical composition by reducing the size of C particles and extending the diffusion time during the smelting of Inconel 625 superalloy.

Abstract: A Cu-20Ni-20Mn alloy was prepared and the aging characteristic of this alloy was investigated in this paper. The experimental results showed that the strength of solution treated Cu-Ni-Mn alloy was enhanced by more than 500MPa when aged for more than 20h. Much more rapid increase of tensile strength was found in the cold rolling samples during the aging process, and the strength was much higher in the range from 1200-1600MPa. The side peaks found beside (200) X-ray diffraction peaks are believed to be corresponding to the Cu-rich and Mn/Ni-rich phase regions formed by up-hill diffusion. It is also known that this fluctuation of composition is usually described by sinusoidal wave, the wave length of which is estimated to be 10-15nm.

Abstract: The present work gives a systematic study on the high temperature and high strain-rate deformation behaviors of a two-phase α/β Cu-Zn-Sn-Al alloy, by combining the split Hopkinson bar experiments and microstructural investigations. The results show that under high strain-rate, both the dislocation slip and deformation twins within the α phase contribute to the plastic strengthening of Cu-Zn-An-Al alloy, resulting in the strain-rate-hardening effect. As the deformation temperature increases, the shapes of the stress-strain curves are mainly influenced by the temperature-softening effect and the dynamic recrystallization of the α phase. Finally, material constants regarding the strain-rate-hardening and temperature-softening effects are determined, based on the Johnson-Cook constitutive model. The results show that compared with other metallic materials, the present Cu-Zn-Sn-Al alloy has a relatively stronger strain-rate-hardening effect and weaker temperature-softening effect.

Abstract: In the present study, a systematic study on both the high strain-rate tensile and compressive deformation behaviors of a specially-made copper sample have been carried out at different high temperatures, by using the split Hopkinson bar experiments. The Johnson-Cook constitutive model was used to model the high strain-rate responses of the specimen at high temperatures. The results showed that compared with other metallic materials, the specially-made copper sample had a relatively stronger strain-rate-hardening effect and weaker temperature-softening effect. Evolution of the microstructure suggests that under high strain-rate, both the dislocation slip and deformation twins contribute to the plastic strengthening of the copper specimen, resulting in the strain-rate-hardening effect. And the dynamic recrystallization behavior plays an important role during the high strain-rate deformation process at the high temperatures.

Abstract: The pure copper tube with strong axial orientation columnar grain and equiaxed grain respectively were rolled using a three-roller precision tube mill. Optical microscope, hardness testing machine and universal testing machine were used to investigate the microstructure and mechanical properties of the pure copper tube rolled with different deformation degrees. The results show that the pure copper tube with two kinds of microstructure can be directly rolled from the dimension of Φ68.5mm×57.9mm to Φ34.7×30.4mm by 8 passes with accumulated deformation degree nearly 80% and without intermediate annealing. With the increasing deformation degree, the primary columnar grain boundaries of the pure copper tube with columnar grain blurred gradually until completely disappeared. However, the microstructures of pure copper tube with equiaxed grain completely transformed into fibrous microstructures when the deformation degree reaches to 79.11%. The columnar grain is without transverse grain boundaries but with small-angle boundary, which is conducive to axial deformation. It is also found that with the increasing deformation degree, the tensile strength of pure copper tube increased steadily, while the elongation decreased gradually. The elongation of pure copper tube with columnar grains is higher and the microhardness is lower than that of equiaxed grains, indicating that the cold deformation properties of pure copper tube with columnar grain was more excellent.