Papers by Author: Li Jia Chen

Abstract: In this study, equilibrium lattice parameters, heat of formation and cohesive energy of four kinds of typical phases with different structure intermetallic compound in Al-Cu-Mg alloy were investigated by first-principles calculations based on density functional theory via CASTEP software. The calculation results are analyzed and show that ternary strengthening phase Al₂CuMg generated first when Mg content is higher, while binary strengthening phase Al₂Cu or Al₃Cu₂ first generated and more stable when Mg content is low in Al-Cu-Mg alloy which indicates that element Cu and Al alloying capacity significantly higher than that of Mg and Al element.

Abstract: High temperature low cycle fatigue properties and fracture behavior of Inconel 625 nickel-base superalloy welding joint at 760^oC were investigated under fully reversed total strain-controlled mode. The fatigue life and cyclic stress-strain data were analyzed to determine the individual strain fatigue parameters. It is noted that the welding joint exhibits the cyclic strain hardening and stability. The fatigue cracks initiate predominantly on the free surface of fatigue specimens and propagate in an intergranular mode or a mixed transgranular and intergranular mode.

Abstract: Serrated flow has been observed in AZ81 alloy during tensile deformation. The observed static strain ageing effect and negative strain rate sensitivity suggest that the serrated flow is due to interaction between dislocations and solute atoms, know as dynamic strain ageing (DSA). The Portevin-Le Chatelier effect is observed at temperatures between 150oC~200oC and 125oC~200oC. In the microstructure of deformed samples dislocations and twins is observed. It is suggested that the occurrence of the dynamic strain aging is associated with interactions between solute atoms and dislocations.

Abstract: The low-cycle fatigue behaviors of permanent-mold cast and die-cast Al-Si-Cu-Mg-Sc alloys at room temperature were investigated. The results show that at the higher total strain amplitudes, both permanent-mould cast and die-cast Al-Si-Cu-Mg-Sc alloys exhibit the cyclic strain hardening. However, at the lowest total strain amplitude, the cyclic strain hardening occurs in the initial and middle stages of fatigue deformation and the stable cyclic stress response is noted in the later stage of fatigue deformation for the permanent-mould cast alloy, while the cyclic strain hardening phenomenon is observed during whole fatigue deformation for the die-cast alloy. At the same total strain amplitude, the die-cast alloy shows the higher cyclic deformation resistance than the permanent-mold cast alloy. Compared with the permanent-mold cast alloy, the die-cast alloy possesses the longer fatigue life at the lower total strain amplitudes. The relationship between both elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior for both permanent-mold cast and die-cast Al-Si-Cu-Mg-Sc alloys

Abstract: Abstract: The fatigue behavior of cast steel ZG20SiMn under strain control mode was investigated, and the influence of imposed total strain amplitude on the cyclic deformation and fracture behavior of the cast steel ZG20SiMn was determined. The experimental results reveal that during fatigue deformation, the cast steel ZG20SiMn may exhibit the cyclic strain hardening and stable cyclic stress response behavior, which mainly depends on the imposed total strain amplitude. The relationship between plastic and elastic strain amplitudes with reversals to failure shows a single linear behavior, and can be well described by the Coffin-Manson and Basquin equations, respectively. At different total strain amplitudes, the fatigue cracks initiate at the free surface of fatigue specimens and propagate for the cast steel ZG20SiMn.

Abstract: Equal channel angular pressing (ECAP) was conducted using the die with a 90° angled channel under routes A, B_C and C for hot extruded AZ91 magnesium alloy. Tensile tests were carried out at 300 °C and initial strain rates ranging from 2×10⁻⁴ to 5×10⁻³ s⁻¹. The experimental results show that different routes have obviously effect on elongation to failure. It is found that the highest elongation to failure is 410 % at a strain rate of 2×10⁻⁴ s⁻¹ for the ECAPed AZ91 alloy with route B_C. At the same strain rate, route B_C can bring the greater superplasticitic deformation compared with routes A and C. Moreover, the strain rate sensitivity coefficient m values of about 0.3 to 0.5 are attained for the ECAPed AZ91 alloys with different routes. For the ECAPed AZ91 alloys, the main superplastic deformation mechanism is the grain boundary sliding, while the main accommodation mechanism is the dislocation creep mechanism controlled by the grain boundary diffusion.

Abstract: Cyclic deformation for two magnesium alloys AZ91 and AM50 with different processing status has been performed under total strain amplitude control mode and at room temperature. A serrated flow can be observed in both tensile and compressive directions of the stress-strain hysteresis loop for as-extruded AZ91 and AM50 magnesium alloys. It means that the so-called dynamic strain aging occurs during cyclic deformation. In addition, the dynamic strain aging phenomenon can also be observed in two extruded magnesium alloys subjected to aging treatment as well as the AZ91 alloy subjected to solution treatment. However, the dynamic strain aging seems not to take place in the extruded AM50 alloy subjected to solution treatment because there exists no significant serrated flow behavior in either compressive or tensile direction of the stress-strain hysteresis loop. It is suggested that the occurrence of the dynamic strain aging is associated with collective behavior of many mobile dislocations as well as interactions between solute atoms and dislocations.

Abstract: In order to identify the influence of solid solution, aging and solid solution plus aging treatments on the low-cycle fatigue behavior of the extruded AZ61 magnesium alloy, the low-cycle fatigue tests were performed at room temperature for the extruded AZ61 magnesium alloy with different treating states. The results indicate that the cyclic stress response behavior of the extruded AZ61 magnesium alloy exhibits both cyclic strain hardening and stability. The solid solution, aging and solid solution plus aging treatments tend to decrease the cyclic deformation resistance of the extruded AZ61 alloy in most conditions. The solid solution treatment can enhance the fatigue lives of the extruded AZ61 alloy at medium total strain amplitudes. In addition, the aging treatment can prolong the low-cycle fatigue lives of the AZ61 alloy at most total strain amplitudes, while the case for the solid solution plus aging treatment is just contrary. For the extruded AZ61 alloy with different treating states, a linear relationship between cyclic stress amplitude and plastic strain amplitude is noted.

Abstract: Totally strain-controlled low cycle fatigue tests with a strain ratio Rε= -1 were carried out on die-cast AZ91HP, AM50HP and AE 42 alloys at a constant strain rate of 2.5×10-3 s-1 and room temperature. The cyclic deformation behavior of the three alloys was investigated through the characteristics of representative hysteresis loops at various total strain amplitudes. Cyclic deformation hardening was observed and the low-cycle fatigue life data were analyzed using the well known Basquin and Coffin-Manson equations. The transition life of the three alloys is considerably low, which can be attributed to the low ductility of these die cast Mg alloys.