Electro-strengthening And Crack-healing Mechanisms In High-strength Alloys

Electropulsing treatment(EPT)is a rapid and non-equilibrium method for metallic materials processing.Thermal effect and athermal effect induced by the interaction between high-density pulsed current and metallic materials lead to the evolutions in microstructure and mechanical properties.Phase transformation,recrystallization,dislocation recovery and defects healing would take place in a short time,as a consequence,microstructural modification and mechanical property recovery and improvement would be realized.Further investigating the mechanisms of EPT effects on metallic materials and developing new methods for microstructure optimization are of significant importance on the theory and application in the field of mechanical property improvement.In this thesis,high-strength metallic materials including 40CrNiMoA steel and 18Ni maraging steel were chosen for the investigation of EPT effects on the solid-solution,precipitation and damage healing behaviors.In addition,cold-rolled CoCrFeMnNi high-entropy alloy was chosen to explore the EPT thermal and athermal effects on the recrystallization behavior.Microhardness evolutions and mechanisms in the 40CrNiMoA steel after EPT were investigated.The samples tempered at 200℃ and 500℃ were chosen for EPT and then XRD、SEM、EBSD and TEM technologies were used for trans-dimensional microstructure characterization.The mechanisms of the strengthening and softening phenomena in the 200℃ tempered samples and the strengthening phenomenon in the 500℃ tempered samples were explored.The effects of EPT on solid-solution behaviors were analyzed from the perspective of thermodynamics and dynamics.The decrease of phase transformation barrier and the increase of atomic diffusion induced by EPT contribute to the rapid phase transformation,and then arouse hardening and softening phenomena in the 40CrNiMoA steel.EPT effects on the precipitation behaviors of 18Ni maraging steel were explored.Single pulsed current was applied to the 18Ni maraging steel to arouse strengthening phenomenon.EPT combined with subsequent short aging treatment was used to further verify the strengthening mechanism.Based on the cluster strengthening and precipitate strengthening mechanism,the strengthening phenomena induced by EPT and aging treatment were discussed.The rapid strengthening aroused by the instantaneous EPT process was attributed to the cluster strengthening.Ni3Ti precipitates characterized with different sizes and distributions were obtained in the aging samples which were treated with different EPT voltages before aging treatment,inducing further strengthening in the 18Ni maraging steel.The influence of EPT acting on the recrystallization process of the cold-rolled CoCrFeMnNi high-entropy alloy was studied.CoCrFeMnNi high-entropy alloy with good strength and plasticity was obtained through the combination of the cold rolling and EPT methods.EPT at room temperature(RT-EPT)and at liquid nitrogen temperature(LN-EPT)were developed to investigate the differences of thermal and athermal effects on the recrystallization behavior at different external temperatures.The microstructure and mechanical property evolutions were observed,and the effects of liquid nitrogen temperature on the Joule heat and electron wind force were theoretically analyzed.The nucleation and growth processes were accelerated by the thermal and athermal effects in EPT,inducing rapid recrystallization.Due to the rapid cooling under LN environment,new grain growth was suppressed and a partially recrystallized CoCrFeMnNi alloy with a high strength was achieved.40CrNiMoA steel with penetrating crack prepared by fatigue test was used to explore the EPT effects on crack healing and fatigue life prolonging.Taking the fatigue crack propagation rate as the measured fatigue property,the crack propagation rate changes before and after EPT were investigated.Through the comparison of crack propagation rates,the observations of crack surfaces and the theoretical analysis of detouring effect and thermal compressive stress,it could be found that local melting on the crack surfaces and the crack deflection induced by thermal compressive stress around the crack tip led to the decrease in effective length and crack propagation rate of the long crack.Therefore,it is suggested that EPT could be a potential method for prolonging fatigue life of metallic materials.