The Mechanical Properties And Microscopic Deformation Mechanisms Of Cu-Ge Alloys Produced By Severe Plastic Deformation

In this work,pure Cu,Cu-0.1Ge,Cu-5.7Ge and Cu-9.0Ge alloys with grain sizes of nanometer and submicrometer were produced by different processing methods,such as forging at the liquid nitrogen temperature,rolling at room temperature,rolling at the liquid nitrogen temperatue,high pressure torsion(HPT)and Split Hopkinson Pressure Bar(SHPB)deformation.Cu and Cu alloys produced by severe rolling were subsequently annealed at low temperature.The microstructures of Cu and Cu-Ge alloys were investigated by X-ray diffraction(XRD)and transmission electron microscope(TEM),and grain size,dislocation density,twin density,microstrain were calculated.The principal methods to investigate the mechanical properties were microhardness and tensile testing at room temperature.The effects of deformation temperature,strain rate,the amount of strain,annealing treatment and stacking fault energy(SFE)on the microstructures and mechanical properties of Cu alloys have been systematically studied.Some results are listed as following:It is demonstrated that the SFE of Cu-Ge alloys decreases with increasing the content of Ge.In order to investigate interrelation of microstructures,mechanical properties and SFE,Cu and Cu-Ge alloys were produced by different plastic deformation methods,such as forging at the liquid nitrogen temperature,rolling at room temperature(RTR),rolling at the liquid nitrogen temperature(LNR),high pressure torsion(HPT+RTR)and Split Hopkinson Pressure Bar deformation(HK+RTR).It can be observed in XRD analysis that a decrease in SFE results in an increase in dislocation density,twin density and microstrain,and a decrease in grain size.The microscopic deformation mechanism transforms from dislocation slipping to deformation twinning,leading to higher twin density.Decreasing SFE makes it difficult for the dislocations to cross-slip and climb,leading to the suppression of dynamic recovery,and thus an increase in the amount of dislocation.Furthermore,deformation twins can serve as locations for dislocation accumulation,resulting in an increase in the dislocation density.With reducing SFE,the grain refinement mechanism of Cu-Ge alloys gradually changes from the dislocation subdivision to twin fragmentation.The twin fragmentation is more effective in microstructure refinement,leading to finer grain size.It is shown from tensile tests that strength and ductility can be improved simultaneously with decreasing SFE,Solution hardening is not the main reason for an increase in strength of materials with low SFE.The increase in strength with decreasing SFE originates from smaller grain sizes and higher defect(dislocation and twin)density.The coherent twin boundaries act as not only the barrier of dislocation movement but also the location of dislocation storage.Furthermore,extensive intersections among profuse dislocations,high-density twins and wide stacking faults lead to the high work hardening rate,thereby improving the ductility.The effect of deformation temperature on the microstructures and mechanical properties can be investigated by comparison of Cu-0.1Ge and Cu-5.7Ge alloys produced by RTR and LNR.It is known that decreasing temperature usually leads to suppression of dynamic recovery and facilitating deformation twinning.It can be seen from XRD analysis that a decrease in deformation temperature results in an increase in dislocation density,twin density and microstrain,and a decrease in grain size.The improvement in strength and ductility of Cu-Ge alloys can be achieved by reducing deformation temperature,which is more obvious for Cu-0.1Ge alloy than Cu-5.7Ge alloy.This may be due to that the influence of deformation temperature on the deformation mechanisms and microstructures varies with SFE.The Cu-0.1Ge with relatively high SFE is more sensitive to the deformation temperature.The effect of strain rate on the microstructures and mechanical properties can be investigated by comparison of Cu-Ge alloys produced by RTR and HK+RTR.The strain rate of RTR and HK+RTR is 5s-1 and 104s-1,respectively.For the Cu-9.0Ge and Cu-5.7Ge alloys,increasing strain rate leads to more twins and dislocations and finer grain sizes,resulting in the improvement of the work hardening rate.Thus,the strength and uniform elongation can be enhanced.For Cu-0.1Ge alloy,the strength and uniform elongation decreasing with increasing strain rate.The effect of the amount of strain on the microstructures and mechanical properties can be investigated by comparison of Cu-Ge alloys produced by RTR and HPT+RTR.The total strain of RTR and HPT+RTR is 3.5 and 10,respectively.The ductility of Cu and Cu-Ge alloys can be improved by increasing the amount of strain.For pure Cu and Cu-0.1Ge with medium SFEs,the strength of the samples increases initially and then decreases slightly.In contrast,for Cu-5.7Ge and Cu-9.0Ge alloys with low SFEs,the strength increases with increasing strain.It is shown that the effect of plastic strain on the strength varied with the amount of SFE because of different dynamic recovery rates.The annealing treatment was applied to the RTR Cu and Cu-Ge alloys at temperatures ranging from 100 to 250 ? for 1 h with an interval of 50 ?.For the Cu-9.0Ge alloy,the tensile strength increases and the ductility decreases after annealing in the temperature range of 100-200 ?,and thus this alloy exhibits unusual anneal hardening behavior.Meanwhile,Cu-9.0Ge alloy exhibits anneal softening after annealing at 250 ?.For Cu-5.7Ge alloy and Cu-0.1Ge alloy,the strength does not change basically after annealing at 100-200 ? and decreases after annealing at 250?.For pure Cu,annealing at 150,200 and 250 ? leads to a decrease in strength and an increase in ductility,exhibiting anneal softening.In order to confirm the anneal hardening behavior,three solid solution alloys of Cu-Al-Zn were rolled at the liquid nitrogen temperature,and then annealed at 200? for 1 h.The anneal hardening behavior can be observed in the two alloys with high solution concentration,and thus low SFE.However,the alloy with high SFE exhibits anneal softening.The anneal hardening can be attributed to anchoring of dislocation by solute segregation and the presence of profuse twins and stacking faults after annealing.