Microstructures And Mechanical Behavior Of Ni-Cr And Ni-Mo Alloys

Ni-based alloys are widely used in various fields including aerospace,naval,petroleum and nuclear engineering.The enhancement of mechanical properties is beneficial for improving the load-bearing and temperature-bearing capabilities of key components,thereby enhancing the overall strength of equipment.Nickel based alloys contain various alloying elements,among which Cr and Mo are the main elements.Alloying elements affect the mechanical properties of nickel-based alloys by affecting the single-phase face-centered cubic（FCC）matrix and nanoscaleγ’/γ’’phases.The focus in current study is the effect of alloying elements on the precipitates and lattice mismatch between the precipitates and the matrix in nickel based alloys.However,there is still a lack of study on matrix strengthening.Therefore,in order to provide a robust theoretical basis and guidance for designing a matrix with excellent strength-ductility combinations,this paper investigates the effect of Cr and Mo elements on the microstructure,mechani cal properties,and deformation mechanism of pure Ni.The main conclusions are as follows:（1）The effect of Cr concentration on the SFE,mechanical behavior and deformation mechanism of Ni-Cr alloys has been investigated.The SFE of pure Ni determined by the experiment is～104 m J/m².The SFE of the Ni-10Cr alloy decreases dramatically to～40 m J/m²;increasing Cr concentration to beyond 10 at.%leads to only slight further reduction,with that of Ni-20Cr,Ni-30Cr,and Ni-40Cr alloys are～31.8,21.2,and 14.6 m J/m²,respectively.As the Cr concentration increases from 0 to40 at.%,the lattice friction stress increases monotonically from 37 MPa to 146 MPa.The strength and ductility are simultaneously improved with increasing Cr concentration,attributing to the change in deformation mechanism from wavy dislocation slip into planar dislocation slip and even twinning.（2）The cold rolling deformation,recovery,recrystallization and grain growth behavior of Ni-Mo alloys have been investigated.With increasing Mo concentration,the hardness increment increases significantly after cold rolling,and this is attributed to the transformation of deformation mechanism.Mo produces a significant solute-drag effect resulting in a higher activation energy for grain grow th.Annealing induced abnormal hardening exists in all cold-rolled Ni-Mo alloys during the recovery annealing.It was found that the governing annealing hardening switches as the concentration of Mo reaches a critical level of 5～7.5 at.%.Below the critica l level,the maximum of annealing induced hardening decreases as Mo concentration increases and the exhaustion of mobile dislocations contributes primarily.Beyond the level,trend reversal occurs and two alternative outweighing mechanisms,including the occurrence of segregation of Mo atoms into stacking faults and formation of atomically ordered structures with different scale in spatial extent,consecutively become predominant.（3）The effect of Mo concentration on the mechanical properties and deformati on mechanism of Ni-Mo alloys have been investigated.With increasing the Mo concentration,the strength-ductility combinations are enhanced collectively.The reason for this is that Mo is much more efficient in generating larger lattice friction stress,stronger grain boundary strengthening effect,and lower stacking fault energy of Ni-Mo alloys.The Ni-20Mo alloys exhibit an excellent combination of strength and ductility surpasses most previously reported alloys because of high lattice friction force（234 MPa）,extremely high Hall-Petch coefficient(1034 MPa·μm^1/2)and excellent strain hardening ability.It is worth mentioning that the fine-grained Ni-20Mo alloy（grain size of 1.7μm）reaches a yield strength of nearly 1047 MPa,ultimate tensile strength of 1301 MPa and total elongation of 37%.The strain hardening ability of the Ni-Mo alloys increases significantly with increasing the Mo contention,and this is mainly due to the for mation of planar slip bands（PSB）,stacking faults（SFs）,and deformation twins（DTs）.There is a tight positive linear correlation between the Hall-Petch coefficient and the volume misfit parameter.The plastic flows in Ni-Mo alloys is dominated by screw dislocation.This finding is surprising because plastic flows in FCC alloys are generally controlled by edge dislocations.