Investigations On Compression And Tension-Tension Fatigue Behaviors Of Coarse-grained Cu-Ni Alloys With High Stacking Fault Energies

As one of the most important plastic deformation mode of crystals,dislocation slip has been given extensive attention.Stacking fault energy(SFE)has been all along considered as the most important factor determining the slip mode in face-centered cubic(FCC)crystals.Differing from most of conventionally investigated Cu alloys like Cu-Zn and Cu-Al alloys,the SFE value of Cu-Ni alloy rises with increasing alloy content,while the degree of short-range clustering(SRC)simultaneously increases.Up to now,investigations on the mechanical behavior and deformation microstructures of Cu-Ni alloys were rarely reported.Particularly,the joint influence of SFE and SRC on the deformation microstructure of the alloy is still lack of systematic understanding.Accordingly,in the present work,compression tests and stress amplitude controlled tension-tension fatigue tests have been conducted on coarse-grained Cu-Ni alloys with different Ni contents.Microstructures and plastic deformation features were systematically observed by transmission electron microscopy(TEM)and scanning electron microscopy(SEM).With increasing Ni content,the compressive yield strength and flow stress of Cu-Ni alloys increase dramatically,resulting from the enhanced alloying strengthening effect,the higher dislocation density and the evolvement of dislocation structures.When the Ni content is low(5 at.%),the characteristic deformation microstructures at a larger strain amount(30%of the engineering strain)are composed of loosely and randomly arranged dislocation cells.When the Ni content increases to 10 at.%,extended cells,which are closely arranged in parallel,dominate the microstructures.In contrast,for the Cu-20at.%Ni alloy,the dislocation structures have been transformed into uniformly distributed cell blocks,which improves the storage capacity of dislocations and the ability to accommodate stress or strain.The work hardening rate curve of Cu-Ni alloys exhibits three different stages of A,B and C,where there exists a clear phenomenon of recovery of work-hardening rate(WSHR)in Stage B.With increasing Ni content from 5 to 20 at.%,The shape of Stage B transforms from a "plateau" to a“bump”,which is in line with dislocation structure transformation(from tangles to typical planar slip bands).This phenomenon strongly demonstrates that SRC can promote planar slip to a great extent,and that a low value of SFE is not necessary for the formation of planar slip structures.During the further deformation,SRC becomes much easier to be damaged by dislocation movements,losing the plane softening ability,and finally resulting in the absence of planar slip structures in Cu-Ni alloys at large strains.Fatigue testing results of Cu-Ni alloys indicate that the curves of fatigue life Nf vs.stress amplitude of Cu-5at.%Ni,Cu-10at.%Ni and Cu20at.%Ni alloys all basically meet the Basquin linear relationship.With increasing Ni content,the Nf and fatigue strength component b of Cu-Ni alloys are significantly improved,resulting from the enhancement of solid solution strengthening and the contribution of SRC to the sliding mode(i.e.planar slip)of dislocations.With increase of Ni content and decrease of stress amplitude,the fatigue crack source and propagation areas on the fracture surface are increasing,while the number and size of secondary cracks increase,and at the same time,the cracking modes on the specimen surface also become varied,weakening the stress or strain concentration and thus enhancing the plastic deformation homogeneity.Furthermore,the fatigue dislocation configurations of Cu-Ni alloys are dominated by dislocation cells,showing that wavy slip acts as the main plastic deformation mode of Cu-Ni alloys.With increase of Ni content,namely increases of SFE and SRC,dislocation configurations are transformed from typical dislocation cells to cells/ill-developed cells/ill-developed slip bands(??/2=65 MPa)and further to cells/ill-developed cells/stacking faults(??/2=85 MPa).Such a slip mode transformation from wavy slip dominating into the combination of wavy slip dominating with a tendency to planar slip indicates that short range structure like SRC is functional enough to promote planar slip in materials with high SFEs.