Grain Boundary Migration In Nanograined Metals And The Influence Of Alloying On Stability

The incorporation of high-density interfaces in nanostructured metallic materials may greatly improve their mechanical properties,such as strength,hardness,fatigue and tribological properties.However,interfaces at nano-scale also provide a strong driving force for grain coarsening,resulting in poor structural stability of nanograined metals compared with their coarse-grained counterparts.Simulations and experiments show that grain boundaries(GBs)in nanograined metals tend to migrate by applying stress or strain at or even below room temperature,which is categorized as mechanically induced GB migration(GBM).The mechanically induced GBM and resulted grain coarsening and softening hinder the technical application of nanocrystalline metals and make it rather hard to refine grains further through plastic deformation.Moreover,grain growth and related softening in nanograined metals is also easy to occur when subjected to thermal stimuli,limiting their technological applications at elevated temperatures.In this work,the intrinsic mechanism of mechanically induced GBM and its relation with dislocation slips are addressed by investigating the orientation dependence of GBM in gradient nanograined Cu.Then,nano-lamellar structures with an enhanced stability are fabricated in pure Cu by modifying the processing parameters.In addition,the effects of alloying on the deformation mechanism and GB relaxation(GBR)behavior are studied,and thermal stability of these nickel alloys is also examined.The main results are as follows:1.Gradient nanograined copper with its average grain size of 60 nm in the topmost layer is prepared by using surface mechanical grinding treatment(SMGT).After uniaxial quasi-static tensile test to 10%uniform strain,obvious grain coarsening can be detected in the surface layer of the sample.The average grain size of the topmost surface increases from 60 nm ± 18 nm in the as-prepared samples to about 103 nm ±32 nm in the as-tensioned ones,indicating that mechanically induced GBM occurs during tensile deformation.Microhardness of nanograins on the topmost surface decreases from 1.8 GPa to 1.56 GPa after tension,exhibiting a softening behavior.EBSD experiments show that the texture changes from weak<111>? TD in the as-prepared samples to<001>? TD.Further analysis shows that GBM is more distinct in nanograins with greater Schmid factors where dislocations are easier to slip,indicating that dislocation motion plays a key role in mechanically induced GBM.As a result,the fraction of low angle GBs between 2° and 15° increased from 8.5%in the as-prepared sample to 16.6%in the as-tensioned sample.2.Based on the understanding of the dislocation activities during mechanically induced GBM,processing parameters of SMGT are modified to suppress the partial dislocation activities in the pure Cu so that deformation process is dominated by full dislocation motions(or mechanically induced GBM).On condition that geometrically necessary dislocations are further promoted and accumulated,nano-laminated(NL)structures can be formed in Cu with medium to low stacking fault energies.The NL structures with an average boundary spacing of 67 nm ± 25 nm show a rather strong{111}<112>shear texture.The microhardness of the surface layer in the NL sample reaches a high level of 2.1 GPa,17%higher than the nanograined(NG)sample with almost the same grain size.The fraction of low angle GBs in the NL sample(30%)is much higher than that of the NG sample(8.5%),while there are only 4.6%of twin boundaries in the NL sample.Transmission electron microscope analysis shows that the straight and sharp interfaces between low angle lamellar are regularly arranged dislocations,which indicates that the lamellar structure is indeed produced by the activity of full dislocations.In addition,the NL structures remain stable even after annealed at 593K(?0.44 Tm,Tm for melting point),while equiaxed grains of about 70 nm in size showed the worst thermal stability and grain coarsening occurs when annealed at temperature lower than 0.3 Tm.NL structures deviate from the typical strength-stability trade-off,showing a combination of high hardness and enhanced thermal stability.3.By using SMGT at liquid nitrogen temperature,extremely fine nanograins with average grain sizes of 8 nm,7.9 nm,16.7 nm,and 14 nm are fabricated in the surface layer of Ni,Ni-5%at.Re,Ni-5%at.Co,and Ni-5%at.Ti,respectively.HCP grains constitute about 5?10%in volume can be detected in the surface layer of pure Ni and Ni-Re,while less than 5%of HCP grains are also detected in Ni-Ti.FCC-HCP transformation is thought to take place through the formation of multiple stacking faults and triggered as an additional mechanism for accommodating strains when most of dislocation activities and GB-mediated processes are inhibited for nanograins below a certain size.In addition,high density of deformation twins and/or stacking faults are presented in Ni,Ni-Re and Ni-Ti.However,no HCP grains can be detected and less twins and stacking faults are involved in Ni-Co sample.It shows that the addition of Re has little effect on the GBR process which is thought to take place via activation of partial dislocations,and the addition of Ti inhibits the GBR process and formation of HCP grains to a certain extent,while the addition of Co significantly inhibits GBR during the deformation process.The annealing experiments show that the coarsening temperature of nanograins in the surface layer of Ni-Re is about 873 K?1073 K,similar with that in pure Ni where GBR is induced.The surface layer of Ni-Co sample,where GBR are significantly inhibited,exhibits the worst thermal stability among these alloys and the apparent grain coarsening temperature is as low as 773 K.The nanograins in the surface layer of Ni-Ti sample start to coarsen at 873 K,while the precipitation of ?-Ti slows down the grain growth process and there are still some grains at sub-micron scale after annealing at 1073 K.