Grain size effect on deformation twinning and detwinning

This article systematically overviews the grain size effect on deformation twinning and detwinning in face-centered cubic (fcc) metals. With decreasing grain size, coarse-grained fcc metals become more difficult to deform by twinning, whereas nanocrystalline (nc) fcc metals first become easier to deform by twinning and then become more difficult, exhibiting an optimum grain size for twinning. The transition in twinning behavior from coarse-grained to nc fcc metals is caused by the change in deformation mechanisms. An analytical model based on observed deformation physics in nc metals, i.e., grain boundary emission of dislocations, provides an explanation of the observed optimum grain size for twinning in nc fcc metals. The detwinning process is caused by the interaction between dislocations and twin boundaries. Under a certain deformation condition, there exists a grain size range where the twinning process dominates over the detwinning process to produce the highest density of twins.     

Grain boundary segregation and interdiffusion effects in nickel-copper alloys: an effective means to improve the thermal stability of nanocrystalline nickel

Nanocrystalline (nc) Ni films show pronounced grain growth and suffer from concomitant deterioration of their mechanical and magnetic properties after annealing at relatively low temperatures (T(ANN) ≥ 475 K). This constitutes a drawback for their applicability as coatings or in components of miniaturized devices. This work reveals that the thermal stability of nc Ni is significantly improved by controllably alloying Ni with Cu, by means of electrodeposition, to form a Ni(1-x)Cu(x) solid solution. To tune the composition of such nc alloys, Ni(1-x)Cu(x) films are deposited galvanostatically using an electrolytic bath containing Ni and Cu sulfate salts as electroactive species, saccharine as grain-refining agent, and applying current densities ranging from -10 to -40 mA cm(-2). The enhanced thermal stability is ascribed to segregation of a Cu-rich phase at the Ni(1-x)Cu(x) grain boundaries, which acts as a shielding layer against grain growth. As a result, high values of hardness (in excess of 6 GPa) remain in nc Ni(1-x)Cu(x) for x ≥ 0.3, even after annealing at T(ANN) ≥ 575 K. From a magnetic point of view, Ni(1-x)Cu(x) films possess lower coercivity values than pure nc Ni films, both in the as-prepared and annealed states, thus offering potential advantages for certain soft magnetic applications.     

The deformation and fracture behavior of an electrodeposited nanocrystalline Ni under compression

A bulk and dense nanocrystalline Ni with an average grain size of 19 nm and a thickness of 5.4 mm was fabricated by an electro-deposition technique. The nc Ni had a preferable (2 0 0) growth texture along the depositing direction. Under compression test, the nc Ni exhibited a high strength of 2920 MPa and an accepted good ductility of 16%. A novel fracture character, i.e., the triple-junction shaped micro-cracks with the size varying from few to several tens of micrometers which run through the holistic fracture body of the nc Ni, was observed. The reason for the formation of such cracks is attributed to GB activities, which leads to the formation of nano-sized void, and the subsequent formation of micro-crack.     

Structure of Crystalline Interfaces

The structure of crystalline interfaces, as observed by transmission electron microscopy, is reviewed with emphasis on the similarity of grain and interphase boundaries of the dislocation type. Small-angle grain boundaries and low misfit interphase boundaries between similar crystal structures largely condense their mismatch into arrays of interfacial dislocations having Burgers vectors in common with dislocations located in the bulk crystals. Large-angle grain boundaries near certain misorientations corresponding to good fit between the abutting grains contain dislocations with Burgers vectors which are not found in the bulk crystal. Partially coherent interphase boundaries between quite dissimilar crystals, for example, f.c.c. and b.c.c., may also contain such dislocations. Principally, because of the difficulties involved in the acquisition of interfacial dislocations, dislocation interphase boundaries, in particular, usually do not have the minimum energy structure.     

Compressive deformation of CoZr and (Co,Ni)Zr intermetallic compounds with B2 structure

B2 type (Co,Ni)Zr compounds which were prepared by arc-melting were deformed in compression at temperatures from liquid nitrogen temperature to 973 K. Their flow stress was anomalously dependent on the testing temperature, decreasing with increasing temperature up to room temperature and then increasing with temperature up to about 673 K, followed by a decrease. The peak of the flow stress was higher for PE specimens than for PA ones which were machined perpendicular and parallel to the direction of grain growth of the ingot, respectively. It is considered that this behaviour of the flow stress is caused, not by the phase transition but by the motion of superlattice dislocations. The ductility of CoZr was lowered by cracking at grain boundaries at which secondary phases were observed. The substitution of nickel for cobalt suppressed the grain boundary cracking and (Co,Ni)Zr had a higher ductility than CoZr.     

Mechanistic model for grain size and temperature dependence of flow stress in 316L austenitic stainless steel

Abstract

During plastic deformation of a polycrystalline material, both the grain interior and the grain boundary regions exhibit distinctly different dislocation behaviours at a given strain and temperature. Studying the variation of experimental flow stress with temperature, it seems that the flow stress of a fine grained polycrystalline material is mainly controlled by dislocation dynamics at and in the vicinity of grain boundaries. At low temperatures in a polycrystalline material, the dislocations are piled up at grain boundaries and the density of dislocations increases significantly in the grain boundary region, while at high temperatures the annihilation of dislocations take place at and in the vicinity of the grain boundaries during deformation. Therefore, the flow stress behaviour of a polycrystalline material can be understood in terms of the process of accumulation and annihilation of dislocations at and in the vicinity of grain boundaries at a given strain and temperature.     

Observation on Emitting Dislocation from Grain Boundary by TEM

The observation on emitting dislocations from grain boundaries by TEM during Cu elongation has been pefformed. It is shown that there exists the "ledge" at the grain boundaries in fcc pure Cu, which is able to emit dislocations into grain under action of stress.     

Twins formation and their role in nanostructuring of zirconium

Twins formation and their role in nanostructuring of zirconium by surface mechanical attrition treatment were investigated. Twins nucleate via successive emission of partial dislocations from grain boundaries or overlapping of stacking faults and partial dislocations in grain interiors. As strain increases, twin nuclei grow up by adding more partial dislocations pairs into either side of the twin boundaries. The interaction between the formed twins and dislocations refines coarse grains into smaller ones, resulting in nanocrystallization of zirconium.     

A floating magnet model for simulating metals and alloys

A floating magnet model [5, 6] originally devised for simulating effects in superconductors has been modified to simulate metal and alloy lattices. The model shows grain boundaries, edge dislocations and point defects in the metal lattice, demonstrating the pinning of dislocations by impurity atoms and the drift of impurities to grain boundaries. Although the alloy was disordered for most concentrations, a stable ordered square lattice was formed readily at equal proportions of the two atoms. Defects, particularly dislocations, were more complicated in the ordered alloy than in the simple metal.     

Microstructural characterization of nanocrystalline nickel produced by surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), nanocrystalline surface layers are produced in pure Ni plates. The average crystallite size, root mean square (r.m.s.) microstrain, dislocation density, and stored elastic energy are determined by X-ray diffraction (XRD) line profile analysis. The average crystallite size obtained by XRD is compared with the grain size observed from transmission electron microscopy (TEM) image. The high-resolution TEM (HRTEM) micrograph confirms the presence of high density of dislocations obtained by XRD, and reveals that most of dislocations distribute at the subgrain boundaries with few inside the subgrains.     

Structure of grains and grain boundaries in cryo-mechanically processed Ti alloy

A Ti5Ta1.8Nb alloy with the major phase as α (hcp) Ti has been subjected to severe plastic deformation by means of cryo-rolling. Significant grain refinement (from ~5 μm to ~60 nm) has been observed. The mechanism of grain refinement was studied by analysis of lattice strain variations with increase in cold work using XRD technique. Various intermediate stages, such as hardening, alignment of dislocations, cell formation and criticality before new grain formation, were identified. Formation of cells with dislocations alignment at the boundaries and then finally forming an ultra-fine grain structure was confirmed by transmission electron microscopy studies. Detailed grain boundary characterisation has been carried out using high-resolution transmission electron microscopy studies and crystallographic texture analysis. The grain-refined structure was found to possess a large fraction of high angle boundaries identified also as special boundaries by evaluating the misorientation angle/axis sets for a pair of adjacent grain boundaries.     

Ni3Al合金硼原子占位及晶界偏聚

依据ＬＩ２型金属间化合物八面体，四面体间隙的特点，并利用硬球模型计算了上述各类间隙的空球半径大小，发现Ｎｉ３Ａｌ合金中６Ｎｉ八面体间空球半径比其他间隙大，使硼原子在更易进入６Ｎｉ八间隙，由于富Ｎｉ－Ｎｉ３Ａｌ合金晶界比Ａｌ－Ｎｉ３Ａｌ合金有更我的６Ｎｉ八面体间隙，所以硼在富Ｎｉ－Ｎｉ３Ａｌ晶界偏聚较多，另外富Ａｌ－Ｎｉ３Ａｌ合金晶界原子间作用力比富Ｎｉ－Ｎｉ３Ａｌ合金晶界原子间作用力大，这也阻碍了     

Effect of stress-induced grain growth during room temperature tensile deformation on ductility in nanocrystalline metals

In the present study defect-free nanocrystalline (nc) Ni-Co alloys with the Co content ranging from 2.4–59.3% (wt.%) were prepared by pulse electrodeposition. X-ray diffraction analysis shows that only a single face-centred cubic solid solution is formed for each alloy and that the grain size reduces monotonically with increasing Co content, which is consistent with transmission electron microscopy (TEM) observations. In the nc Ni-Co alloys, both the ultimate tensile strength and the elongation to failure increase as the Co content increases. The TEM observations reveal that stress-induced grain growth during tensile deformation is significantly suppressed for the nc Ni-Co alloys rich in Co in sharp contrast to those poor in Co. We believe that sufficient solutes could effectively pin grain boundaries making grain boundary motions (e.g. grain boundary migration and/or grain rotation) during deformation more difficult. Thus, stress-induced grain growth is greatly suppressed. At the same time, shear banding plasticity instability is correspondingly delayed leading to the enhanced ductility.     

A study of the influence of grain boundaries on short crack growth during varying load using a dislocation technique

The propagation of short cracks in the neighbourhood of grain boundaries have been investigated using a technique were the crack is modelled by distributed dislocation dipoles and the plastic deformation is represented by discrete dislocations. Discrete dislocations are emitted from the crack tip as the crack grows. Dislocations can also nucleate at the grain boundaries. The influence on crack growth characteristics of the distance between the initial crack tip and the grain boundary has been studied. It was found that crack growth rate is strongly correlated to the dislocation pile-ups at the grain boundaries.     

A strain-gradient plasticity theory of bimodal nanocrystalline materials with composite structure

For the purpose of evaluating the mechanical property of bimodal nanocrystalline (nc) materials, a new composite constitutive model comprised of coarse grains evenly distributed in the nc matrix with respect to strain gradient has been developed. Due to their dissimilar properties and mismatch between the two phases, dislocation-controlling mechanism based on the statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) was analyzed and extended to consider the different influences of two parts in the composite model. We firstly built a stress–strain relation for strain gradient plasticity to predict the effect of grain size distribution on the flow stress. To describe the strain strength quantitatively, a strain-hardened law determined from strain gradient and a nanostructure characteristic length parameter were developed. The strain-hardened law and nanostructure characteristic length parameter were not the same as described in classical strain gradient theory.     

Deformation behavior and mechanisms of a nanocrystalline multi-phase aluminum alloy

A nanocrystalline (nc) Al–Fe–Cr–Ti alloy containing 30 vol.% nc intermetallic particles has been used to investigate deformation behavior and mechanisms of nc multi-phase alloys. High compressive strengths at room and elevated temperatures have been demonstrated. However, tensile fracture strengths below 300 °C are lower than the corresponding maximum strengths in compression. Creep flow of the nc fcc-Al grains is suppressed even though rapid dynamic recovery has occurred. It is argued that the compressive strength at ambient temperature is controlled by propagation of dislocations into nc fcc-Al grains, whereas the compressive strength at elevated temperature is determined by dislocation propagation as well as dynamic recovery. The low tensile fracture strengths and lack of ductility at temperatures below 300 °C are attributed to the limited dislocation storage capacity of nanoscale grains. Since the deformation of the nc Al-alloy is controlled by dislocation propagation into nc fcc-Al grains, the smaller the grain size, the higher the strength. This new microstructural design methodology coupled with ductility-improving approaches could present opportunities for exploiting nc materials in structural applications at both ambient and elevated temperatures.     

HRTEM study of [001] low-angle tilt grain boundaries in fiber-textured BaTiO3 thin films

Low-angle tilt grain boundaries in [001] fiber-textured BaTiO₃ thin films were investigated by high-resolution transmission electron microscopy. Extensive observation revealed a very high density of low-angle tilt grain boundaries in the film. The low-angle tilt grain boundaries can be described as periodical arrays of dislocations on {100} and {110} boundary planes. The boundaries with (100) plane on {100} planes are composed of perfect dislocations with Burgers vectors b = a < 100 > (a = lattice constant of BaTiO₃: 0.3992 nm), while the boundaries with (110) plane on {110} planes are composed of the dissociated dislocations with Burgers vectors a/2 < 110 >. It was thus found that the difference in the boundary plane leads to different dislocation structures along the low-angle grain boundaries.     

Sliding and Migration of Tilt Grain Boundaries in a Mg–Zn–Y Alloy

Sliding and migration of tilt grain boundaries in a Mg–Zn–Y alloy have been investigated on the atomic scale using aberration‐corrected scanning transmission electron microscopy. Grain boundary sliding is accommodated by non‐basal dislocations moving along the grain boundary; grain boundary migration is induced by the motion of grain boundary dislocations with synchronized grain boundary diffusion. Simultaneous sliding and migration of tilt boundaries take place in both Mg matrix and long period stacking ordered phases. These results provide evidence for occurrence of grain boundary motion, which may play a role in plasticity of this kind of Mg alloys.