Temperature dependence of the Peierls-Nabarro forces in body-centered cubic metals

Based on the Kuhlmann-Wilsdorf theory, an expression for the temperature dependence of the Peierls-Nabarro forces was derived. It was shown that this dependence is in qualitative agreement with experimental data on the influence of temperature on the yield point of bcc metals.     

Dislocation junctions as indicators of elementary slip planes in body-centered cubic metals

In body-centered cubic (bcc) metals, an unambiguous determination of the elementary slip planes remains difficult owing to several possible interpretations of the glide activity, of slip steps on the specimen surface or features of the dislocation microstructure. In this article, a method is proposed to determine the elementary slip planes in bcc metals based on the line directions of sessile junctions resulting from the interaction of mobile dislocations with \({a}/2\langle 111\rangle \)  Burgers vector. The proposed method allows to determine slip activity inside a material and not at its surface, where other effects may play a role. It is in principle applicable to determining the elementary slip plane in any crystalline material. Particularly, it may help to resolve a long-standing debate of the nature of the elementary slip planes in bcc metals.     

Point-defect properties in body-centered cubic transition metals with analytic EAM interatomic potentials

Analytic embedded-atom method (EAM) interatomic potentials for body-centered cubic (bcc) transition metals have been constructed. The total energy is regarded as consisting of a pair potential part, an embedding interaction part and a modification term. All these parts are analytic functions of the atomic separations only and are fitted to various bulk properties, such as cohesive energy, vacancy formation energy, lattice constant and elastic constants. The present potentials are shown to predict a more realistic pressure–volume relationship so that interactions at separations smaller than that of the first-nearest neighbors can be treated. Interstitial formation energies are calculated for various configurations, using quenched molecular dynamics. Vacancy, surface and phonon spectra have been also studied and satisfactory agreement with available experimental data has been found.     

Investigation of the elastic moduli of face and body-centered cubic crystals

By the moment method the elastic moduli of anharmonic face and body-centered cubic crystals are considered. The analytic expressions for the elastic moduli as the Young's modulus E, the bulk modulus K, the rigidity modulus G are obtained. The obtained results are applied to Ag, Al, Fe, W, Nb metallic crystals and compared with the experimental data.     

Determination of nanoindentation size effects and variable material intrinsic length scale for body-centered cubic metals

It is well-known by now that the micro and nanoindentation hardness of metallic materials displays a strong size effect. The objective of this work is to formulate a micromechanical-based model for Temperature and Rate Indentation Size Effects (TRISE) for body centered cubic (BCC) metals encountered in nanoindentation experiments. In this regard, two physically based models are proposed here in order to capture the TRISE in single and polycrystalline materials by considering different expressions of the geometrical necessary dislocations (GNDs) density.The gradient plasticity theory formulates a constitutive framework on the continuum level that bridges the gap between the micromechanical plasticity and the classical continuum plasticity by incorporating the material length scale. A micromechanical-based model of variable material intrinsic length scale is also developed in the present work. The proposed length scale allows for variations in temperature and strain rate and its dependence on the grain size and accumulated plastic strain.The results of indentation experiments performed on niobium, tungsten, and single- and polycrystalline commercially pure iron (very similar to iron alloys) are used here to implement the aforementioned framework in order to predict simultaneously the TRISE and variable length scale at different temperatures, strain rates and various distances from the grain boundary. Numerical analysis is performed using the ABAQUS/VUMAT software with a physically based viscoplastic constitutive model.     

An analysis on nanovoid growth in body-centered cubic single crystalline vanadium

Molecular dynamics simulations were performed to analyze nanovoid growth in single crystalline vanadium under tension. Radial distribution function at the first nearest neighbor distance was calculated to find out the critical strain rate below which the deformation of specimen was static. Then a tensile stress was exerted on both void contained box and intact box under two constraint conditions. Homogenous dislocations were nucleated in intact box at yield point; while for void contained box with void radius twice the lattice constant, 〈1 1 1〉{1 1 0} shear loops were punched out from void surface. The formation of shear loops was the result of the splitting of purely screw cores on three non-planar planes, as well as their transformations to more stable two-fold non-planar dislocations under tension. The asymmetry of loops was influenced by both strain rate and triaxiality of system. It is also found that, in lower rate cases the yield point and peak stress point coincided; however, the two points separated at higher rate due to the inadequate void growth rate. Mean square displacement of void surface atoms were given out to geometrically depict the void evolution. Moreover, simulations with different initial porosity and box size were performed respectively. It is shown that when void reduced to contain only one vacancy, dislocations can be nucleated independently of the void; when porosity was large enough, the interactions between void and its periodic images were noticeable. Also, when both the void and box were large, triangular prismatic loops on {1 1 0} planes were observed at void surface, which may be contributed to a combined effect of the intersection of shear loops and the ledges along the void surface. Finally, the results of our MD simulations agreed well with that from Lubarda equation.     

Orientation dependence of the dislocation microstructure in compressed body-centered cubic molybdenum

The orientation dependence of the deformation microstructure has been investigated in commercial pure molybdenum. After deformation, the dislocation boundaries of compressed molybdenum can be classified, similar to that in face-centered cubic metals, into three types: dislocation cells (Type 2), and extended planar boundaries parallel to (Type 1) or not parallel to (Type 3) a {110} trace. However, it shows a reciprocal relationship between face-centered cubic metals and body-centered cubic metals on the orientation dependence of the deformation microstructure. The higher the strain, the finer the microstructure is and the smaller the inclination angle between extended planar boundaries and the compression axis is.     

X-ray analysis of sputtered films of beta-tantalum and body-centered cubic tantalum

The X-ray diffraction patterns obtained from thin films of β-Ta and b.c.c.-Ta are complicated by variations in type and degree of preferred orientation, and by variations in cell parameters. A discussion of the effects of these variations on the modified Debye-Scherrer X-ray diffraction patterns is given. The diffraction patterns from the commonly observed (200) β-Ta and (110) and (111) b.c.c.-Ta preferred orientations are illustrated, as well as those from mixtures of the two phases. For the case in which the film is a mixture of β-Ta and b.c.c.-Ta phases it is shown that the unit cells of the two phases expand in parallel. The difficulty in determining the orientation and relative amount of a phase present in a film containing both phases on the basis of diffractometer traces alone is emphasized.     

Recent investigations on some cubic Laves phase compounds of rare earth and transition metals

Pseudobinary alloy, Tb_0·27Dy_0·73Fe_2−δ , belonging to C-15 cubic Laves phase having MgCu₂-type structure, possesses large magnetostriction and high magnetomechanical coupling coefficient. The advantages of this material over existing piezoelectric materials, particularly for SONAR applications, are highlighted. Recent results on the influence of cobalt (Co) on magnetic and magnetomechanical properties of Tb_0·27Dy_0·73Fe₂ are discussed.     

Simultaneous enhancement of strength and ductility of body-centered cubic TiZrNb multi-principal element alloys via boron-doping

Body-centered cubic(BCC)multi-principal element alloys(MPEAs)have intrinsic high strength but poor ductility,which greatly limits their potential applications.Here we present the boron-doping strategy to enhance the strength and ductility of TiZrNb MPEAs simultaneously.The yield strength and ductility of the TiZrNb MPEA with boron addition of 500 ppm are increased by 19.0％and 48.7％compared to the boron-free TiZrNb MPEA,respectively.Boron-doping induced high efficiency in grain refinement from～96.0 pm to～16.2 pm is the main factor for strengthening.Dislocation dominated deformation mechanism involving cross slip and dislocation pining in the TiZrNb containing 500 ppm boron serves to enhance the strain-hardening capacity,resultant the enhancement of ductility from 7.8％to 11.6％.While the planar slip of dislocations is the dominated deformation mechanism for the boron-free TiZrNb.     

Reaction-matrix theory of quantum solids and its application to3He and4He in body-centered cubic phases

The self-consistent reaction-matrix theory of quantum solids is applied to bcc³He and⁴He. The two-body correlation function is solved numerically in terms of partial waves. The results for ground-state energies, pressures, sound velocities, and phonon-dispersion curves are compared with those obtained by theS-wave correlation function. In both helium isotopes better agreement with experiments is obtained by the partial-wave analysis.On leave of absence from Institute of Physics, College of General Education, University of Tokyo, Tokyo, Japan.     

Contribution of ultrasonic surface rolling process to the fatigue properties of TB8 alloy with body-centered cubic structure

The effect of ultrasonic surface rolling process(USRP) as a severe plastic deformation technology was investigated on the evolution of microstructure, residual stress and surface morphology of TB8 alloys with body-centered cubic structure. Stress-controlled rotating-bending fatigue tests indicated increased fatigue strength in USRP samples prepared using different number of passes compared to the base material, which was attributed to the presence of gradient structure surface layers. Five subsequent USRP passes resulted in the highest fatigue strength, due to the optimal surface properties including higher extent of grain refinement, larger compressive residual stresses, "smoother" surface morphology and increased micro-hardness. However, the effect of USRP technology on improving fatigue strength of TB8 alloy was not significant in comparison with that of other titanium alloys(for example, Ti6 Al4 V), which was attributed to the notable surface residual stresses relaxation revealed from measurements on postfatigued USRP samples. Electron backscatter diffraction analysis confirmed that fatigue crack initiation occurred in the larger grains on the surface with high Schmid factor. Small cracks were found to propagate into the core material in a mixed transgranular and intergranular mode. Further analysis indicated that grain growth existed in post-fatigued USRP-treated TB8 samples and that the average geometrically necessary dislocations value reduced after fatigue loading.     

Microhardness and interatomic binding in some cubic crystals

Vicker’s microhardness measurements have been carried out on single crystals of CaF₂, SrF₂, BaF₂, CdF₂, PbF₂, EuF₂, ThO₂, NaClO₃, NaBrO₃, Bi₄ (GeO₄)₃, Bi₄(SiO₄)₃, Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀. The hardness values are discussedvis-a-vis the interatomic binding in these crystals. While most of the fluorite-type crystals are highly ionic, covalency is indicated in the bismuth compounds studied.     

Characteristics of face-centered cubic metals processed by equal-channel angular pressing

This review surveys the characteristics of face-centered cubic (fcc) metals and alloys processed by equal-channel angular pressing (ECAP). The significance of the Hall–Petch relationship for ultra-fine grained structures is examined and the dependence of the saturated stress obtained in ECAP on the absolute melting temperature is described and discussed. In addition, the flow processes at low temperatures in ultrafine-grained materials and the microstructural evolution of the dislocation densities and precipitates in some alloys of practical importance are also considered briefly.     

Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate

Realizing high work hardening and thus elevated strength–ductility synergy are prerequisites for the practical usage of body-centered-cubic high entropy alloys(BCC-HEAs). In this study, we report a novel dynamic strengthening mechanism, martensitic twinning transformation mechanism in a metastable refractory element-based BCC-HEA(TiZrHf)₈₇Ta₁₃(at.%) that can profoundly enhance the work hardening capability, leading to a large uniform ductility and high strength simultaneous...     

Ultrafine grain formation in face centered cubic metals during severe plastic deformation

The evolution mechanisms of new high-angle boundaries as well as ultrafine grains at large strains were studied by means of multidirectional forging (MDF) of pure copper at low temperature and aluminum alloy at high temperature, where dynamic recovery operates as a main restoration process. The structural changes can be characterized by the evolution of deformation bands such as microshear or kink bands at moderate strains. Multidirectional forging accelerates the evolution of many mutually crossing microshear or kink bands developed in various directions. The misorientations between (sub)grains increased gradually with increasing cumulative strain, finally leading to the development of a new fine-grained structure. The dynamic grain formation can be resulted from in situ or continuous dynamic recrystallization which is discussed in detail.     

On the behaviour of body-centred cubic metals to one-dimensional shock loading

The response of metallic materials to shock loading, like all loading regimes, is controlled largely by factors operating at the microscopic or atomic levels. Over the past few years, face-centred cubic (fcc) metals have received a level of attention where the role of features such as stacking fault energy and precipitation hardening have been investigated. We now turn our attention to body-centred cubic (bcc) metals. In the past, only tantalum, tungsten, and their alloys have received significant attention at high strain-rate conditions due to their use by the ordnance community. In particular, this investigation examines the shear strength of these materials at shock loading conditions. Previous results on tantalum, tungsten, and a tungsten heavy alloy are reviewed, and more recent experiments on niobium, molybdenum, and Ta–2.5 wt% W presented. Results are discussed in terms of known deformation mechanisms and variations of Peierl’s stress.