Mo Single Crystals Research Articles

The effect of annealing on micro-hardness of molybdenum single crystals

Molybdenum (Mo) crystals exhibit excellent mechanical properties such as high yield strength, high elastic modulus, very high melting point, and excellent corrosion resistance, making them a suitable choice for various industrial applications. The mechanical behaviour of these crystals has been investigated through tension, compression, and indentation experiments. The indentation experiments on annealed Mo single-crystal thin film show an increase in hardness with increase in annealing temperature from 400 °C–650 °C. However, it is not clear whether this behaviour is specific to the Mo thin films or these are applicable to the bulk samples of single crystals also. In order to address these issues, the Vickers indentation experiments are performed on as-received and annealed samples of (100) -, (110) -, and (111) -oriented Mo single crystals. The results show that the low-temperature annealing leads to an increase in micro-hardness by 9% for (100) oriented crystals. The x-ray diffraction (XRD), electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) studies have shown that there is a drop in the number of dislocation sources or dislocation density ( ρ ) with no high-angle grain boundary formation or any microstructural changes or any oxide formation. This necessitates to enhance the applied stress required for significant yielding (or yield strength) which results in an increase in the hardness. Based on experimental observations, a hardening mechanism governed by dislocation annihilation during annealing has been proposed in the present study.

Single crystalline molybdenum nanostructure for generating reflective micro-focal spot X-rays: Structure-controllable preparation and characteristic-selectable radiation

The use of properly designed nanostructured anode targets enables direct generation of high performance X-rays. In particular, this scheme has attracted widespread research interest for obtaining micro-focal spot X-ray radiation. In this study, we developed a thermal evaporating deposition technique and achieved controllable preparation of different single crystalline molybdenum (Mo) nanostructures by exploiting the lattice matching relationship between the substrate and the product crystal orientation. When used as an anode target for a reflective X-ray radiation device, these nanostructured Mo single crystals exhibit advantages over traditional targets at larger radiation angles. Further experimental and simulation results demonstrate the geometric characteristics of these Mo nanostructures and their correlation with the radiation intensity, focal spot size, and brightness of the generated X-rays. Among them, a unique frustum of square pyramid single crystalline Mo nanostructure stands out, due to its combination of three-dimensional body feature of nanostructures and radiation plane feature of planar targets. The research results not only provide a class of nanostructured anode target materials for reflective X-ray sources with different radiation angle requirements, but also make it possible to design and develop miniaturized devices for X-ray sources with selectable radiation performance and micro-focal spot.

Improving mechanical properties and achieving structural stability of large-size molybdenum single crystal during hot deformation by adding Nb element

The occurrence of recrystallization will cause catastrophic damage to the structure of the metallic single crystal, which can be avoided by alloying reasonably. In this work, a new strategy is proposed to improve mechanical properties and achieve the structural stability of Mo single crystal during hot deformation. Thermal simulation tests show that the mechanical property of Mo single crystal is significantly improved by adding the Nb element. The Nb element is beneficial to inhibit the dynamic recrystallization of Mo single crystals through blocking the development of high-angle grain boundaries and dislocation cells, leading to Mo-Nb alloy single crystals still retaining monocrystal structure.

Структура волны сжатия при плоском ударном деформировании монокристалла молибдена [100] с различной начальной плотностью дислокаций

The features of the evolution of the elastic-plastic compression wave in Mo single crystals along the crystallographic plane [100], having a different initial dislocation structure formed by small deformations of varying degrees of static compression at room temperature, have been studied. The analysis of wave profiles recorded using the VISAR laser interferometer in samples of different thicknesses showed a non-monotonic change in the hugoniot elastic limit depending on the initial dislocation density – pre-strain of a single crystal by compression reduces the hugoniot elastic limit by 3 times.

Compression wave structure under plane impact deformation of a molybdenum single crystal [100] with different initial density of dislocations

The features of the evolution of the elastic-plastic compression wave in Mo single crystals along the crystallographic plane [100], having a different initial dislocation structure formed by small deformations of varying degrees of static compression at room temperature, have been studied. The analysis of wave profiles recorded using the VISAR laser interferometer in samples of different thicknesses showed a non-monotonic change in the Hugoniot elastic limit depending on the initial dislocation density pre-strain of a single crystal by compression reduces the Hugoniot elastic limit by 3 times. Keywords: Single crystal, small deformations, compression wave evolution, wave profile, VISAR interferometer.

Peak states of molybdenum single crystals shock compressed to high stresses

To determine crystal anisotropy effects at high stresses, peak states behind the plastic shock waves were examined in BCC single crystals. Using plate impact experiments, molybdenum (Mo) single crystals were shock compressed up to 190 GPa elastic impact stress along [100], [110], and [111] orientations. Laser interferometry was used to measure wave velocities and particle velocity profiles at the Mo–LiF window interface. These data were analyzed to obtain in-material quantities in the peak states. The Hugoniots for [100] and [110] orientations were comparable, but the Hugoniot for the [111] orientation was different from the other two orientations. Also, these Mo single crystal Hugoniots display differences from the polycrystalline Mo Hugoniots. Although none of the differences can be considered large, the present results demonstrate that, unlike FCC metal single crystals (Cu, Al), some anisotropy is preserved in Mo single crystal Hugoniots even at high stresses.

Effect of plastic bending on high temperature creep resistance of molybdenum single crystals

Using molybdenum single crystals, it is shown how plastic bending multiplies the high-temperature strength of products for pipelines and tube shells of small diameter (10–30 mm), which are subjected to high internal pressure. It is very convenient to do this when the plastic bending process is included in the manufacturing technology of such products, for example, in the form of spiral winding a strip into a pipe. It has been established that during 1000 h, the creep rate of a pre-bent Mo single crystals at 1633 K and an applied stress of 10 MPa permanently decreases by 4–5 orders of magnitude as compared to an unstrengthened single crystal. Using light microscopy, X-ray diffractometry, and transmission electron microscopy, changes in the dislocation structure and the mechanism leading to such strengthening have been found. During the creep test, the edge components of dislocations are depleted and disappear, and the cross slip of screw dislocations is blocked by numerous loops and helicoids that were formed under bending and transformed during the creep process. As a result, dislocation creep gradually turns into diffusion creep during one test. It is safe to assume that similar effects will be observed in polycrystals when the dominant texture direction will correspond to the required bending axis.

Shock compression of molybdenum single crystals to 110 GPa: Elastic–plastic deformation and crystal anisotropy

To investigate the role of crystal anisotropy on the elastic–plastic deformation of BCC single crystals at high shock stresses, molybdenum (Mo) single crystals were shock compressed along the [100], [111], and [110] orientations at elastic impact stresses between 20 and 110 GPa. Laser interferometry was used to measure shock wave velocities and particle velocity histories. Along the [100] and [111] orientations, elastic–plastic waves (two wave structure) were observed up to 110 GPa. Along the [110] orientation, the two wave structure was observed only up to 90 GPa. The measured elastic wave amplitudes were analyzed to determine crystal anisotropy effects, impact stress dependence, and the activated slip systems on the Hugoniot elastic limit. The findings from our work have provided insight into the role of crystal anisotropy on the elastic–plastic deformation under shock compression at high stresses.

Composition of ω-Phase Particles in Ti(Mo) Alloys Studied by Anomalous X-ray Diffraction

Nanoparticles of hexagonal ω phase in bcc-Ti(Mo) single crystals ( β phase) occur due to a diffusionless athermal β to ω transformation and they grow during follow-up ageing at elevated temperatures, while the alloying atoms (Mo in our case) are expelled from the nanoparticle volumes. We investigated the Mo content in growing ω nanoparticles by anomalous X-ray diffraction and demonstrate that the Mo expulsion from the ω phase is not full; a thin shell of a nanoparticles where the β to ω transformation is not complete still contains a considerable amount of Mo atoms.

Anomalous X-ray diffraction from ω nanoparticles in β-Ti(Mo) single crystals.

Anomalous X-ray diffraction (AXRD) is a technique which makes use of effects occurring near the energy of an absorption edge of an element present in the studied sample. The intensity of the diffracted radiation exhibits an anomalous decrease when the primary beam energy matches the energy needed to excite an electron from an atomic orbital. The characteristics of this step are sensitive to the concentration of the `anomalous' element and its spatial distribution in the sample. In the present investigation, AXRD was employed to study ω particles in a metastable β titanium alloy Ti-15Mo (in wt%). The experiments were done in an energy range around the Mo K edge at 20.0 keV, allowing investigation of the distribution of Mo in the material, which is rejected from the volume of ω particles during their diffusion-driven growth. This paper deals with diffuse scattering patterns around the (006)β diffraction maximum. It was observed that different regions of the diffuse scattering exhibited different variations of diffracted intensity with the incident photon energy near the absorption edge. Numerical simulations of diffuse scattering patterns as well as of energy dependences of the scattered intensity were performed. It was found that the observed patterns and their dependence on the primary beam energy can be explained by taking into account (a) elastic deformation of the β matrix arising from the presence of slightly misfitting ω particles and (b) the presence of a `cloud' of a higher Mo concentration around ω particles.

Elastic-plastic deformation of molybdenum single crystals shocked to 12.5 GPa: Crystal anisotropy effects

To understand crystal anisotropy effects on shock-induced elastic-plastic deformation of molybdenum (Mo), results from high-purity single crystals shocked along [110] and [111] orientations to an elastic impact stress of 12.5 GPa were obtained and compared with the [100] results previously reported [A. Mandal and Y. M Gupta, J. Appl. Phys. 121, 045903 (2017)]. Measured wave profiles showed a time-dependent response, and strong anisotropy was observed in the elastic wave attenuation with the propagation distance, elastic limits, shock speeds, and overall structure of the wave profiles. Resolved shear stresses on {110}〈111〉 and {112}〈111〉 slip systems provided insight into the observed anisotropy in elastic wave attenuation and elastic limits and showed that shear stresses, and not longitudinal stresses, are a better measure of strength in shocked single crystals. Under shock compression, resolved shear stresses at elastic limits were comparable to the Peierls stress of screw dislocations in Mo. Elastic wave attenuation was rapid when shear stresses were larger than the Peierls stress. Large differences in the elastic limits under shock and quasi-static loading are likely a consequence of the large Peierls stress value for Mo. Numerically simulated wave profiles, obtained using the dislocation-based plasticity model described in the [100] work, showed good agreement with all measured wave profiles but could not differentiate between the {110}〈111〉 and {112}〈111〉 slip systems. Overall, experimental results and corresponding numerical simulations for the three crystal orientations have provided a comprehensive insight into shock-induced elastic-plastic deformation of Mo single crystals, including the development of a continuum material model.

Shape-Controlled Synthesis of Ultrafine Molybdenum Crystals via Salt-Assisted Reduction of MoO2 with H2

In the current investigation, the salt-assisted hydrogen reduction of MoO2 was proposed to prepare ultrafine Mo single crystals with controllable shape and size. By adjusting the type and amount of...

Shear relaxation behind the shock front in [formula omitted] molybdenum – From the atomic scale to continuous dislocation fields

In this work we study shock-induced plasticity in Mo single crystals, impacted along the 〈1 1 0〉 crystal orientation. In particular, the shear relaxation behind the shock front is quantitatively inspected. Molecular dynamics (MD) simulations are employed to simulate the deformation during shock, followed by post-processing to identify and quantify the dislocation lines nucleated behind the shock front. The information on the dislocation lines is ensemble averaged inside slabs of the simulation box and over different realizations of the MD simulations, from which continuous dislocation fields are extracted using the Discrete-to-Continuous method. The continuous dislocation fields are correlated with the stress and strain fields obtained from the MD simulations. Based on this analysis, we show that the elastic precursor overshoots the shear stress, after which dislocations on a specific group of slip planes are nucleated, slightly lagging behind the elastic front. Consequently, the resolved shear stress is relaxed, but the principal lateral stress increases. The latter leads to an increase in the resolved shear stress on a plane parallel to the shock wave, resulting in an additional retarded front of dislocation nucleation on planes parallel to the shock front. Finally, the two-stage process of plasticity results in an isotropic stress state in the plane parallel to the shock wave. The MD simulation results are employed to calculate the dislocation densities on specific slip planes and the plastic deformation behind the shock, bridging the gap between the information on the atomic scale and the continuum level.

Specimen size and shape dependent yield strength in micropillar compression deformation of Mo single crystals

The deformation behavior of [1¯25]-oriented Mo micropillars with rectangular cross-sections having the [111] primary slip direction parallel to one of the two orthogonal side-surfaces has been investigated as a function of specimen size and shape (with varying ratios for two side-edge lengths of the cross-section). Micropillars with square-shaped cross-sections exhibit a size dependence of the critical resolved shear stress τCRSS approximated by τCRSS∝L−m with a power-law exponent of m ∼0.5 (L: cross-section side length). For a series of micropillars with a fixed length for the side-edge perpendicular to [111] (CE series), the CRSS also exhibit a power-law relationship with a relatively small exponent of m = 0.3. In contrast, an unexpected size-dependence is observed with two strength plateaus being connected by a strongly size dependent region (m ∼ 1) for a series of micropillars having a fixed length for the other cross-section side (CS series). The observed specimen size and shape dependence of CRSS of Mo micropillars is discussed based on the stochastic dislocation source length model and the dislocation kinetics model.

Elastic-plastic deformation of molybdenum single crystals shocked along [100

To understand the elastic-plastic deformation response of shock-compressed molybdenum (Mo) - a body-centered cubic metal, single crystal samples were shocked along the [100] crystallographic orientation to an elastic impact stress of 12.5 GPa. Elastic-plastic wave profiles, measured at different propagation distances ranging between ∼0.23 to 2.31 mm using laser interferometry, showed a time-dependent material response. Within the experimental scatter, the measured elastic wave amplitudes were nearly constant over the propagation distances examined. These data point to a large and rapid elastic wave attenuation near the impact surface, before reaching a threshold value (elastic limit) of ∼3.6 GPa. Numerical simulations of the measured wave profiles, performed using a dislocation-based continuum model, suggested that {110}⟨111⟩ and/or {112}⟨111⟩ slip systems are operative under shock loading. In contrast to shocked metal single crystals with close-packed structures, the measured wave profiles in Mo single crystals could not be explained in terms of dislocation multiplication alone. A dislocation generation mechanism, operative for shear stresses larger than that at the elastic limit, was required to model the rapid elastic wave attenuation and to provide a good overall match to the measured wave profiles. However, the physical basis for this mechanism was not established for the high-purity single crystal samples used in this study. The numerical simulations also suggested that Mo single crystals do not work harden significantly under shock loading in contrast to the behavior observed under quasi-static loading.

Influence of Mo Addition on High Temperature Deformation Behavior of L12 Type Ni3Al Intermetallics

The high temperature deformation behavior of Ni3Al and Ni3(Al,Mo) single crystals that were oriented near was investigated at low strain rates in the temperature range above the flow stress peak temperature. Three types of behavior were found under the present experimental conditions. In the relatively high strain rate region, the strain rate dependence of the flow stress is small, and the deformation may be controlled by the dislocation glide mainly on the {001} slip plane in both crystals. At low strain rates, the octahedral glide is still active in Ni3Al above the peak temperature, but the active slip system in Ni3(Al,Mo) changes from octahedral glide to cube glide at the peak temperature. These results suggest that the deformation rate controlling mechanism of Ni3Al is viscous glide of dislocations by the {111} slip, whereas that of Ni3(Al,Mo) is a recovery process of dislocation climb in the substructures formed by the {001} slip. The results of TEM observation show that the characteristics of dislocation structures are uniform distribution in Ni3Al and subboundary formation in Ni3(Al,Mo). Activation energies for deformation in Ni3Al and Ni3(Al,Mo) were obtained in the low strain rate region. The values of the activation energy are 360 kJ/mol for Ni3Al and 300 kJ/mol for Ni3(Al,Mo).

A tale of two mechanisms: Strain-softening versus strain-hardening in single crystals under small stressed volumes

Pre-straining defect-free single crystals will introduce heterogeneous dislocation nucleation sources that reduce the measured strength from the theoretical value, while pre-straining bulk samples will lead to strain hardening. Their competition is investigated by nanoindentation pop-in tests on variously pre-strained Mo single crystals with several indenter radii (∼micrometer). Pre-straining primarily shifts deformation mechanism from homogeneous dislocation nucleation to a stochastic behavior, while strain hardening plays a secondary role, as summarized in a master plot of pop-in strength versus normalized indenter radius.

Dynamic abnormal grain growth in tantalum

Dynamic abnormal grain growth (DAGG) is a phenomenon that produces one or more very large, abnormal grains during plastic deformation of polycrystalline material at high temperatures. DAGG was previously observed in commercial-purity molybdenum (Mo) and was used to produce large Mo single crystals of centimeters in length. The present investigation is the first to demonstrate DAGG in commercial-purity tantalum (Ta) sheet, another body-centered-cubic refractory metal. DAGG occurs in Ta at temperatures from 1450–1850°C across strain rates from 3×10−5 to 5×10−4s−1. Grain boundary migration rates during DAGG in Ta are on the order of 10mm/min. DAGG produces large abnormal grains preferentially oriented with the 〈101〉 direction approximately parallel to the tensile axis. A unique observation of this investigation is a preponderance of Σ3 special boundary character along the boundaries of large abnormal grains produced in Ta through DAGG. The propensity toward this special boundary character is a result of a relatively large grain size and strong texture in the polycrystalline material prior to DAGG and the typically low energy of Σ3 boundaries, which suppress boundary migration.

Structural fragmentation in TiNi(Fe, Mo) single crystals during intense plastic deformation

The structural transformations in TiNi(Fe, Mn) alloy single crystals during intense plastic deformation (cold rolling and omnidirectional pressing) are studied by electron microscopy as a function of the strain. The micromechanisms of plastic deformation and fragmentation are analyzed in the formation of nanocrystalline and quasi-amorphous states.

The features of energy transfer to the emission centers in ZnWO4 and ZnWO4:Mo

The process of energy transfer to the emission centers is studied in ZnWO4 and ZnWO4:Mo single crystals in the temperature range 10–300K. A numerical simulation describing the process is presented; the temperature dependence of energy transfer efficiency to the intrinsic emission centers in ZnWO4 is shown to be defined by the modification of Onsager sphere radius. A competitive role of radiative relaxation channel related to the impurity of molybdenum is studied in ZnWO4:Mo. It is shown that below 60K the energy transfer to the MoO6 emission centers by free charge carriers terminates due to the self-trapping of holes at WO6 complexes.