C. Woodward

Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy

Compression properties of a refractory multi-component alloy, Ta20Nb20Hf20Zr20Ti20, were determined in the temperature range of 296–1473xa0K and strain rate range of 10−1–10−5xa0s−1. The properties were correlated with the microstructure developed during compression testing. The alloy was produced by vacuum arc melting, and it was hot isostatically pressed (HIPd) and homogenized at 1473xa0K for 24xa0h prior to testing. It had a single-phase body-centered cubic structure with the lattice parameter axa0=xa0340.4xa0pm. The grain size was in the range of 100–200xa0μm. During compression at a strain rate of έxa0=xa010−3xa0s−1, the alloy had the yield strength of 929xa0MPa at 296xa0K, 790xa0MPa at 673xa0K, 675xa0MPa at 873xa0K, 535xa0MPa at 1073xa0K, 295xa0MPa at 1273xa0K and 92xa0MPa at 1473xa0K. Continuous strain hardening and good ductility (εxa0≥xa050%) were observed in the temperature range from 296 to 873xa0K. Deformation at Txa0=xa01073xa0K and έxa0≥xa010−3xa0s−1 was accompanied by intergranular cracking and cavitation, which was explained by insufficient dislocation and diffusion mobility to accommodate grain boundary sliding activated at this temperature. The intergranular cracking and cavitation disappeared with an increase in the deformation temperature to 1273 and 1473xa0K or a decrease in the strain rate to ~10−5xa0s−1. At these high temperatures and/or low-strain rates the alloy deformed homogeneously and showed steady-state flow at a nearly constant flow stress. Partial dynamic recrystallization, leading to formation of fine equiaxed grains near grain boundaries, was observed in the specimens deformed at 1073 and 1273xa0K and completed dynamic recrystallization was observed at 1473xa0K.

The Chemistry of Deformation: How Solutes Soften Pure Metals

Solutes have been added to strengthen elemental metals, generating usable materials for millennia; in the 1960s, solutes were found to also soften metals. Despite the empirical correlation between the “electron number” of the solute and the change in strength of the material to which it is added, the mechanism responsible for softening is poorly understood. Using state-of-the-art quantum-mechanical methods, we studied the direct interaction of transition-metal solutes with dislocations in molybdenum. The interaction increases dramatically with increasing electron number and strongly influences the mechanisms responsible for plasticity in these materials. Our quantitative model explains solution softening of metals by using changes in energy and stress scales of plasticity from solutes.

Quantitative prediction of solute strengthening in aluminium alloys

Despite significant advances in computational materials science, a quantitative, parameter-free prediction of the mechanical properties of alloys has been difficult to achieve from first principles. Here, we present a new analytic theory that, with input from first-principles calculations, is able to predict the strengthening of aluminium by substitutional solute atoms. Solute-dislocation interaction energies in and around the dislocation core are first calculated using density functional theory and a flexible-boundary-condition method. An analytic model for the strength, or stress to move a dislocation, owing to the random field of solutes, is then presented. The theory, which has no adjustable parameters and is extendable to other metallic alloys, predicts both the energy barriers to dislocation motion and the zero-temperature flow stress, allowing for predictions of finite-temperature flow stresses. Quantitative comparisons with experimental flow stresses at temperature T=78 K are made for Al-X alloys (X=Mg, Si, Cu, Cr) and good agreement is obtained.

Phase-field modeling of bimodal particle size distributions during continuous cooling

Abstract Microstructures in Nickel-base alloys typically contain a two-phase mixture of γ/γ′. The microstructure having a bimodal size distribution of γ′ is of particular interest because it has important property consequences [1] . In this paper, the phase-field method with an explicit nucleation algorithm is employed to investigate the microstructural development during a continuous cooling with various cooling rates. It is demonstrated that bimodal particle size distributions can be achieved at an intermediate cooling rate due to a coupling between diffusion and undercooling, in which the system experiences two peaks of well-isolated nucleation events. It is suggested that this is caused by soft impingement, followed by a renewal of driving force for nucleation, followed by a subsequent soft impingement. Under very high cooling rates, the microstructure becomes unimodal, because undercooling always outruns diffusion and the microstructure never reaches soft impingement.

Atomistic simulation of cross-slip processes in model fcc structures

Abstract Three-dimensional cross-slipped core structures of (a/2)[110] screw dislocations in model fcc structures are simulated using lattice statics within the embedded-atom method (EAM) formalism. Two parametric EAM potentials fitted to the elastic and structural properties of fcc Ni were used for the simulations. The two- and three-dimensional Greens function techniques newly developed by Rao et al, are used to relax the boundary forces in the simulations. Core structures and energetics of the constrictions occurring in the cross-slip process are studied. The core structure of the constrictions are diffuse, as opposed to a point constriction as envisaged by Stroh. The two constrictions formed by cross-slip onto a cross {111} plane have significantly different energy profiles, at variance with classical continuum theory of Stroh. This suggests that self-stress forces and atomistics dominate the energetics of the cross-slip process; the far-field elastic-energy contribution to cross-slip appears to be m...

Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy

Isothermal oxidation behavior of a refractory high-entropy NbCrMo0.5Ta0.5TiZr alloy was studied during heating at 1273xa0K for 100xa0h in flowing air. Continuous weight gain occurred during oxidation, and the time dependence of the weight gain per unit surface area was described by a parabolic dependence with the time exponent nxa0=xa00.6. X-ray diffraction and scanning electron microscopy accompanied by energy-dispersive X-ray spectroscopy showed that the continuous oxide scale was made of complex oxides and only local (on the submicron levels) redistribution of the alloying elements occurred during oxidation. The alloy has a better combination of mechanical properties and oxidation resistance than commercial Nb alloys and earlier reported developmental Nb–Si–Al–Ti and Nb–Si–Mo alloys.

Quasi-periodic events in crystal plasticity and the self-organized avalanche oscillator

When external stresses in a system—physical, social or virtual—are relieved through impulsive events, it is natural to focus on the attributes of these avalanches. However, during the quiescent periods between them, stresses may be relieved through competing processes, such as slowly flowing water between earthquakes or thermally activated dislocation flow between plastic bursts in crystals. Such smooth responses can in turn have marked effects on the avalanche properties. Here we report an experimental investigation of slowly compressed nickel microcrystals, covering three orders of magnitude in nominal strain rate, in which we observe unconventional quasi-periodic avalanche bursts and higher critical exponents as the strain rate is decreased. Our experiments are faithfully reproduced by analytic and computational dislocation avalanche modelling that we have extended to incorporate dislocation relaxation, revealing the emergence of the self-organized avalanche oscillator: a novel critical state exhibiting oscillatory approaches towards a depinning critical point. This theory suggests that whenever avalanches compete with slow relaxation—in settings ranging from crystal microplasticity to earthquakes—dynamical quasi-periodic scale invariance ought to emerge.

Effects of focused ion beam induced damage on the plasticity of micropillars

Abstract : The hardening effects of focused ion beam (FIB) induced damage produced during the fabrication of micropillars are examined by introducing a surface layer of nanosized obstacles into a dislocation dynamics simulation. The influence of the depth and strength of the obstacles as a function of pillar diameter is assessed parametrically. We show that for a selected set of sample sizes between 0.5 and 1.0 micrometer, the flow strength can increase by 10 20%, for an obstacle strength of 750 MPa, and damage depth of 100 nm. On the other hand, for sizes larger and smaller than this range, the effect of damage is negligible. Results show that the obstacles formed during the FIB milling may be expected to alter the microstructure of micropillars, however, they have a negligible effect on the observed size-strength scaling laws.

Ab initio simulations of molten Ni alloys

Convective instabilities responsible for misoriented grains in directionally solidified turbine airfoils are produced by variations in liquid–metal density with composition and temperature across the solidification zone. Here, fundamental properties of molten Ni-based alloys, required for modeling these instabilities, are calculated using ab initio molecular dynamics simulations. Equations of state are derived from constant number-volume-temperature ensembles at 1830 and 1750 K for elemental, binary (Ni–X, X=Al, W, Re, and Ta) and ternary (Ni–Al–X, X=W, Re, and Ta) Ni alloys. Calculated molar volumes agree to within 0.6%–1.8% of available measurements. Predictions are used to investigate the range of accuracy of a parameterization of molar volumes with composition and temperature based on measurements of binary alloys. Structural analysis reveals a pronounced tendency for icosahedral short-range order for Ni–W and Ni–Re alloys and the calculations provide estimates of diffusion rates and their dependence ...

Atomistic simulations of cross-slip nucleation at screw dislocation intersections in face-centered cubic nickel

The Escaig model for thermally activated cross-slip in face-centered cubic (fcc) materials assumes that cross-slip preferentially occurs at obstacles that produce large stress gradients on the Shockley partials of the screw dislocations. However, it is unclear as to the source, identity and concentration of such obstacles in single-phase fcc materials. Embedded atom potential, molecular-statics simulations of screw character dislocation intersections with 120° forest dislocations in fcc Ni are described that illustrate a mechanism for cross-slip nucleation. The simulations show how such intersections readily produce cross-slip nuclei and thus may be preferential sites for cross-slip. The energies of the dislocation intersection cores are estimated and it is shown that a partially cross-slipped configuration for the intersection is the most stable. In addition, simple three-dimensional dislocation dynamics simulations accounting for Shockley partials are shown to qualitatively reproduce the atomistically determined core structures for the same dislocation intersections.