Extra Dislocation Research Articles

Lath decomposition and dispersion induced by heterostructure deformation and associated interactions in a lamellae metastable-β Ti alloy during subtransus processing

The mechanisms of lath decomposition and dispersion were hitherto rarely explained due to limited understanding into the heterostructure deformation between two phases and associated interactions. In this work, the lath decomposition and dispersion were systematically investigated in a wide subtransus processing range of 0.001 s−1–1 s−1 and 745 °C–820 °C with the consideration into the heterostructure deformation and associated interactions. The relationship between flowing behavior and microstructure evolution were discussed. Results indicated that the initial buckling of α laths caused high peak strength. The rotation and twist of α laths are helpful to decrease the flow stress, but it caused serious extra deformation and dislocation storage in the β matrix. The lath decomposition dominated by the ablation was greatly contributed by the adiabatic temperature rise (ATR) near boundaries with high energies. The α laths could rotate under low strain rates, and numerous residual laths (RLs) could be obtained. The increment in strain rates to 0.1 s−1 resulted in earlier lath decomposition and softening, as well as to disperse αp particles and homogenize the β matrix. But an overhigh strain rate of ∼1 s−1 caused adiabatic shear bands. The low temperatures are unfavorable to the lath decomposition, while over-high temperatures diminished the extra deformation and ATR. On this basis, a deformation mechanism map for the subtransus processing of lamellae Ti-55531 alloy was established. This investigation sheds deeper insight into the tailoring of microstructure and prediction of flowing behavior for the subtransus processing of lamellae metastable-β Ti alloys.

Strength softening mitigation in bimodal structured metals

Polycrystalline metals become stronger with a decrease in the constitute grain size, yet strength softening takes over at the nanometer regime. Here, we find that this softening rate can be tailored and mitigated by tuning structural heterogeneity and grain size uniformity. With reducing the small grain size in bimodal structures, the strength reduction is markedly alleviated due to the enhanced strengthening of large grains. This mitigation of strength softening is mediated by promoted intragranular slip, extra dislocation storage, and reduced grain boundary sliding in bimodal structures. Our findings signifying the role of grain size non-uniformity on crystal plasticity shed light on a broad class of heterogeneous structured materials with improved strength and ductility.

Grain size effect on tensile deformation behaviors of pure aluminum

Grain refinement is a very effective method to improve the mechanical properties of materials and attracts widespread interests among researchers. However, the grain size effect on the mechanical properties is still unclear due to the undesirable microstructure in ultrafine grained (UFG) materials. In the present work, series of ideal materials with average grain sizes range from 0.7 μm to 30.0 μm containing high fraction of high angle grain boundaries (HAGBs), equiaxed grains and low density of dislocations were produced by friction stir processing (FSP). It was found that the Hall-Petch relationships could be classified into three stages as the grain size reduced from coarse grain to UFG regimes, which were decided by the strengthening mechanisms during tensile deformation. The strengthening effect of HAGBs (71 MPa⋅μm1/2) was almost three times of low angle grain boundaries (25 MPa⋅μm1/2), resulting in the positive deviation of Hall-Petch slope in fine grain regime by the increased specific surface area of HAGBs. The further positive deviation of the Hall-Petch slope in UFG regime was affected by the occurrence of an extra dislocation source limited strengthening mechanism, which was up to 29 MPa and reached to about 20% of the yield strength. The increased recovery rate of dislocations at HAGBs contributed to the decrease of mobile dislocation density, leading to the losing of work hardening in UFG regime during tensile deformation.

Extra strengthening in a coarse/ultrafine grained laminate: Role of gradient interfaces

The interfaces introduced in metals by heterostructural design play crucial roles in mechanical behaviors. Here the effect of gradient interfaces on mechanical behavior was investigated in a laminated Cu–30Zn sample composed of coarse-grained and ultrafine-grained layers. Tensile tests revealed a superior strength-ductility synergy with extraordinary strengthening and work hardening. By combining the measurements of height contour and strain distribution using digital image correlation, the development of strain gradient was detected in the near-interface zone during tension, which was caused by the mechanical incompatibilities across interfaces and the synergetic constraint between layers. The intensity of strain gradient in the near-interface zone increased with tensile strain, which was accommodated by the accumulation of geometrically necessary dislocations, thereby resulting in extra back stress and dislocation strengthening.

A step deformation method for superplasticity improvement of coarse-grained Ti–15V–3Cr−3Sn−3Al

Ti–15V–3Cr–3Sn–3Al (Ti–15–3), a kind of metastable beta titanium which has high specific strength and good cold-formability, is highlighted for applications in the aerospace manufacture industry. However, the technique for improving its formability at elevated temperatures is still a challenge at present. In this work, a step deformation method is proposed for superplasticity improvement of coarse grained Ti–15–3 plates at temperatures around its beta transus. The effects of the strain rate and the strain at the first stage on the superplasticity are investigated. The results show an increase of the strain rate sensitivity and a decrease of the flow stress under the step deformation mode compared to those obtained under constant strain rates at 780 °C. The maximum strain to failure obtained in the step mode is 93% higher than that deformed in the constant strain rate mode. Strain rates, strains at the first stage, and temperatures have influences on the superplasticity improvement. The deformation mechanism is concluded as subgrain formation accommodated by grain boundary sliding rate-controlled by dislocation climb. The improved m value in the step deformation is accounted to the extra dislocation density produced during the strain rate reduction.

Quasicontinuum simulations of geometric effect on onset plasticity of nano-scale patterned lines

Onset plasticity of metallic nano-lines or nano-beams is of considerable scientific and technological interest in micro-/nano- mechanics and interconnects of patterned lines in electronic devices, where capability of resistance to deformation is important. In this study, a multiscale quasicontinuum (QC) method was used to explore such an issue in a nano-scale copper (Cu) line protruding from a relatively large single crystal Cu substrate during compression. The results show that the yield stress of a rectangular beam on the substrate can be greatly reduced compared with that of a flat surface of the same area. For the rectangular line, the aspect ratio (width/height) affects dislocation morphology at the onset plasticity without much change of yield stress. However, for the trapezoidal line, the yield stress decreases with the base angle (α), especially when the α is over 54.7°. As the sidewall orientation changes from 〈100〉 at α = 0°, then to 〈111〉 at α = 54.7° and finally to 〈110〉 at α = 90°, a higher surface energy could enable easier dislocation formation and lower yield stress. Meanwhile, it is found that the interaction between the line and the support substrate also shows a great effect on yield stress. Moreover, although it is possible to open two extra dislocation slip planes inside from the two bottom corners of the Cu line with the α over 54.7°, dislocation nucleation derived from them is only observed at α = 90°.

The interface effect of a nano-inhomogeneity on the fracture behavior of a crack and the nearby edge dislocation

The interface effect of a nano-inhomogeneity on the elastic–plastic fracture behavior of a crack with the influence of nearby edge dislocation is studied by applying the surface/interface stress model. When the inhomogeneity and the crack become nano-sized, the bimaterial interface contributes considerable stresses to the crack surfaces, which are usually ignored in the traditional micro/macro crack problems. With the existence of nano-inhomogeneity, an extra dislocation near the crack may be either absorbed into the crack (so the crack grows bigger) or repelled away by the crack and the inhomogeneity. The Zener–Stroh mechanism is employed to model the nano-sized fracture behavior and the generalized Irwin plastic zone correction is taken to improve the fracture analysis at the crack tips. Using the complex potential functions and distributed dislocation method, the stress intensity factor, plastic zone size, and crack tip opening displacement are evaluated by solving the Cauchy singular integral equations. The numerical examples prove that the values of stress intensity factors, plastic zone size and crack tip opening displacement are sensitive to the interface properties of the current nano-scaled problem, especially when the inhomogeneity is small. When the extra edge dislocation is close to the sharp crack tip, the stress intensity factor increases significantly, which indicates the crack has a potential to absorb the nearby dislocation. Further investigations find that mixed mode loading conditions always induce much higher stress intensity factors, plastic zone size, and crack tip opening displacement comparing to pure mode I loading problem.

On the nonlinear anelastic behaviour of AHSS

It has been widely observed that below the yield stress the loading/unloading stress-strain curves of plastically deformed metals are in fact not linear but slightly curved, showing a hysteresis behaviour during unloading/reloading cycles. In addition to the purely elastic strain, extra dislocation based micro-mechanisms are contributing to the reversible strain of the material which results in the nonlinear unloading/reloading behaviour. This extra reversible strain is the so called anelastic strain. As a result, the springback will be larger than that predicted by FEM considering only the recovery of the elastic strain. In this work the physics behind the anelastic behaviour is discussed and experimental results for a dual phase steel are demonstrated. Based on the physics of the phenomenon a model for anelastic behaviour is presented that can fit the experimental results with a good accuracy.

On the plastic zone correction of a Zener–Stroh crack interacting with a nearby inhomogeneity and an edge dislocation

The investigation on the interaction among a Zener–Stroh crack, a nearby inhomogeneity/inclusion and an extra edge dislocation has been carried out. A Zener–Stroh crack is formed by pileup of net dislocations. With the existence of a nearby inclusion, an extra dislocation near the crack may be either absorbed into the crack (so the crack grows bigger) or repelled away by the crack and the inclusion. The result depends on the force on the dislocation from the crack and inclusion. In our current study, the plastic zone correction at the crack tips is adopted, where the plastic zone size (PZS) is determined by our newly developed generalized Irwin approach since the stress fields around the crack tips are mixed modes. With the distributed dislocation method, the expressions for the stress intensity factor, PZS and crack tip opening displacement (CTOD) are obtained for different sets of geometric and material parameters. Numerical examples show that when the extra single edge dislocation is located near the sharp tip of the Zener–Stroh crack, the normalized PZS and normalized CTOD become very sensitive to the position of the edge dislocation. It is also observed that the mixed mode problem usually has larger PZS and CTOD comparing to the pure mode I loading problem.

Advantage in solar cell efficiency of high-quality seed cast mono Si ingot

We have grown 50 cm2 mono Si ingots by the seed cast technique. The carbon and oxygen concentrations of the ingots have been significantly reduced by improving the gas flow condition and coating. The dislocation density was also reduced by eliminating the extra dislocation generation sources. Owing to these developments, the lifetime of wafers has reached 465 µs. Finally, the efficiency of 18.7% has been achieved, which is comparable to 18.9% of the reference Czochralski (CZ) Si wafer.

On the formation mechanism of faulted dipolar loops in gamma -TiAl

] Some new geometrical features of 1/6<112] faulted dipolar loops have been observed in a Ti-52 at.% Al alloy deformed at 77 K. It was observed that the 1/6<112] dipole could switch its alignment from <101] to <011] directions, and vice versa; also the loops in alignment along both <101] and <011] could be created by the same parent superdislocation along its line and, on further movement of the parent dislocation, the two loops could attach to one another at the meeting point. No extra dislocation debris or segments were created at either the turning point or the attachment site. These particular features have been discussed on the basis of the available models for the formation mechanism of the dipoles. It is predicted that the particular features can be created only under a favourable stress and no such features are expected for 1/3<211]faulted dipolar loops.

The concentration dependence of Ir distribution and dislocation structure of Mo-Ir single crystals

Core-like radial inhomogeneities rather than axial striations or cells were shown to be the source of extra dislocation modifying the substructure of high purity Mo single crystals when doped with Ir in the range of 0.01 to 1.5 at%. From this result the observed non-monotonic dependence of perfection on mean Ir concentration can be explained.

Grain boundary dislocations and the mechanism of sliding in symmetrical [011] tilt Al bicrystals

SUMMARYObservations of grain boundary dislocations in [011] tilt bicrystals of aluminium after high temperature shear tests have led to a concept for grain boundary sliding that involves extrinsic dislocations moving through an array of structural dislocations. Each extrinsic dislocation consists of a group of structural dislocations with closer than normal network spacing caused by the presence of an extra dislocation with the same Burgers vector. The extrinsic dislocation exhibits a long range strain field and can move individually through the array of structural dislocations. The movement involves displacements in the structural network analogous to atom displacements during crystal slip. The width of the extrinsic dislocation on the grain boundary plane is dependent on the degree of accommodation of the extra dislocation by the structural array and this appears to increase with increasing density of structural dislocations.

Bypassing of dislocations past particles by a climb mechanism

The bypassing of dislocations past coherent, mismatch-free precipitates during creep conditions by a climb mechanism has been quantitatively analysed. Such passage occurs under the influence of an opposing force due to the extra dislocation line length produced during the bypassing. The analysis showed that the opposing stress varies proportionally to the applied stress, and furthermore that this relationship was quite insensitive to the interparticle spacing and the particle size. It is suggested that this process is the cause of breaks in the stress-creep rate relationship frequently observed in precipitation hardened alloys.

Dynamic strain ageing in mild steel

The grain size dependence of the yield and flow stresses for mild steel have been investigated over the temperature range 20°C2–400°C and dislocation densities of fine grained specimens have also been measured over the same temperature interval. It is found that the mechanisms by which strain ageing increases the flow stresses are the creation of an extra dislocation density and precipitation on the dislocations. Generation of high dislocation densities is associated with jerky stress-strain curves and also with smooth stress-strain curves when precipitates on dislocations grow to a visible size. The temperature at which the strain ageing peak occurs in the flow stress of mild steel is shown to depend on the nitrogen content of the steel. It is concluded that the grain size dependence of the flow stress during dynamic strain ageing arises from the difficulty of propagating yielding through contiguous grains in which dislocation sources are rapidly relocked by solute atoms.