Strong Dislocation Interactions Research Articles

New strategy in light-weight and ultrastrong Ti40Nb15Mo30(NbC)15 refractory complex concentrated alloy

Traditional refractory high-entropy alloys (RHEAs) generally exhibit a trade-off between high-temperature strength and light weight. In present work, a novel design strategy based on tailoring element distribution is proposed to achieve excellent high-temperature strength at a density lower than 7 g·cm-3. Specifically, a Ti40Nb15Mo30(NbC)15 composite was designed and prepared by powder metallurgy. The composite is found to be composed of two ultrafine-grained (UFG) phases including a body-centered cubic (bcc) solid-solution phase and a face-centered cubic (fcc) ceramic reinforcement phase (Ti, Nb)C. The as-sintered composite shows a uniform and UFG microstructure where two phases are interconnected. Due to this unique microstructure, the Ti40Nb15Mo30(NbC)15 composite displays superb specific yield strengths among surveyed RHEAs, complex concentrated alloys, and metal-matrix composites from 800°C (243 MPa·g-1·cm3) to 1000°C (127 MPa·g-1·cm3). The outstanding high-temperature compressive strength was found to be associated with high resistance to dislocation motion and strong dislocation interactions in both the bcc and fcc phases. The phase interface after hot compression remained semi-coherent, vindicating its high stability. The high-density of stable phase interfaces not only retards the dislocation motion due to the large image force near the phase boundary but also induces a high value of activation energy for diffusion. The high activation energy can further achieve significant microstructure stability even after a long-term annealing (36 h) at 1000°C. This work provides new perspectives for the design and application of light and ultrastrong refractory complex concentrated alloys (RCCAs) by comparison to the insufficient strength of many traditional and light RCCAs.

Quantifying chemical fluctuations around medium-range orders and its impact on dislocation interactions in equiatomic CrCoNi medium entropy alloy

Equiatomic multi-principal element alloys manifest unique mechanical properties derived from the single-phase solid solutions with compositional disorders. Recently discovered medium-range orders (MROs) with a size of 1–5 nm in CrCoNi medium entropy alloy acclaimed measurable impact on mechanical properties, which, however has been questioned by density functional theory calculations. Here we report that the formation of MROs is accompanied by the redistribution of constituent elements, and the subtle chemical composition fluctuations could by directly probed and quantified by using a state-of-the-art aberration-corrected transmission electron microscope. The presence of such MROs contributes to an appreciable increase in yield strength (∼40 MPa) with a higher work hardening rate, originating from the strong dislocation interactions with MROs at the incipient plastic deformation. These findings demonstrate that MROs have a significant impact on reducing the mean free path of full/partial dislocations along a specific slipping plane, offering a new avenue for strengthening of equiatomic element alloys by tuning local composition and atomic configurations.

Enhanced tensile properties of CrMnFeCoNi0.8 high entropy alloy with in-situ TiC particles

In-situ TiC particulate reinforced CrMnFeCoNi0.8 high entropy alloy (HEA) matrix composites were prepared by induction melting method. The effect of TiC content upon the microstructure and tensile properties of as-cast samples were analyzed and discussed. The HEA matrix has single-phase face-centered cubic (FCC) structure, determined by X-ray diffraction (XRD) pattern. Compared to the CrMnFeCoNi0.8 matrix, 7.5 vol%TiC/CrMnFeCoNi0.8 composite possessed remarkable yield strength (517.1 MPa vs 240.9 MPa), ultimate tensile strength (926.2 MPa vs 527.5 MPa) and excellent uniform elongation (38.0%). Strong dislocation interactions, load-bearing effect and Orowan mechanism afforded by in-situ TiC particles, together with solid-solution strengthening via C and Ti elements, are responsible for the strength enhancement.

Thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars with high-density dislocations

Micropillar compression testing was performed on bcc solid-solution single-crystal micropillars with two diameter (d) ranges (2–3 and 5–6 μm) prepared on the surfaces of fully annealed and subsequently 10% cold-rolled 18Cr ferritic steel specimens. This was done to investigate the effect of initial dislocations on the plastic deformation of micron-sized crystals. The 10% cold rolling process significantly increased the stress level (provided by an activated single slip system) required to initiate slip in the annealed micropillars with different diameter ranges. In the case of the annealed micropillars, the strain-rate sensitivity (m) of the yield stress becomes lower for larger specimens (the m value decreased from 0.12 to 0.04). However, the 10% cold-rolled micropillars exhibited a much lower strain-rate sensitivity regardless of the specimen size. The activation volume (v*) of the annealed micropillars showed a significant specimen size dependency (v*∝d1.47), consistent with published data on the micropillar compression experiments of single-crystals of the bcc metals and alloys. For the 10% cold-rolled micropillars, the high v* values (88b3 and 112b3) showed a slight dependency on the specimen size (v*∝d0.33). The v* values appeared somewhat higher but were equivalent to those of bulk-size specimens, suggesting that the initial high-density dislocations would provide sufficient sources for dislocation multiplication in the micron-sized specimens under compression. The observed high v* values might be associated with the strong dislocation interactions in the α-(Fe, Cr) solid-solution matrix.

Primary hot working characteristics of an as-cast and homogenized nickel base superalloy DMR-742 in the sub and super-solvus temperature regime

Primary deformation behaviour of a medium alloyed nickel base superalloy DMR-742 in the as-cast and homogenized condition (furnace cooled) was evaluated in the sub-solvus and super-solvus temperature regimes. The flow stress in the sub-solvus regime was found to be highly sensitive to temperature. The contribution of back stress to the flow stress sensitivity was estimated using the microstructure dependent analytical equations and compared with the back stress estimated using experimental steady state flow stress. It was established that Orowan looping or by-passing mechanism contributes to the back stress resulting in high flow stress sensitivity in the sub-solvus regime. In the super-solvus regime, the material was relatively temperature insensitive and exhibited discontinuous dynamic recrystallization (DDRX) with a characteristic power dissipation value of 38–44% in three different temperature-strain rate regimes. Disregarding the industrially non-viable low strain rate regime, the recrystallization kinetics in the moderate and high strain rate regime was estimated using grain size factor compensated grain orientation spread maps. A higher rate of recrystallization observed in the material deformed at high temperature-strain rate was attributed to relatively strong dislocation interactions and weak recovery. The true stress exponent, activation volume and energy estimated using back stress and elastic modulus compensated flow stress revealed that diffusion controlled climb and cross-slip as the rate controlling deformation mechanism. Side-pressing under high secondary tensile stress conditions was used to evaluate the workability and validate the experimental findings.

Tuning element distribution, structure and properties by composition in high-entropy alloys.

High-entropy alloys are a class of materials that contain five or more elements in near-equiatomic proportions1,2. Their unconventional compositions and chemical structures hold promise for achieving unprecedented combinations of mechanical properties3-8. Rational design of such alloys hinges on an understanding of the composition-structure-property relationships in a near-infinite compositional space9,10. Here we use atomic-resolution chemical mapping to reveal the element distribution of the widely studied face-centred cubic CrMnFeCoNi Cantor alloy2 and of a new face-centred cubic alloy, CrFeCoNiPd. In the Cantor alloy, the distribution of the five constituent elements is relatively random and uniform. By contrast, in the CrFeCoNiPd alloy, in which the palladium atoms have a markedly different atomic size and electronegativity from the other elements, the homogeneity decreases considerably; all five elements tend to show greater aggregation, with a wavelength of incipient concentration waves11,12 as small as 1 to 3nanometres. The resulting nanoscale alternating tensile and compressive strain fields lead to considerable resistance to dislocation glide. In situ transmission electron microscopy during straining experiments reveals massive dislocation cross-slip from the early stage of plastic deformation, resulting in strong dislocation interactions between multiple slip systems. These deformation mechanisms in the CrFeCoNiPd alloy, which differ markedly from those in the Cantor alloy and other face-centred cubic high-entropy alloys, are promoted by pronounced fluctuations in composition and an increase in stacking-fault energy, leading to higher yield strength without compromising strain hardening and tensile ductility. Mapping atomic-scale element distributions opens opportunities for understanding chemical structures and thus providing a basis for tuning composition and atomic configurations to obtain outstanding mechanical properties.

Orientation dependence in nanocutting of Fe single crystals: A molecular-dynamics study

We study systematically the nanocutting of an Fe single crystal in dependence of the surface orientation and cut direction. We find that in the general case – where the edge dislocations formed at the cutting edge glide obliquely to it – a complex, three-dimensional dislocation network is created. If, however, the edge dislocations formed align with the cutting edge direction, then a simple (two-dimensional) cutting mechanism is realized, in which dislocations glide consecutively without interaction. The shape of the chip is governed by the direction of the activated glide systems. For strong dislocation interaction, a grain boundary may form separating the chip from the workpiece. A cutting hardness is defined; the calculated values are in good agreement with the ploughing hardness measured in scratching simulations. For selected orientations, we find that instead of dislocation plasticity, twinning is activated. This requires smaller cutting forces and the created chip surfaces are smooth.

A minimum principle for microstructuring in rigid-viscoplastic crystalline solids

A minimum plastic power principle is proposed for a rigid-viscoplastic crystalline domain subdivided into two sets of lath-shaped regions, called bands. The lattice orientation in each band is assumed uniform and to differ infinitesimally from that in the other band. The proposed minimum principle yields the slip activity in the bands and semi-analytical expressions for the misorientation axis and orientation of band boundaries. These band boundary characteristics are predicted for f.c.c. lattice orientations near the ideal rolling texture components. Surprisingly, it found that the predicted band boundary characteristics closely match those of microstructural features called cell block boundaries reported in the experimental literature, except when the dislocations of activated slip systems are known to interact very strongly. This suggests that except when precluded by strong dislocation interactions, continuum extremum principles may also govern microstructural characteristics.

Local rules for approximating strong dislocation interactions in discrete dislocation dynamics

We consider the problem of approximating the strong dislocation interactions which occasionally take place during the numerical simulation of dense dislocation systems. Local rules are proposed to govern the behaviour of those tracking points that find themselves in an under-resolved environment. It is shown that these rules, utilizing only nearest-neighbour information, satisfactorily approximate more fully resolved calculations of localized dislocation–dislocation interactions, of extended structures such as junctions, and of higher order processes such as dislocation–junction and junction–junction interactions. Use of these rules should make it possible to do efficient large-scale numerical dislocation simulations, while still retaining the important qualitative effects arising from strong local interactions.

Accuracy and convergence of parametric dislocation dynamics

In the parametric dislocation dynamics (PDD), closed dislocation loops are described as an assembly of segments, each represented by a parametric space curve. Their equations of motion are derived from an energy variational principle, thus allowing large-scale computer simulations of plastic deformation. We investigate here the limits of temporal and spatial resolution of strong dislocation interactions. The method is demonstrated to be highly accurate, with unconditional spatial convergence that is limited to distances of the order of interatomic dimensions. It is shown that stability of dislocation line shape evolution requires very short time steps for explicit integration schemes, or can be unconditionally stable for implicit time integration schemes. Limitations of the method in resolving strong dislocation interactions are established for the following mechanisms: dislocation generation, annihilation, dipole and junction formation, pileup evolution.

The effect of electropulsing on dislocation structures in [233] coplanar double-slip-oriented fatigued copper single crystals

The effect of high-current-density electropulsing on dislocation structures in a coplanar double-slip-oriented copper single crystal that had previously been fatigued is reported. The results show that, after electropulsing, vein structures are transformed to cell structures with some dark regions. It is proposed that the thermal compressive stress caused by electropulsing activates a coplanar slip system and leads to strong dislocation interactions between primary and coplanar slip systems, thereby forming cell structures. Partial recrystallization may occur by electropulsing, leading to the appearance of some dark regions.

Study of the lattice strain relaxation in the Ga1-XAl X Sb/GaSb system by x-ray topography and high resolution diffraction

High resolution x-ray diffraction and topographic methods have been used to study lattice strain relaxation in the Ga1-XAl X Sb/GaSb system. Samples with layer thickness ranging between 0.1 and 6 μm and with Al concentration x=0.402±0.005 have been grown by molecular beam epitaxy at 550 °C on (0 0 1) oriented undoped GaSb LEC substrates. A first critical thickness (tC1), related to the misfit dislocation generation, has been found to be between 0.16 < tC1 < 0.20 μm. Due to the weak sensitivity of the rocking curve to the onset of relaxation, this result has been obtained by means of a double crystal topographic technique. A “plateau” region in the curve of the residual strain versus thickness has been observed for t ranging between 0.2 and 0.5 μm. The residual strain ɛres shows a dependence close to t–0.5 above a second critical thickness value tC2 slightly larger than 0.5 μm. Finally, in the last region above a layer thickness of 3 μm, strong dislocation interaction effects seem to affect the relaxation. A comparison with theoretical models has been made.

Anisotropie creep behavior of a nickel-based superalloy compared with single phase nickel solid solution and γ′ phase single crystals

The creep behavior of three different single crystal alloys with various orientations at a temperature of 1123 K was investigated. The anisotropic behavior of the nickel-based superalloy CMSX-4 has been reported recently [1–3]. These observations are compared with the anisotropic creep behavior of the isolated phases which build up the two phase superalloy. The matrix material is given by a nickel solid solution single crystal, whereas the γ′ phase is represented by the alloy Ni 75Al 22Ti 1Ta 2. A detailed analysis of the deformed samples with respect to phase nucleation and dislocation configurations has been carried out. Single phase specimens show strong dislocation interactions during primary creep, resulting in a more homogeneous distribution of activated glide planes and an increasing amount of non-screw dislocations in the secondary creep stage.

Planar colony of needle precipitates formed during solidification of a ferritic stainless steel

Micropillar compression testing was performed on bcc solid-solution single-crystal micropillars with two diameter (d) ranges (2–3 and 5–6 μm) prepared on the surfaces of fully annealed and subsequently 10% cold-rolled 18Cr ferritic steel specimens. This was done to investigate the effect of initial dislocations on the plastic deformation of micron-sized crystals. The 10% cold rolling process significantly increased the stress level (provided by an activated single slip system) required to initiate slip in the annealed micropillars with different diameter ranges. In the case of the annealed micropillars, the strain-rate sensitivity (m) of the yield stress becomes lower for larger specimens (the m value decreased from 0.12 to 0.04). However, the 10% cold-rolled micropillars exhibited a much lower strain-rate sensitivity regardless of the specimen size. The activation volume (v*) of the annealed micropillars showed a significant specimen size dependency (v*∝d1.47), consistent with published data on the micropillar compression experiments of single-crystals of the bcc metals and alloys. For the 10% cold-rolled micropillars, the high v* values (88b3 and 112b3) showed a slight dependency on the specimen size (v*∝d0.33). The v* values appeared somewhat higher but were equivalent to those of bulk-size specimens, suggesting that the initial high-density dislocations would provide sufficient sources for dislocation multiplication in the micron-sized specimens under compression. The observed high v* values might be associated with the strong dislocation interactions in the α-(Fe, Cr) solid-solution matrix.

Low cycle fatigue of a copper bicrystal under constant plastic strain control

Symmetrical tension-compression fatigue tests in an axial plastic strain range of 0.12 × 10 −3 to 2.75 × 10 −3 were performed on a copper bicrystal containing two highly misoriented component crystals one of which is oriented for single slip and another is oriented very closely for double slip. The grain boundary (GB) is roughly perpendicular to stress axis. Experimental results showed that this bicrystal exhibited a very high saturation stress and did not exhibit an apparent plateau in the cyclic stress-strain (CSS) curve. It seems that no persistent slip bands formed under higher strain amplitudes according to the analysis of the change of fatigue loop shape. Surface observation showed that at least three slip systems operated in both of the components of the bicrystal due to constraints of GB. The occurrence of multiple slip, which may result in very strong dislocation interaction, is considered to be responsible for the features of the CSS curve of the bicrystal. Intergranular microcracks and relatively large intergranular macro-cracks were frequently observed on the specimen surfaces even though small cumulative strains were applied. Apparently, the GB plays the most important role in the fatigue failure of the bicrystal to be studied.

Static and dynamic strain ageing in D0 3 — Ordered Fe 3Al

Starting from room temperature there exist three temperature regions in the deformation behaviour of Fe 3Al single crystals characterized by different TO and SRS values, the character of the stress strain curve and the slip line structure. In the intermediate region (80 °C to 500 °C) static and dynamic strain ageing was found in combination with strong dislocation interactions. This can be inferred from the clustering behaviour of slip bands and stress serrations (PLC effect). Stress relaxation experiments indicate that the ageing process in the “low” temperature region is based on pipe diffusion of either vacancies or solutes. At elevated temperatures (2 SOOT) the slip system has changed, no localisation of slip and no stress serrations are found.