Dislocation Slip Deformation Research Articles

High strength and high work hardening rate in oxygen gradient Ti-15Mo alloy

The low work hardening is a prominent deficiency for high-strength titanium (Ti) alloys. The gradient design of oxygen content was adopted to realize the coupling deformation of {332}<113> twinning and dislocation slip in the Ti-15Mo alloy. This oxygen gradient alloy exhibited an optimal balance of yield/tensile strength (700 and 848 MPa) and elongation (25 %), with remarkable work hardening behavior. The dominated dislocation slip deformation and the solution strengthening of oxygen atoms in the oxygen-rich region resulted in a remarkable increase in yield strength. The successive formation of {332}<113> twins and piled-up geometrically necessary dislocations around the twin boundaries in the oxygen-free region induced remarkable back stress strengthening, maintaining the high work hardening rate, which resulted in a stable increase in strength. The twins and dislocations formed at the crack tips effectively hindered the cracking behavior, avoiding premature necking. The present study provides a novel idea for designing oxygen layer-distributed Ti alloys, which further improves the strength–ductility tradeoff.

Microstructure and dynamic mechanical behavior of wire-arc additive manufactured high-strength steel

The working environment of high-strength steel components produced by wire-arc additive manufacturing (WAAM) often faced challenges of explosion and impact, but the microstructure evolution and dynamic mechanical behavior of WAAM produced high-strength steel were still unclear. In this study, low alloy high-strength steel （HSS） components were produced by WAAM. Split Hopkinson Pressure Bar (SHPB) tests at different strain rates were carried out on WAAM-HSS, and the specimens before and after dynamic compression were characterized by techniques such as SEM, EDS, XRD, EBSD, etc. The results showed that： The as-built WAAM-HSS plate mainly consists of five typical structures: lath martensite (ML), acicular ferrite (AF), polygonal ferrite (PF), martensite-austenite (M-A), and precipitated phase δ. The central area of the precipitated phase is Si and Mn, and the C element is mainly distributed in the surrounding area of the precipitated phase. Under dynamic compression, the as-built WAAM-HSS exhibited obvious yield phenomenon, and the yield stress increased with the increase of strain rate. The yield-strengthening mechanism is mainly contributed by grain refinement strengthening mechanism and dislocation density strengthening mechanism. Under dynamic compression, the martensite transformation occurred in the WAAM-HSS. The deformation mechanism is dislocation slip and twinning, which compete with each other. Dislocation slip deformation is dominant at low strain rate and twinning deformation is more dominant at high strain rate.

Effect of Cu-15Sn on the microstructure and mechanical properties of pressureless sintered Fe-Co-Cu alloy

Fe-Co-Cu alloys were prepared by pressureless sintering at 875 °C. The effects of pre-alloyed Cu-15Sn sintering aids on the microstructure and mechanical properties of Fe-Co-Cu alloys were investigated. The results show that the Cu-15Sn aids makes the sintering densification process of Fe-Co-Cu alloy changes from solid phase sintering to instantaneous liquid phase sintering, which improves atomic diffusion rate and relative density of alloys, and also makes the matrix grains grow up. The relative density, hardness and bending strength of Fe-10 wt% Co-15 wt% Cu alloy can be significantly improved by adding 7 wt% Cu-15Sn, which can reach 96.34 %, 85 HRB, and 1857 MPa, respectively. The volume fraction of Cu-rich phase, the relative density of the alloy and the grain size have great influence on the hardness and bending strength of the alloy. When there are too many liquid phases, the relative density of the alloy decreases, making it difficult to continue to improve the strength of the alloy. Intergranular brittle fracture occurs mainly in α-Fe phase grains, but dislocation slip deformation occurs in Cu-rich phase grains. With the increase of volume fraction of Cu-rich phase and relative density, the alloy gradually exhibits macroscopic plasticity.

Influence of interface with mismatch dislocations on mechanical properties of Ti/Al nanolaminate

As a representative boundary, interphase-interface may affect the strength or ductility of multilayered composites dramatically. However, the effect of the interface with mismatch dislocations on the mechanical behavior of multilayered composites is still not clear. In the present work, we performed molecular dynamics simulations to investigate the effect of interface structures and layer spacing on the mechanical properties of the Ti/Al nanolaminate. The results indicate that there are two transitions of the plastic deformation mechanism in the Ti layer with the increase of layer spacing in the sample with a coherent interface. The plastic deformation mechanism evolves from one that is dominated by dislocation to the phase transformation from the hcp-Ti to the fcc-Ti mode, which transfers to the dislocation slip deformation again. For the samples with an incoherent interface, the plastic deformation is dominated by the transformation from hcp-Ti to fcc-Ti, regardless of the variation of layer spacing, while the plastic deformations in the Al layers are mainly dislocations confined in the layer slip in the samples with both coherent and incoherent interfaces. When the layer spacing is larger than 6.6 nm, an obvious second hardening is observed due to the superior dislocation storage ability of the Ti/Al laminate with the incoherent interface. Meanwhile, extraordinary ductility is obtained when optimal layer spacing is employed in the Ti/Al laminate. Moreover, the phase transformation mechanism of hcp-Ti to bcc-Ti has also been explicated in the present work. The general conclusions derived from this work may provide a guideline for the design of high-performance Ti/Al multilayer and alloy devices.

Effects of Strain Rate and Texture on the Tensile Behavior of Pre-strained NiCr Microwires

The stress–strain behavior and strain rate sensitivity of pre-strained Ni80Cr20 (Ni20Cr) were studied at strain rates from 4.8×10–4 s–1 to 1.1×10–1 s–1. Specimens were prepared through cold drawing with abnormal plastic deformation. The texture of the specimen was characterized using electron backscatter diffraction. Results revealed that the ultimate tensile strength and ductility of the pre-strained Ni20Cr microwires simultaneously increased with increasing strain rate. Twinning-induced negative strain rate sensitivity was discovered. Positive strain rate sensitivity was present in fracture flow stress, whereas negative strain rate sensitivity was detected in flow stress values of σ0.5% and σ1%. Tensile test of the pre-strained Ni20Cr showed that twinning deformation predominated, whereas dislocation slip deformation dominated when twinning deformation reached saturation. The trends observed in the fractions of 2°-5°, 5°-15°, and 15°-180° grain boundaries confirmed that twinning deformation dominated the first stage.

Devitrification-induced an Ultrahigh Strength Al-based Composite Maintaining Ductility

A new Al-based composite consisting of submicron-sized α-Al matrix embedded with precipitated intermetallic phases was developed by controlling the devitrification process of an Al–Ni–Y–Co–La amorphous alloy. Such a homogeneous composite structure presented an ultrahigh strength of about 1.34 GPa and a large compressive plastic strain up to 22%. The unique mechanical properties during compression are mainly attributed to the dislocation slip deformation of ductile α-Al matrix and the shear-induced refinement of strengthening intermetallic phases.

Effect of thermo-mechanical treatment on superelastic behavior of Ti–19Nb–14Zr (at.%) shape memory alloy

The effect of thermo-mechanical treatment on shape memory effect and superelastic behavior of Ti–19Nb–14Zr (at.%) shape memory alloy is investigated. The annealed specimens are aged at 300 °C for different times from 10 min to 20 h followed by ice water quenching. Both reverse and forward martensitic transformation peaks are identified in the annealed and aged specimens. Fine ω particles with a dimension of ∼20 nm in β grains are clearly observed in the TEM micrograph. The strains of the annealed specimen are completely recovered by heating, starting from the 2nd cycle to the 5th cycle due to the reverse martensitic transformation. Non-martensitic superelasticity strain of about 6% is obtained in the aged-10 min specimen. It is noted that the ω phase hinders the formation of stress-induced martensite from the β phase, suggesting that the non-martensitic superelasticity should be attributed to the presence of ω + β phases. Also, the non-martensitic superelasticity can be improved through increasing the critical stress for dislocation slip deformation by controlling the size and amount of ω phase and/or introducing plastic strain. It is found that this non-martensitic superelasticity can be obtained from the present alloy with combined ω + β phases by introducing additional precipitation hardening and/or work hardening. The elastic modulus of the aged-10 min specimen after aging and cyclic training is approximately of 14 GPa, which is close to that of cortical bone of human beings (7–35 GPa).

Plastic Deformation and Damage Behavior of AL6XN Super-Austenitic Stainless Steels

Tensile and compressive deformation and damage behaviors of Al6XN super-austenitic stainless steels were examined at different strain rates. The deformation and fracture surfaces were characterized by scanning electron microscopy (SEM). It was found that the uniaxial deformation (tensile or compressive) behaviors of Al6XN stainless steel shows a low strain rate sensitivity over the range of 10-4s-1 - 10-2s-1. The tensile and compressive yield strengths measured are nearly comparable. The steel shows a good tensile plasticity. Dislocation slip deformation is the main characteristic of uniaxial deformation. All fracture surfaces induced by tensile deformation at different strain rates can be divided into two parts, i.e., fibrous zone and shear lip zone. The fibrous zone consists of dimples with a bimodal size.

Surface Deformation Features in Ultrafine-Grained Copper Cyclically Stressed at Different Temperatures

The surface deformation features in ultrafine-grained copper produced by equal channel angular (ECA) pressing, which was cyclically deformed at temperatures between room temperature and 573 K under a constant stress amplitude of 200 MPa, were investigated. It was found that the surface deformation features and damage behaviour are strongly dependent upon the testing temperature. For examples, large-scale shear bands (SBs) formed at room temperature, whereas finer and discontinuous SBs, instead of large-scale SBs, were found to become the dominant feature with increasing temperature (below recrystallization), resulting from the reduction in quantity and volume fraction of grain boundaries as a consequence of grain growth and the enhanced dislocation slip. When the temperature is above recrystallization, no clear SBs were observed and dislocation slip deformation within grains governed the plastic deformation of UFG copper, causing nucleation of cracks along slip bands in grains or along grain boundaries, in contrast to the nucleation along SBs at temperatures below recrystallization.

Investigation of mechanical properties in cryomilled dispersion strengthened aluminium

In the present work dispersion strengthened (DS) commercial pure aluminium (cryomilled) has been studied into further detail with respect to deformation behaviour. During uniaxial tensile straining both deformation temperature and initial applied strain rate have been varied. The present work applied DS aluminium as rolled sheets. A thorough investigation is carried out with respect to grain structure and deformation structure of the cold worked material. The uniaxial tensile straining experiments revealed that the rolled sheet exhibit properties that are usual for this kind of processed material, i.e. good elevated temperature properties. Additionally, the rolled sheet achieved very good ductility for specific test parameters. While dislocation slip was identified as the most important deformation mechanism in the lower temperature regime, in the higher temperature regime the material exhibits a combination between grain boundary sliding and dislocation slip deformation mechanism. For the rolled sheet material, there were indications of dynamic grain growth or grain boundary migration for the specimens exhibiting the highest fracture strains. The latter aspect showed to be dependent on applied strain rate variations and the preprocessing procedure of the material.

Common features of intragranular dislocation slip and intergranular sliding

Abstract An analogy between the occurrence of intragranular dislocation slip and intergranular (grain-boundary) sliding at different structural levels is discussed. Observation of in-situ superplastic tensile deformation in a scanning electron microscope on a Pb-62wt.%Sn eutectic alloy showed that the phenomenon of intergranular sliding on the scale of the entire deformed volume and on the scale of grain groups is similar to the occurrence of intragranular dislocation slip deformation at these microstructural levels.