Deformation heterogeneity in copper oligocrystals using high-resolution stereo DIC

Effect of annealing treatment on microstructure and mechanical properties of lightweight steels micro-alloyed with vanadium and niobium

In situ neutron diffraction revealing the achievement of excellent combination of strength and ductility in metastable austenitic steel by grain refinement

Elucidating the deformation behavior of as-cast B2 CoTi intermetallic using in-situ tensile testing

A crystalline orientation dependent transformation model has been developed and finite element simulations clarify the effects of the crystalline orientation on the deformation and transformation behavior of TRIP steels. The results revealed that the deformation and transformation behavior of single crystal substantially depend on the maximum value of the Schmid factors of all slip systems. Decreasing the value of the Schmid factors raises the yield stress and drives the transformation in the early stage of deformation. In the later stage of deformation, the transformation behavior depends on the rotation of crystalline orientation due to the non-uniform deformation in the crystal grain. Meanwhile, the deformation and transformation behaviors of polycrystalline is strongly affected by not only individual crystalline orientation but also the situation of the distribution of crystalline orientation. The increase in the deviation of the distribution of the crystalline orientation causes significant non-uniform deformation due to misalignment of crystalline orientation with respect to the neighboring crystals, which promotes the transformation. Therefore, the transformation and deformation behavior may not be reproduced by the simple superposition of those of single crystals.

A study of ultra-thin gold films and thin-walled nanoboxes has confirmed that length scales in terms of dislocation spacing can predict flow stress. Initial stages of deformation conform to linear hardening with average dislocation spacing controlled by the number of geometrically necessary dislocations in a pile-up. Later stages of deformation exhibit parabolic behavior with Taylor hardening interpreted in terms of a dislocation density described by the total line length of prismatic loops per unit volume. Comparisons of 20 and 40 nm thick planar films could be made to 205 nm high hollow gold nanoboxes with a wall thickness of 24 nm. These highly constrained, ultra-thin planar films demonstrated increased hardness from about 2 to 10 GPa with strains of 20 percent while less constrained nanoboxes increased from 0.8 to 4 GPa for the same strain magnitude.

Plastic deformation process of Ti alloys depends on the competition of active deformation modes: slip and twinning. The specific deformation mechanism depends on the availability of the specific deformation modes as well as on the critical resolved shear stress for slip and the twin activation stress. Effort has been made to provide information on the different slip systems other than prismatic slips, which get activated during the channel die compression process. It is found that, though at the initial stage of deformation, prismatic slip is prevalent, but other slip systems, especially pyramidal types, also appear at the later stage of deformation. Transmission electron microscopy results confirm the presence of pyramidal slips beside prismatic ones. It is also observed that twining does not take place in those grains having the most stable orientation as per the hexagonal close packed system. Information on the critical resolved shear stresses of different glide systems for T40 is also provided.

A study was conducted of the metal flow and fracture in a forging process involving both extrusion flow into a hub and lateral flow into a flange. In the initial stage of deformation, depending on the die angle and friction, either the extrusion mode of flow or the upsetting mode occurs, but never both. If the upsetting mode acts initially, tensile strains occur on the hub surface, which lead to fracture. The extrusion mode results in compressive strains and fracture does not occur. Slip-line field analysis is utilized to predict the combinations of die angle and friction coefficient that delineate between the initial extrusion and upsetting modes of deformation. In addition, an experimental study was performed using sintered powder billets of 601 AB aluminum alloy as the workpiece material. Measurements of the surface strains and workpiece deformation profiles confirm the slip-line field analytical predictions of initial deformation mode, even though the material properties do not match those required for slip-line analysis. After the initial stage of deformation, both extrusion and upsetting flow occur. Surface strain states on the hub surface that were compressive in the initial stage of deformation now become tensile and lead to fracture in the later stages of deformation. The strains at fracture obey a fracture criterion established previously by the authors. Results of this study can be utilized to aid preform and die design in extrusion-forging processes. In addition, the experimental techniques may aid the analysis of other deformation processes.

An investigation of slip transmission during uniaxial tensile deformation of a coarse-grained copper via EBSD characterization and crystal plasticity modelling

Copper-2.5 at%Al alloy single crystals oriented for a single slip system were deformed in tension at 600 and 700°C. In the later stage of deformation, the condition of steady state deformation was satisfied; namely the specimens have been deformed at a constant stress, independent of strain. In this steady state deformation stage, the internal stress and the effective stress were determined by the stress dip test. The dislocation substructure formed in this stage were observed by the etch pit technique. The results obtained are as follows:(1) The tensile axis rotated into the direction predicted by the single glide on the (111)[\bar101] slip system. When the tensile axis approached the [001]-[\bar111] symmetry line, the steady state deformation was observed.(2) The strain rate is proportional to the nth power of the steady state stress and the n value is about 5.(3) The ratio of the internal stress to the applied stress is 0.6∼0.9 and depends on the applied stress. The effective stress increases with the applied stress.(4) The parameter m* describing the effective stress sensitivity of dislocation glide velocity was estimated assuming the mobile dislocation density to be proportional to the square of the applied stress and the m* value was found to be 1.1.(5) The subgrains elongating in the [1\bar21] direction are found on the (111) plane. On the (1\bar11) plane the rows of dislocations are found along the direction roughly parallel to the trace of the primary slip plane, and the high dislocation density regions, approximately perpendicular to the slip trace, are observed.

The influence of nitrogen on the mechanical properties of two high Ni containing advanced austenitic stainless steels with low stacking fault energies is investigated. The results show that increase of nitrogen content greatly increases both strength and elongation of the steel at the same time. At the cryogenic temperature, the steels show a twin induced plasticity behavior. Ab initio calculations indicate that the increase of nitrogen slightly increases the stacking fault energy and consequently the critical shear stress for twin initiation in the steel. However, addition of nitrogen significantly increases the flow stress. This leads to a smaller critical strain for twin initiation and promotes deformation twinning in the high nitrogen steel. This is confirmed by the microstructure investigation. Deformation in steels is a competitive process between slip and twinning. Dislocation slip is dominant at low strain range, but formation of stacking fault and twinning become important in the later stages of deformation. At cryogenic temperature, it is mainly deformation twinning. The influence of nitrogen addition on magnetic property and its effect on deformation twinning are also discussed. The present study increases the understanding for the development of high‐performance and low‐cost advanced austenitic stainless steels.

Orientation evolution in Hadfield steel single crystals under combined slip and twinning

The Eastern Highlands shear zone (EHSZ) is located on Cape Breton Island where it separates rocks of the Bras-d'Or and Aspy terranes within Ganderia in the Appalachian accretionary belt. It records late Silurian to Early Devonian shear with east-over-west kinematics (D1) and a second deformation event (D2) with oblique dextral and west-over-east kinematics of unknown timing. Field mapping and thin section observations identified a third deformation event (D3) with brittle ductile fabrics, brecciation and hydrothermal alteration of a granite on the Aspy side of the EHSZ. These new mapping results contribute to our understanding of the later stages of deformation and their interplay with hydrothermal alteration. They also provide the framework for further studies attempting to reconstruct the postorogenic reactivation of the EHSZ and the associated magmatic-hydrothermal processes that may have contributed to the mineral endowment of the orogen.

A framework for quantifying the thermomechanical response of polymer bonded explosives (PBX) at the microstructural level is developed using a cohesive finite element method (CFEM). This framework allows the contributions of individual constituents, fracture and frictional contact along failed crack surfaces to heating to be analyzed and tracked. Digitized micrographs of actual PBX materials and idealized microstructures with various distributions of grain sizes are used in the analysis. The analysis concerns impact loading of HMX/Estane with strain rates on the order of 104 – 105 s-1. Issues studied include large deformation, thermomechanical coupling, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating. The Estane matrix is described by a thermo-elasto-viscoelastic constitutive formulation, accounting for temperature dependence, strain rate sensitivity and strain hardening. The HMX crystals are assumed to be elastic under the conditions analyzed. Energy localization leading to formation of local hot spots as potential ignition sites is primarily due to the viscoelastic dissipation in the matrix in early stages of deformation and frictional heating along crack surfaces in later stages of deformation. Microstructure-response relations that can be used in the design of soft energetic composites are established.

Room temperature creep was investigated for a commercial Ti-6Al-4V alloy containing 80 ppm to 720 ppm hydrogen. It was shown that dissolved hydrogen promoted the creep of this alloy at room temperature, markedly increasing both the creep strain and rate in the primary stage. The results indicated that dissolved hydrogen does not change the primary creep mechanism, but rather increases mobility of glide dislocations. It is suggested that dissolved hydrogen atoms soften the alloy at the beginning of deformation due to this increase in dislocation mobility. The initial deformation results in hydrogen build-up near the subboundaries due to the “sweeping” effect of the glide dislocations, thereby causing hydrogen embrittlement in a later stage of deformation.

Inhomogeneity of plastic deformation and fracture modes in two-phase alloys