Study of yield stress anomaly of Fe2MnAl single crystal by in situ TEM straining

L2₁-structured Fe₅₉Mn₁₇Al₂₄ shows a yield stress anomaly with a peak yield stress at 700?K. The aim of the work reported here was to determine the dislocation mechanisms involved in this anomalous behaviour by performing in situ straining on Fe₅₉Mn₁₇Al₂₄ single crystals in a transmission electron microscope at 300–900?K. Cross-slip of ?111? dislocations was frequently found to occur between {110} and {211} planes at all temperatures tested. At 300?K, dislocations were four-fold dissociated and the partials moved together under stress. At 700–800?K, partial dislocations with a Burgers vector of a/2?111? dominated the deformation. These partial dislocations moved independently in the ordered matrix in a jerky manner, with rapid motion between long periods of locking. X-ray diffraction measurements showed that the degree of L2₁-order slightly decreased with increasing temperature in the range 300–700?K, and dropped rapidly above 700?K. At 900?K, the material was B2-ordered. The increased yield stress at elevated temperatures is primarily attributed to the uncoupling of a/2?111? partial dislocations trailing shear-induced anti-phase boundaries.     

A new micromechanics-based scale transition model for the strain-rate sensitive behavior of nanocrystalline materials

An original two-step “three phase” elastic–viscoplastic scale transition model is developed based on the combined self-consistent and Mori–Tanaka schemes. A coated inclusion is embedded within a matrix, wherein the inclusion represents grain interiors and the coating of the inclusion mimics the effects of grain boundaries and triple junctions. The predominant behavior within the grain interiors is captured through dislocation glide, whereas grain boundary (GB) dislocation emission and absorption, as well as thermally assisted GB sliding, describe the deformation processes within the coating describing the GB affected zone. Furthermore, an imperfect interface is assumed between the inclusion and the coating to account for viscoplastic grain boundary sliding along a stick-slip mechanism. Results and discussion focus on the competitive roles of GB sliding, GB dislocation emission/absorption, dislocation sweep in grain cores and collective dislocation plasticity, and the origins of the pronounced strain rate sensitivity of fcc NC materials.     

In situ TEM observations of dislocation/anti-phase boundary interactions

L2₁-ordered Fe₅₉Mn₁₇Al₂₄ (in at%) single crystals were in situ strained at either 300?K or 700?K in a transmission electron microscope. At 300?K, the strain was accommodated by the glide of four-fold dissociated super-dislocations, whereas, at 700?K, the strain was accommodated by the glide of a/2?111? partials. Dislocation pile-ups occurred at a/4?111? thermal anti-phase boundaries (APBs). Screw super-dislocations frequently cross-slipped when they encountered the thermal APBs, while mixed dislocations tended to be pinned at them. This impediment is attributed to the creation of new APB segments when dislocations pass through the curvy thermal APBs.     

Evolution of dislocation density in a hot rolled Zr–2.5Nb alloy with plastic deformation studied by neutron diffraction and transmission electron microscopy

Abstract

The evolution of dislocation density and microstructure of a hot rolled Zr–2.5Nb alloy under compressive plastic strain, at room temperature, was analysed using neutron diffraction and transmission electron microscopy (TEM). The dislocation densities of type 〈a〉, 〈c + a〉 and 〈c〉 dislocations at different plastic strains in the elastic–plastic transition regime and plastic regime have been measured by diffraction line profile analysis (DLPA). TEM microstructure characterization revealed the operation of different slip systems. It has been found that slip of type 〈a〉 dislocations contributed to most of the plastic strain at the early stage of deformation, and strong pyramidal 〈c + a〉 slip did not occur until the deformation was fully plastic. Unambiguous evidence of basal slip occurring at room temperature in Zr is provided. Loading along a plate direction with more basal poles favoured the operation of basal and pyramidal slip. Dislocation features including relative edge:screw character of 〈c + a〉 dislocations are shown to be different under tension and compression loading, providing a mechanistic driver for the previously observed asymmetry in critical resolved shear stress for 〈c + a〉 slip.     

Rapid dislocation‐density mapping of as‐cut crystalline silicon wafers

Rapid quantification of structural defects, especially dislocations, is desired for characterization of semiconductor materials. Herein, we outline and validate a low‐cost approach for dislocation‐density quantification in silicon, involving a high‐resolution commercial dark‐field imaging device, a flatbed scanner. This method requires minimal surface preparation and can be performed on as‐cut 15.6 × 15.6 cm²wafers in less than 5 minutes. The method has been tested at a spatial resolution down to 250 µm. At 1 mm resolution, the average root mean square of the normalized error was 0.39.

     

Plastic size effect analysis of lamellar composites using a discrete dislocation plasticity approach

Plastic size effect analysis of lamellar composites consisting of elastic and elastic-plastic layers is performed using a discrete dislocation plasticity approach, which is based on applying periodic homogenization to the superposition method for discrete dislocation plasticity. In this approach, the decomposition of displacements into macro and perturbed components circumvents the calculation of superposing displacement fields induced by dislocations in an infinitely homogeneous medium, resulting in two periodic boundary value problems specialized for the analysis of representative volume elements. The present approach is verified by analyzing a model lamellar composite that includes edge dislocations fixed at interfaces. The plastic size effects due to dislocation pile-ups at interfaces are also analyzed. The analysis shows that, strain hardening in elastic-plastic layers arises depending on two factors, namely the thickness and stiffness of elastic layers; and the gap between slip planes in adjacent elastic-plastic layers. In the case where the thickness of elastic layers is several dozen nm, strain hardening in elastic-plastic layers is restrained as the gap of the slip planes decreases. This particular effect is attributed to the long range stress due to pile-ups in adjacent elastic-plastic layers.     

Interaction between edge dislocations and amorphous interphase in carbon nanotubes reinforced metal matrix nanocomposites incorporating interface effect

Dislocations mobility and stability in the carbon nanotubes (CNTs)-reinforced metal matrix nanocomposites (MMNCs) can significantly affect the mechanical properties of the composites. However, current processing techniques often lead to the formation of coated CNT (amorphous interphase exists between the reinforcement and metal matrix), which have large impact upon the image force exerting on dislocations. Even though the importance of the interphase zone formed in metal matrix composites has been demonstrated by many studies for elastic properties, the influence of interphase on the local elastoplastic behavior of CNT-reinforced MMNCs is still an open issue. This paper puts forward a three-phase composite cylinder model with new boundary conditions. In this model, the interaction between edge dislocations and a coated CNT incorporating interface effect is investigated. The explicit expressions for the stress fields and the image force acting on an edge dislocation are proposed. In addition, plastic flow occurring around the coated reinforcement is addressed. The influences of interface condition and the material properties of coated CNT on the glide/climb force are clearly analyzed. The results indicate that the interface effect becomes remarkable when the radius of the coated reinforcement is below 10 nm. In addition, different from the traditional particles, the coated CNT attracts the adjacent edge dislocations, causing pronounced local hardening at the interface between the interphase and the metal matrix under certain conditions. It is concluded that the presence of the interphase can have a profound effect on the local stress field in CNT-reinforced MMNCs. Finally, the condition of the dislocations stability and the equilibrium numbers of dislocations at a given size grain are evaluated for considering the interface effect.     

Wedge-shaped fracturing in the pull out of FRP stiffeners from quasi-brittle substrates

Fiber-Reinforced-Polymer (FRP) strips can be glued to the surface of concrete or masonry structures to improve their strength. Pull-out tests on FRP bonds have shown a progressive failure of the adhesive joint involving early-stage cracking parallel to the axis of the FRP stiffener, and an inclined crack initiating at the free end of the stiffener and extending into the quasi-brittle substrate in the latest stage. The subsurface crack produces a characteristic wedge-shaped spall. There is no consensus on the reasons for the transition from cracking along the bond to cracking within the substrate. Therefore a Linear Elastic Fracture Mechanics model problem is presented here that accounts for and provides improved understanding of the formation of the subsurface crack. The boundary value problem is solved analytically using the distributed dislocation technique. Competition between crack extension along the adhesive joint and into the substrate is quantified using a quantized crack propagation criterion, whereby the crack does not advance in infinitesimal continuous increments, but instead in finite steps of length comparable to the characteristic dimensions of the material microstructure. The model predicts results that are in good agreement with experimental evidence.