SCRIPTA

Formation and evolution of basal-prismatic/prismatic-basal (BP/PB) interfaces in a Mg alloy subjected to cyclic deformation were studied and modeled, according to atomic-resolution experimental observations and theory of interfacial defects. It is found that BP/PB interfaces could be formed by emission of 60° basal 1/3<112_0> dislocations (referred to as <a₆₀>),  <c + a₆₀> and {112_0} twinning disconnections (TDs) from asymmetric tilt grain boundaries near the {112_0} twin orientation. Transformation of BP/PB interfaces into {112_0} twin boundaries could occur by emission of <a₆₀> and <c + a₆₀> dislocations, resulting in nucleation of {112_0} twins. Moreover, each side of BP/PB facets could emit {112_0} TDs to facilitate migration of BP/PB interfaces. Our experimental results may provide insights into the dislocation-assisted mechanisms of formation and evolution of BP/PB interfaces in hexagonal-close-packed materials.

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A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of a surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

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In this paper, the composition distribution characteristics in adiabatic shear bands (ASBs) of an α/β dual phase Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy and its influence on dynamic recrystallization (DRX) were investigated via atomic probe tomography. The results showed that in the ASB transition region, a clear grain boundary (GB) in α/β interface was observed. However, in the ASB central region, a “fuzzy GB” was formed in α/β interface, and abundant dislocations accumulated in the “fuzzy GB”. In addition, β stabilizing elements (Mo/Cr) became rather sparse in some local regions of β grains to form finer α grains, while in some regions of α grains, local β stabilizing elements were enriched to form finer β grains, thus separating the original grains into finer DRX grains. The unique composition distribution characteristics in α/β grains and α/β interface were defined as composition redistribution, which was the combined result of localized plastic deformation and adiabatic temperature rise.

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It has long been established that drawn pearlitic steel wires can achieve ultra-high strengths; however, this is usually achieved to the detriment of tensile and torsional ductility. Here, we employed a modified microstructure to overcome these trade-offs through a process involving simple multiple drawing and annealing steps. In this microstructure, the conventional pearlitic cementite plates are replaced by cementite nano-particles bridged by grain boundaries enriched with substitutional elements. After being subject to this modified processing route, 0.92%C steel wires were found to exhibit 2300 MPa tensile strength, 6.4% uniform elongation and 0.73 uniform torsion strain. The wires processed by the new route had higher ultimate strength than wires prepared by the traditional process to the same strain, but with much superior tensile and torsional ductility – indeed they match the ductility of strain-free pearlitic steel rod.

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We successfully formed the first prominent crystallographic texture of tungsten using laser powder bed fusion (LPBF). It is difficult even to manufacture highly dense tungsten products using LPBF because of its extremely high melting point and high thermal conductivity. By tuning the laser process parameters, we succeeded in fabricating almost fully dense pure tungsten parts with a relative density of 99.1%, which is the highest value yet to be reported. More importantly, a single crystalline-like prominent crystallographic texture evolved, in which <011> preferentially oriented in the scanning direction. This texture was formed to reduce the crystal misorientation at the melt pool center, at which the solidification fronts from the right and left halves of the melt pool encounter. This texture formation mechanism is similar to that of conventional alloys with ordinary thermal properties; however, the crystal growth directionality that governs the crystallographic orientation differs according to the melt pool morphology.