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The faulted dipoles in face-centered cubic materials have been well proposed theoretically to comprise two opposing-sign Shockley dislocations and two opposing-sign sessile stair-rod dislocations connected by three stacking faults; however, studies on them have hitherto received limited attention. In the present study, the faulted dipoles in a nanostructured Al alloy produced via high-pressure torsion were observed and identified using high resolution transmission electron microscopy. The mechanisms underlying the formation of the faulted dipoles were discussed. As strong barriers to dislocation motion, the sessile faulted dipoles present a potential to entrap dislocations, enhancing work-hardening ability and ductility of nanostructured metals and alloys. [ABSTRACT FROM AUTHOR]

Abstract Recently Lomer-Cottrell (L-C) locks as barriers to pin and entrap dislocations and thus to enhance work-hardening and ductility of nanostructured (NS) metals and alloys have received attention. This letter reports on a new type of L-C locks with multiple stair-rod dislocations, enabling them to exhibit higher structural stability resistant to dissociation and thus to be stronger barriers to dislocation motion than conventional L-C locks. The new type of L-C locks were observed and identified using high resolution transmission electron microscopy in a NS 7075 Al alloy processed by high-pressure torsion (HPT). The L-C locks could be formed via reactions between Shockley dislocations enclosing the intersected SFs on different {111} slip planes during HPT. The combination of nanoscale grain size, high applied stress during HPT and solute atoms enable the aforementioned dislocation reactions and the retention of the SFs with widths on the order of nanometers in the resultant L-C locks. [ABSTRACT FROM AUTHOR]

Experimental observations on dislocation-stacking fault (SF) interactions in nanostructured (NS) Al and Al alloys have received very limited attention albeit some available theoretical studies using molecular dynamic simulations in the published scientific literature; consequently, the effect of the SFs on strength and ductility remains poorly understood. In an effort to address this question, the present study reports a type of configurations comprising one or several full dislocation(s) and a SF in a NS Al alloy processed by high-pressure torsion. The configurations were identified and studied using high resolution transmission electron microscopy. The calculations reveal that the two partial dislocations enclosing the SF generate high magnitudes of components of stress in the position of the full dislocation(s), indicative of strong full dislocation-SF interactions. This finding suggests the potential of the SFs to enhance both strength and ductility of NS Al and Al alloys by impeding dislocation slip and entrapping dislocations. [ABSTRACT FROM AUTHOR]

This paper reports a study on mechanical behavior during tensile testing of nanostructured (NS) Al and ultra-fine grained (UFG) Al containing nanoscale oxide particles that were produced via consolidation of nano-sized Al powders using spark plasma sintering (SPS) only and SPS plus extrusion, respectively. Our results show that the NS Al exhibits ∼370 MPa in 0.2% offset yield strength and ∼0.9% in elongation to failure, whereas the UFG Al presents ∼430 MPa in 0.2% offset yield strength and ∼1.5% and ∼1.9% in uniform elongation and elongation to failure, respectively. Based on the tensile stress vs. strain curves, microstructural features and fractured surface characteristics, mechanical behavior of NS Al and UFG Al was analyzed and discussed in detail. We suggest that the lower yield strength and ductility in the NS Al than those in UFG Al can be attributed to the strong stress concentration at grain boundaries (GBs) as a result of (i) the presence of highly segregated nanoscale oxide particles and of residual porosity at GBs to induce, and (ii) the absence of dislocation sources and thus of activated dislocation slip at nano-sized grain interiors (GIs) to release, the stress concentration. Immediately after yield in the as-SPSed Al, the stress concentration debonded GBs and further tensile straining rapidly led to connection between debonded GBs and the occurrence of intergranular fracture. In contrast, in the UFG Al the extent of oxide particle segregation and the porosity at GBs were reduced due to materials flow during extrusion, avoiding the occurrence of intergranular fracture at a very low strain level and thus sustaining plastic straining; as a result, yield strength and ductility were improved. However, the residual porosity and nano-sized/ultra-fine grains still induced early strain localization shortly followed by transgranular fracture via growth, coalescence and connection of nano-sized pores, leading to limited ductility. [ABSTRACT FROM AUTHOR]

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material consolidated at a high pressure (e.g. 500 MPa), grain coarsening occurs via a combination of thermally activated grain boundary (GB) migration, stress-coupled GB migration and grain rotation-induced grain coalescence. In contrast, in the case of the material consolidated at a low pressure (50 MPa), grain growth occurs primarily via thermally activated GB migration. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stress-induced grain growth in nanostructured Al–Al2O3 at 400°C was studied. [•] A model that can study stress-induced grain growth with particles was suggested. [•] Stress-induced grain growth in nanostructured Al–Al2O3 was theoretically analyzed. [ABSTRACT FROM AUTHOR]

This paper studies the widths of the stacking faults (SFs) formed by the dissociation of screw and 60° full dislocations in a nanostructured Al alloy processed via high-pressure torsion. The average width of the SFs from the former is several times of that from the latter. The underlying mechanisms are discussed. [ABSTRACT FROM AUTHOR]

We report on a study of the microstructure and mechanical behavior of a bulk nanostructured (NS)/ultra-fine grained (UFG) aluminum fabricated by cryomilling and high-pressure spark plasma sintering (SPS). The compressive yield stress of the consolidated material is determined to be 380 MPa, which is significantly higher than that of commercial strain hardened aluminum (124 MPa). Microstructural studies reveal that the nanometric grains are embedded inside a matrix of ultra-fine grains in the bulk material. The corresponding average grain size is approximately 125 nm and with a distribution of grains that are smaller than 500 nm. Moreover, chemical analysis of the powder particles and the consolidated samples indicates that carbon and oxygen levels remain unchanged, and that there is a slight decrease in the nitrogen level and a significant reduction in the hydrogen level after spark plasma sintering. Semi-quantitative analysis suggests that the mechanisms that contribute to the strength of the consolidated material include: grain boundary strengthening, second phase strengthening and dislocation strengthening. [ABSTRACT FROM AUTHOR]