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In industrial production, highly alloyed tool steels (HATSs) are usually fabricated by powder consolidation using hot isostatic pressing (HIP) technology; HIP's primary disadvantage is very high production cost. However, a well-developed cost-effective powder consolidation technology, termed as vacuum hot-pressing (VHP) that is also able to produce HATSs in an industrial scale, has heretofore attracted few attentions, primarily attributable to nonuniform density distribution in VHP consolidated HATSs. In this paper, an innovative VHP processing approach was designed as follows, aiming not only to overcome the aforementioned VHP's disadvantage but also to enhance mechanical properties relative to those of its counterpart produced by HIP approach: first, inert gas-atomized prealloyed powders are consolidated via VHP purposely at a temperature lower than those used during HIP, in order to retain very fine carbides, but which likely yields partial densification with non-uniform density distribution and incomplete metallurgical bond between powders; then, identical full densification and complete metallurgical bond are achieved throughout the consolidated HATS by using hot-working at a temperature no higher than the VHP temperature, minimizing carbide growth. A HATS, T15 steel with nominal composition of Fe-1.5C–12W–4Cr–5V–5Co (wt%) as model material, was produced to assess and validate the designed VIP processing approach. The experimental results reveal submicrometric average carbide size in the as-VHP T15 steel smaller than that in its as-HIP counterpart, together with essentially unchanged carbides relative to those in the as-VHP T15 steel, identical full densification and complete metallurgical bond in the entire VHP plus hot-rolled (HR) T15 steel. After quenching and tempering (QT), the VHP + HR + QT T15 steel still retains submicrometric average remaining carbide size and displays refined average prior austenite grain size, with both smaller than, and exhibits bend fracture strength higher than, accompanied by hardness similar to, those in its counterpart produced by HIP approach. The mechanisms underlying microstructural evolutions associated with carbides, densification and metallurgical bond in the designed VHP processing approach were discussed. The aforementioned mechanical property characteristics were rationalized. VHP plus subsequent hot-working at lower temperatures provides a cost-effective commercialized avenue to manufacture HATSs with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]

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]

The objective of the present study is to explore the feasibility of producing 7075 Al alloy ribbons with the increased microhardness via rapid solidification processing, namely melt spinning. Our results show that the melt spun+aged 7075 Al alloy ribbons exhibit approximately 17% (307 MPa) higher microhardness than that of the coarse-grained (CG) counterpart solid-solution treated+aged under the same conditions. The higher microhardness of the melt spun+aged 7075 Al alloy stems from its microstructural features, including (i) sub-micrometric/micrometric sized grains, (ii) intragranular subgrains, and (iii) nanoscale precipitates with inter-precipitate distance slightly smaller than that in the CG counterpart. In an effort to understand the microstructure in melt spun+aged 7075 Al alloy, the microstructural evolution during aging of the as-melt spun 7075 Al alloy was analyzed and discussed. By quantifying the contributions of precipitation, grain boundary, dislocation, solid solution and subgrain boundary strengthening to the yield strength based on the microstructural analysis, the higher microhardness of the melt spun+aged 7075 Al alloy ribbons was rationalized. [ABSTRACT FROM AUTHOR]

Cast Al–Mg alloys with high Mg contents (>10 wt%) have received particular attention due to much lower density than those of other cast Al alloys. However, high amounts of coarse dendritic Al–Mg intermetallic phases substantially deteriorate both strength and ductility. This work aims to reduce amount and size of Al–Mg intermetallic phases by rapid solidification (RS) and to improve both strength and ductility in a cast Al-15 wt%Mg alloy. This paper reports a study on Al-15 wt%Mg alloy processed by a RS technique, namely twin-roll casting (TRC). The results of optical microscopy, electron backscatter diffraction and scanning electron microscopy analysis show refined Al grains and refined granular Al 3 Mg 2 particles, and the results of X-ray diffraction analysis reveal the increased Mg solid solubility and correspondingly the decreased amount of Al 3 Mg 2 phase, in the TRC Al-15 wt% alloy, as compared to the conventionally mold cast (CMC) counterpart. Consequently, the tensile testing results show 0.2% offset yield strength of 243 MPa and ultimate tensile strength of 355 MPa, accompanied by uniform elongation of 3.6%, in the TRC Al-15 wt% alloy, relative to premature fracture in the CMC counterpart. The microstructural features and the mechanisms underlying improved strength and ductility in the TRC Al-15 wt%Mg are analyzed and discussed. [ABSTRACT FROM AUTHOR]

This paper reports a study on multi-scale metallic materials containing ultra-fine lamellar structure, which have received very limited attention in the published literature. In the present study, a multi-scale Al–Mg alloy, which comprises ultra-fine lamellar structure with a high density of dislocations, as well as nearly dislocation-free equiaxed ultra-fine, fine and coarse grains, was produced via partial recrystallization of the heavily cold-rolled coarse-grained (CG) counterpart. The multi-scale Al–Mg alloy achieves both high strength (~320 MPa in 0.2% offset yield strength in comparison with 145 MPa in 0.2% offset yield strength for the annealed CG counterpart) and high ductility (~11% in uniform elongation). Our results show that the ultra-fine lamellar structure takes a volume fraction as high as 42.5% and presents a high strength (~450 MPa in 0.2% offset yield strength), enabling it to make a substantial contribution to the high overall strength of the multi-scale alloy. Interestingly, the ultra-fine lamellar structure concurrently exhibits a reasonably high ductility (~6% in uniform elongation), which benefits for the multi-scale alloy to attain the high ductility. [ABSTRACT FROM AUTHOR]

This paper reports a study on the microstructure and hardness of an as-melt spun Al–Mg alloy as a potential material used for micro-manufacturing. Our results show that the microstructure is characterized by a nearly single-phase fine-grained Al-based supersaturated solid solution containing a relatively high density of dislocations and the as-melt spun Al–Mg alloy exhibits a much higher microhardness relative to that of the coarse-grained counterpart. The quantitative relationship between microhardness and microstructure was established by quantifying the contributions of various strengthening mechanisms. [ABSTRACT FROM AUTHOR]

To attain an unusual combination of low density and high specific strength in lightweight materials, Ti–Mg heterostructured (HS) materials comprising soft Mg domains embedded in hard Ti domain matrix were developed. Unlike hitherto reported HS materials, Ti–Mg HS materials are characterized by: (i) significant difference in coefficient of thermal expansion (CET) between soft Mg and hard Ti domains, and (ii) soft Mg domains having lower ductility. The influences of the two features on strength and ductility of HS materials remain poorly understood. To address these questions, this paper reports studies on microstructure and mechanical properties of the Ti–Mg HS materials. The Ti–Mg HS materials were prepared by spark plasma sintering of the mixture of Ti and Mg elemental powders. Ti–12Mg achieves tensile yield strength higher than that calculated using the rule-of-mixture and uniform elongation higher than that of cast Mg. Ti–15Mg obtains tensile yield strength higher than that calculated using the rule-of-mixture but uniform elongation lower than that of cast Mg. Each of Ti–20Mg and Ti–29Mg exhibits tensile yield strength lower than that calculated using the rule-of-mixture and uniform elongation lower than that of cast Mg. The mechanisms underlying strength and ductility of these Ti–Mg HS materials were revealed by analyzing the combined effects of heterogeneous deformation induced strengthening and work-hardening with stress concentration originating from the presence of cracks at Ti/Mg domain boundaries as a result of notable difference in CET between Ti and Mg. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports a novel strategy to produce an ultra-fine grained (UFG) structure in an Al–4Mg (weight percent, wt%) alloy via discontinuous dynamic recrystallization under moderate straining (true strain <2.5) as imposed during conventional extrusion at 400°C. The mechanisms underlying the occurrence of discontinuous dynamic recrystallization and the formation of the UFG structure were analyzed and discussed in detail. The UFG Al–4wt%Mg alloy exhibits both high tensile yield strength and elongation. [Copyright &y& Elsevier]

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 influence of microstructure on the mechanical behavior of bulk nanoporous Cu fabricated by chemical de-alloying of Cu 50 Al 50 , Cu 40 Al 60 , Cu 33 Al 67 and Cu 30 Al 70 (at%) alloys. The precursor Cu–Al alloys were fabricated using arc melting and bulk nanoporous Cu was obtained by subsequent de-alloying of Cu–Al alloys in 20 wt% NaOH aqueous solution at a temperature of 65 °C. We studied the microstructure of the precursor Cu–Al alloys, as well as that of the as de-alloyed bulk nanoporous Cu, using X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Moreover, the compressive strength of bulk nanoporous Cu was measured and the relationship between microstructure and mechanical properties was studied. Our results show that the microstructure of bulk nanoporous Cu is characterized by bi-continuous interpenetrating ligament-channels with a ligament size of 130±20 nm (for Cu 50 Al 50 ), 170±20 nm (for Cu 40 Al 60 ) and 160±10 nm (for Cu 33 Al 67 ). Interestingly the microstructure of de-alloyed Cu 30 Al 70 is bimodal with nanopores (100's nm) and interspersed featureless regions a few microns in size. The compressive strength increased with decreasing volume fraction of porosity; as porosity increased 56.3±2% to 73.9±2%, the compressive strength decreased from 17.18±1 MPa to 2.71±0.5 MPa. [ABSTRACT FROM AUTHOR]

This paper presents a method of dealloying combined molten salt electrolysis (MSE) to prepare porous Ti alloy. The oxidation produced in dealloying process can be reduced efficiently by MSE. Meanwhile, the grain grew and the second phase of acicular nanocrystals was precipitated to enhance the mechanical properties during MSE process. [ABSTRACT FROM AUTHOR]

Cast Al–Mg alloys with high Mg content have attracted considerable interest as a result of exceptionally low mass density and enhanced Mg solid-solution strengthening effect. However, poor ductility in this class of cast Al–Mg alloys originating from the increased amount of brittle Al 3 Mg 2 phase presents a challenge. This work aims to improve ductility of a cast Al-10 wt%Mg alloy by prolonging time and/or increasing temperature during solid-solution treatment, hence minimizing the amount of the Al 3 Mg 2 phase. The amounts of Al 3 Mg 2 decrease and the concentrations of solute Mg increase with prolonging time and/or increasing temperature, improving both ductility and strength. Solid-solution treatment at 425 °C for 9 h or at 438 °C for 3 h induces complete dissolution of the Al 3 Mg 2 phase, achieving an average uniform elongation of ~27% accompanied by an average 0.2% offset yield strength of ~186 MPa. The mechanisms improving both ductility and strength are discussed. [ABSTRACT FROM AUTHOR]