Structure and mechanical properties of nanostructured Al–Mg alloys processed by severe plastic deformation

Structural features, microhardness, and mechanical properties of three binary Al–Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion at room temperature were comparatively investigated using transmission electron microscopy, high-resolution transmission electron microscopy, and quantitative X-ray diffraction measurements. Average grain sizes measured by dark-field images are in the range 71–265 nm while the sizes of coherent domains decreased tremendously from 86 to 46 nm as the Mg content increased from 0.5 to 4.1 wt%. The average dislocation density in the deformed alloys is in the range 0.37 × 10¹⁴–4.97 × 10¹⁴ m^?2. Both the microhardness and tensile strength of all the deformed alloys increased dramatically as compared to the undeformed counterparts. The yield strength with values ranging from 390 to 690 MPa in the deformed alloys is typically five to seven times higher than that of the same undeformed alloys. Calculations based on the Hall–Petch and Taylor equations suggest that the strengthening mechanisms contributing to the very high strength may depend not only on the conventional mechanisms of grain size strengthening and dislocation strengthening, but also on the additional mechanisms related to the contributions from stacking faults and nanotwins, and nonequilibrium GBs observed in the deformed alloys.     

The effect of friction stir processing on the microstructure and mechanical properties of equal channel angular pressed 5052Al alloy sheet

In this study, equal channel angular pressed (ECAP) 5052Al alloy sheet was friction stir processed (FSP). This was carried out to understand the effect of FSP on the microstructure and mechanical properties of the ECAP sheet. FSP led to further grain refinement and a tighter distribution of grains. Fraction of high-angle grain boundaries changed from 15% in ECAP condition to more than 70% after FSP. Although FSP caused lowering of yield strength (YS) and ultimate tensile strength (UTS), it resulted into a substantial improvement in uniform deformation region of the tensile sample (from 1.3% in as-received condition to 12.9% in FSP condition). Strain hardening rate (SHR) analysis showed lowering of recovery rate on FSP. A static grain growth model correctly predicted the average grain size obtained after FSP. Existing grain boundary, solid solution, and dislocation strengthening models were used to estimate the YS of 5052Al alloy in both the conditions. The strengthening model was able to predict the YS of the alloy in as-received and FSP conditions very well.     

Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al₄Ca phases distributed as a network along the grain boundary and Al₃(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al₄Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.

     

Fabrication of large-bulk ultrafine grained 6061 aluminum alloy by rolling and low-heat-input friction stir welding

In this study, the ultrafine grained (UFG) 6061 Al alloys fabricated by cold rolling were friction stir welded (FSW) with different rotation rates under both air cooling and rapid cooling in water. Low-heat-input parameters of 400 rpm rotation rate in water (400-Water) could effectively inhibit the coarsening of recrystallized grains, reduce the precipitation rate, and retain more dislocations of the UFG 6061 Al parent metal. 400-Water joint showed high lowest-hardness value, narrow low-hardness zone, and high tensile strength, attributing to the effect of dislocation, grain boundary, solid-solution, and precipitation hardening. This work provides an effective strategy to fabricate large-sized bulk UFG Al alloy by cold rolling with large deformation and low-heat-input FSW.     

Grain size dependence of tensile behavior in nanocrystalline Ni–Fe alloys

The tensile behaviors of FCC Ni–Fe alloys were investigated within three grain size regimes: >100 nm, 15–100 nm, and <15 nm. The results show that the nanocrystalline metals demonstrated large strain hardening rates, which increase with decreasing the grain size. With the similar grain size, lowing the stacking-fault energy (SFE) by addition of alloying element increases the yield strength and strain hardening ability. The “low” tensile elongation of nanocrystalline metals is due to the basic tradeoff between the strength and tensile elongation, i.e. nanostructured metals are not inherently brittle. Both the tensile results and fracture surface observations suggest that the tensile ductility increases with increasing the grain size. Furthermore, within the large grain size regime, the fracture surface exhibited the real void structure; while the fracture surface showed the concave and convex features when the grain size is less than the critical value.     

Joint effect of quasicrystalline icosahedral and L12-strucutred phases precipitation on the grain structure and mechanical properties of aluminum-based alloys

Dispersoid hardening is a key factor in increasing the recrystallization resistance and mechanical strength of non-heat treatable aluminum-based alloys.Mn and Zr are the main elements that form dispersoids in commercial Al-based alloys.In this work,the annealing-induced precipitation behavior,the grain struc-ture,and the mechanical properties of Al-3.0Mg-1.1 Mn and Al-3.0Mg-1.1 Mn-0.25 Zr alloys were studied.The microstructure and the mechanical properties were significantly affected by annealing regimes after casting for both alloys.The research demonstrated a possibility to form high-density distributed quasicrystalline-structured I-phase precipitates with a mean size of 29 nm during low-temperature annealing of as-cast alloys.Fine manganese-bearing precipitates of Ⅰ-phase increased recrystallization resistance and significantly enhanced the mechanical strength of the alloys studied.The estimated strengthening effect owing to Ⅰ-phase precipitation was 150 MPa.Due to the formation of L12-structured Al3Zr dispersoids with a mean size of 5.7 nm,additional alloying with Zr increased yield strength by about 90 MPa.The L12-phase strengthening effect was estimated through the dislocation bypass looping and shearing mechanisms.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Relationships of strength to composition and phase constitution for three aged beta titanium alloys

X-ray diffraction techniques were used to study properties of three beta titanium alloys in the alpha aged condition. The alloys studied were Beta 111 (Ti-11.5 Mo-6 Zr-4.5 Sn), Beta C (Ti-3 Al-8 V-6 Cr-4 Mo-4 Zr), and 8-8-2-3 (Ti-8 V-8 Mo-2 Fe-3 Al). The volume percentage of alpha phase present and the lattice parameters of both the alpha and beta structures were determined for different ageing treatments. Ultimate tensile strength is related to both alpha content and beta unit cell size in these alloys. However, at high strength levels, beta unit cell size is a more sensitive indicator of tensile strength than percentage of alpha phase. The effects of precipitation hardening mechanisms and alloy partitioning on strengthening are discussed.     

β钛合金的强化机理

β钛合金是超高强钛合金的理想选择,其强化取决于该合金具有的晶粒和亚晶粒尺寸以及第二相的性质、体积比、分布特征等微观组织结构因素.其具体的强化机制包括位错强化、固溶强化、细晶强化及第二相强化,综述了这4种强化机制及其相互之间的影响.     

High strength and high creep resistant ZrB2/Al nanocomposites fabricated by ultrasonic-chemical in-situ reaction

In this study, the ZrB₂/Al nanocomposites were fabricated via in-situ reaction of the Al-K₂ZrF₆-KBF₄ system, assisted with ultrasonic vibration and spiral electromagnetic stirring. Microstructure, tensile property and creep behavior of the fabricated nanocomposites were further investigated. Microstructure observation showed that the ultrasonic vibration could prevent the fast growth as well as break the clusters of in-situ synthesized nanoparticles in melt, resulted in smaller size (10–50 nm) and relatively more uniform distribution of the in-situ nanoparticles located on the boundary of and/or inside the aluminum matrix grains in the final composites. The fabricated nanocomposites exhibited an enhancement in both strength and ductility, due to the elevated work hardening ability, i.e., improved dislocation propagating ability and decreased dynamic recovery of the existing dislocations induced by the in-situ nanoparticles. Meanwhile, the nanocomposites exhibited excellent creep resistance ability, which was about 2–18 times higher than those of the corresponding aluminum matrix. The stress exponent of 5 was identified for the fabricated nanocomposites, which suggested that their creep behavior was related to dislocation climb mechanism. The enhanced creep resistance of the nanocomposites was attributed to the Orowan strengthening and grain boundary strengthening induced by the ZrB₂ nanoparticles. Thus, the ultrasonic-chemical in-situ reaction promises a low cost but effective way to fabricate aluminum nanocomposites with high strength and high creep resistance.     

D6A钢在轧制过程中的强韧化机理EI北大核心CSCD

通过轧制-热处理工艺能够使D6A钢的强度显著提高。为了探究其强韧化机理,本实验采用热轧及两相区温轧退火工艺,获得微米级D6A合金钢样品,微观组织为铁素体基体及粒状渗碳体。通过室温拉伸实验、SEM、X射线衍射、EBSD等手段对实验钢的显微组织和力学性能进行表征,结果表明:随着变形量的增加,晶粒尺寸由4.5μm细化为1.5μm,渗碳体的含量逐渐增加,小角度晶界的比例升高,屈服强度和抗拉强度不断增加,伸长率略有降低,说明轧制过程使亚晶粒的尺寸不断降低,晶界面积增加,位错滑移受到的阻力增大。同时,本研究对不同轧制变形量实验钢的位错密度进行计算,当轧制变形量为88%时,位错密度最高,此时加工硬化的程度最高。随着变形量的增加,第二相强化和晶粒细化引起的强度增量呈不断上升的趋势,位错强化引起的强度增量先升高后降低,D6A钢的主要强化方式为第二相强化、细晶强化及位错强化。     

复合微合金化对Al-Mg合金组织与性能的影响

研究了Sc和Ti复合微合金化对Al-Mg合金显微组织与拉伸性能的影响.结果表明:Sc和Ti复合微合金化可以显著提高Al-Mg合金的强度,并可细化铸态合金的晶粒组织.微量Sc和Ti的加入可使合金中形成大量细小弥散的球形Al3(Ti,Sc)粒子,这些Al3(Ti,Sc)粒子对位错和亚晶界具有强烈地钉扎作用,因而能强烈抑制合金的再结晶.Sc和Ti复合微合金化的Al-Mg合金的强化作用主要来源于Al3(Ti,Sc)粒子的析出强化和亚结构强化以及细晶强化.     

Development of ultrafine grained high strength age hardenable Al 7075 alloy by cryorolling

High strength age hardenable Al 7XXX series alloys are difficult to process by many of the severe plastic deformation processes at room temperature. The Al 7075 alloy has been processed at cryogenic temperature and room temperature up to different rolling strains, in the present work, with the objective of developing a processing strategy to obtain ultrafine grained microstructure with enhanced mechanical properties in the alloy. It has been identified that the Al 7075 alloy samples can be successfully cryorolled to higher strains (up to 3.4) if the reduction per pass is less than 0.3 mm, however it was found to be difficult to deform the samples at room temperature. A cryorolling strain of 3.4 has been found to be desirable for producing the ultrafine grained Al 7075 alloys with the high angle grain boundaries. However, the subgrains are not recrystallized up to this strain in the case of room temperature rolled Al alloys. The strength and hardness of the cryorolled Al 7075 alloy samples are higher than that of the room temperature rolled samples as observed in the present work. The improved strength and hardness of cryorolled samples are due to the grain size effect and higher dislocation density. The reduction in dimple size of cryorolled Al 7075 alloy upon failure confirms the grain refinement and strain hardening mechanism operating in the heavily deformed samples.     

Structure and properties of bulk nanostructured alloys synthesized by flux-melting

Nanomaterials can easily be prepared as thin films and powders, but are much harder to prepare in bulk form. Nanostructured materials are prepared mainly by consolidation, electrodeposition, and deformation. These processing techniques have problems such as porosity, contamination, high cost, and limitations in refining the grain size. Since most bulk engineering metals are initially prepared by casting, we developed a casting technique, flux-melting and melt-solidification, to prepare bulk nanostructured alloys. The casting technique has such advantages as simplicity, low cost, and full density. In our method, Ag–Cu alloys were melted in B₂O₃ flux, which removed most of the impurities, mainly oxides, in the melts. Upon solidifying the melt at a relatively slow cooling rate on the order of 10¹–10² K/s a large undercooling of ∼0.25 T _m (where T _m is the melting temperature) was achieved. This large undercooling leads to the formation of bulk nanostructured Ag–Cu alloys composed of alternative Ag/Cu lamella and nanocrystals, both ∼50 nm in dimension. Our liquid-processed alloys are fully dense and relatively free from contamination. The nanostructured Ag–Cu alloys have similar yield strength in tension and in compression. The as-quenched alloys have yield strength of 400 MPa, ultimate tensile strength (UTS) of 550 MPa, and plastic elongation of ∼8%. The UTS was further increased to ∼830 MPa after the as-quenched alloy rod was cold drawn to a strain of ∼2. The nanostructured Ag–Cu alloys show a high electrical conductivity (∼80% that of International Annealed Copper Standard), a slight strain hardening (strain-hardening coefficient of 0.10), and a high thermal stability up to a reduced temperature of 2/3 T _m. Some of these behaviors are different than those found in previous bulk nanostructured materials synthesized by solid state methods, and are explained based on the unique nanostructures achieved by our flux-melting and melt-solidification technique.     

Modeling the hardness and yield strength evolutions of aluminum alloy with rod/needle-shaped precipitates

For heat-treatable aluminum alloy containing rod/needle-shaped precipitates, a strengthening model for aging hardening behavior has been developed to describe the variations of dimension and volume fraction of the precipitates, and the yield strength and hardness with aging time. The model incorporates both the strengthening mechanisms of dislocation cutting the precipitates at underaged stage and bypassing the precipitates at overaged stage. It has been shown that the model predictions broadly agree with the experimental data for Al–Mg–Si alloys at varied aging time and aging temperatures. Thus, the model can be used to predict the variation of yield strength and hardness with aging time and aging temperature.     

Enhanced tensile ductility in a nanostructured Al–7·5%Mg alloy

Abstract

This paper reports work on the enhanced tensile ductility in a nanostructured Al–7·5%Mg alloy with a mean grain size of 90 nm processed via consolidation of cryomilled Al–Mg powders. An annealing treatment at a temperature of 773 K for 2·5 h modified the extruded microstructure slightly without causing significant grain growth, as revealed by TEM and XRD patterns. The annealing treatment significantly improved the ductility, with a remarkably small loss in strength. The observed high thermal stability of the cryomilled Al alloy was attributed to the existence of impurity elements introduced during cryomilling and the presence of a supersaturated solid solution. The reported phenomenon of enhanced tensile ductility was attributed to a mechanism involving dislocation activity in submicron grains during plastic deformation.     

High‐Strength Nanotwinned Al Alloys with 9R Phase

Light‐weight aluminum (Al) alloys have widespread applications. However, most Al alloys have inherently low mechanical strength. Nanotwins can induce high strength and ductility in metallic materials. Yet, introducing high‐density growth twins into Al remains difficult due to its ultrahigh stacking‐fault energy. In this study, it is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned (nt) columnar grains with high‐density 9R phase in Al(Fe) solid solutions. The nt Al–Fe alloy coatings reach a maximum hardness of ≈5.5 GPa, one of the strongest binary Al alloys ever created. In situ uniaxial compressions show that the nt Al–Fe alloys populated with 9R phase have flow stress exceeding 1.5 GPa, comparable to high‐strength steels. Molecular dynamics simulations reveal that high strength and hardening ability of Al–Fe alloys arise mainly from the high‐density 9R phase and nanoscale grain sizes.     

Deformation behavior of aluminum alloy matrix composites reinforced with few-layer graphene

Microstructure and mechanical properties of aluminum alloy 2024 (Al2024)/few-layer graphene (FLG) composites produced by ball milling and hot rolling have been investigated. The presence of dispersed FLGs with high specific surface area significantly increases the strength of the composites. The composite containing 0.7 vol.% FLGs exhibits tensile strength of 700 MPa, two times higher than that of monolithic Al2024, and around 4% elongation to failure. During plastic deformation, restricted dislocation activities and the accumulated dislocation at between FLGs may contribute to strengthening of Al2024/FLG composites.     

The study of grain boundary density effect on multi-grain thin film under tension

The grain-size effect on the yield strength and strain hardening of thin film at sub-micron and nanometer scale closely relates to the interactions between grain boundary and dislocation. Based on higher-order gradient plasticity theory, we have systematically investigated the size effect of multi-grain thin film arising from the grain boundary density under tensile stress. The developed formulations employing dislocation density and slip resistance have been implemented into the finite element program, in which grain boundary is treated as impenetrable interface for dislocations. The numerical simulation results reasonably show that plastic hardening rate and yield strength are linear to the grain boundary density of multi-grain thin film. The aspect ratio of grain size and orientation of slip system have distinct influence on the grain plastic properties. The research of slip system including homogeneous and nonhomogeneous distribution patterns reveals that the hardening effect of low-angle slip system is greater than that of high-angle slip system. The results agree well with the experimentally measured data and the solutions by discrete dislocation dynamics simulation.     

Influence of boron carbide content on the microstructure,tensile strength and fracture behavior of boron carbide reinforced aluminum metal matrix composites

In the present work, an indigenously developed low cost modified stir casting technique is developed for the processing of 6061 Al‐B₄C composites containing high‐volume fraction of boron carbide particles (up to 20 vol. %). The influence of varying reinforcement content on the spatial distribution of boron carbide in the aluminum matrix is qualitatively characterized using scanning electron microscope. At a lower volume fraction of reinforcement, wide particle free zone and large interparticle spacing were observed in the matrix while the composite with high reinforcement content displayed relatively homogeneous and discrete particle distribution. X‐ray diffraction analysis confirms the presence of only aluminum and boron carbide diffraction peaks, indicating that no significant reaction occurs during composite processing. The tensile behavior of composites revealed that strength and ductility are influenced by varying particulate content. The quantitative analysis of strengthening mechanism in the casted composites showed that higher volume fraction of boron carbide lead to larger values of thermal dislocation strengthening, grain size and strain gradient strengthening. The morphology of fracture surfaces reveals the presence of dimple network and the average size of dimples gradually decreases with the increase in particulate content, which indicates the co‐existence of ductile and brittle fracture.