Experiments and modeling of double-peak precipitation hardening and strengthening mechanisms in Al-Zn-Mg alloy

The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatures based on the modified Langer-Schwartz approach. The double aging peaks are present in the long time age-hardening curves of Al-Zn-Mg alloys. The physically-based model, while taking explicitly into account nucleation, growth, coarsening of the new phase precipitations and two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing）, was used for the analysis of precipitates evolution and precipitation hardening during aging of Al-Zn-Mg alloy. Model predictions were compared with the measurements of Al-Zn-Mg alloy. The systematic and quantitative results show that the predicted hardness profiles of double peaks via adding a shape dependent parameter in the growth equation for growth and coarsening generally agree well with the measured ones. Two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing） were considered operating simultaneously in view of the particle size-distribution. The transition from shearing to bypassing strengthening mechanism was found to occur at rather early stage of the particle growth. The bypassing was found to be the prevailing strengthening mechanism in the investigated alloys.

Microstructure evolution and strengthening mechanisms of spray-formed 5A12 Al alloy processed by high reduction rolling

The extrusion preform of the spray-formed5A12Al alloy was hot rolled using high reduction rolling technology.By means of transmission electron microscopy(TEM),electron backscatter diffraction(EBSD)and energy dispersive spectroscopy(EDS),the microstructure evolution was studied and the strengthening and toughening mechanism was thereby proposed.The results indicate that discontinuous and continuous dynamic recrystallization occurred during the hot rolling deformation of the spray-formed5A12Al alloy.The grain size was significantly refined and the micro-scale grains formed.Partial dynamic recrystallization leads to a significant increase of dislocation density and cellular structure.The Mg atoms were distributed in the Al matrix mainly in the presence of solid solution rather than the formation of precipitate.High solid solution of Mg atoms not only hindered the dislocation motion and increased the density of dislocation,but also exhibited a remarkable solid solution strengthening effect,which contributes to the high strength and high toughness of the as-rolled sheets.The tensile strength and elongation of spray formed5A12Al alloy at room temperature after3passes hot rolling were622MPa and20%,respectively.

Analysis of local microstructure and strengthening mechanisms in adjustable-gap bobbin tool friction stir welds of Al-Mg

The bobbin tool friction stir welding process was used to join 6 mm thick 5A06 aluminum alloy plates.Optical microscope was used to characterize the microstructure.The electron backscatter diffraction(EBSD)identified the effect of non-homogeneous microstructure on the tensile properties.It was observed that the grain size in the top of the stir zone(SZ)is smaller than that in the centre region.The lowest ratio of recrystallization and density of the geometrically-necessary dislocations(GNDs)in the SZ was found in the middle near the thermo-mechanically affected zone(TMAZ)being 22%and 1.15×10^(−13) m^(−2),respectively.The texture strength of the heat-affected zone(HAZ)is the largest,followed by that in the SZ,with the lowest being in the TMAZ.There were additional interfaces developed which contributed to the strengthening mechanism,and their effect on tensile strength was analysed.The tensile tests identified the weakest part in the joint at the interfaces,and the specific reduction value is about 93 MPa.

Strengthening mechanisms of indirect-extruded Mg-Sn based alloys at room temperature

The strength of a material is dependent on how dislocations in its crystal lattice can be easily propagated.These dislocations create stress fields within the material depending on their intrinsic character.Generally,the following strengthening mechanisms are relevant in wrought magnesium materials tested at room temperature:fine-grain strengthening,precipitate strengthening and solid solution strengthening as well as texture strengthening.The indirect-extruded Mg-8Sn(T8)and Mg-8Sn-1Al-1Zn(TAZ811)alloys present superior tensile properties compared to the commercial AZ31 alloy extruded in the same condition.The contributions to the strengthen of Mg-Sn based alloys made by four strengthening mechanisms were calculated quantitatively based on the microstructure characteristics,physical characteristics,thermomechanical analysis and interactions of alloying elements using AZ31 alloy as benchmark.

Strengthening mechanisms of reduced activation ferritic/martensitic steels:A review

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic(RAFM)steels.High-angle grain boundaries,subgrain boundaries,nano-sized M_(23)C_(6),and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength.M23C6 carbides are easily coarsened under high temperatures,thereby weakening their ability to block dislocations.Creep properties are improved through the reduction of M23C6 carbides.Thus,the loss of strength must be compensated by other strengthening mechanisms.This review also outlines the recent progress in the development of RAFM steels.Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength.Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel.The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries.This procedure increases the creep life of TMT(thermo-mechanical treatment)9Cr-1W-0.06Ta steel by~20 times compared with those of F82H and Eurofer 97 steels under 550℃/260 MPa.

Development and strengthening mechanisms of a hybrid CNTs@SiCp/Mg-6Zn composite fabricated by a novel method

The hybrid addition of CNTs was used to improve both the strengths and ductility of SiCp reinforced Mg matrix composites.A novel method was developed to simultaneously disperse SiCp and CNTs in Mg melt.Firstly,new CNTs@SiCp hybrid reinforcements were synthesized by CVD.Thus,CNTs were well pre-dispersed on the SiCp surfaces before they were added to Mg melt.Therefore,the following semisolid stirring and ultrasonic vibration dispersed the new hybrid reinforcements well in Mg-6Zn melt.The hybrid composite exhibits some unique features in microstructures.Although the distribution of SiCp was very uniform in the Mg-6Zn matrix,most CNTs distributed along the strips in the state of micro-clusters,in which CNTs were bonded very well with Mg matrix.Most of the CNTs kept their structure integrity during fabrication process.All these factors ensure that the hybrid composite have much higher strength and elongation than the mono SiC/Mg-6Zn composites.The dominant strengthening mechanism is the load transfer effect of CNTs.Apart from grain refinement,the CNTs toughen the composites by impeding the microcrack propagation inside the material.Thus,the hybrid CNTs@SiCp successfully realizes the reinforcing advantage of“1+1>2”.

Microstructure and strengthening mechanisms of Mg-6Al-6Nd alloy

The microstructure and strengthening mechanisms of as-cast Mg-6Al-6Nd alloy were studied. The results show that the addition of 6 wt.% Nd into Mg-6Al alloy leads to the precipitation of Al11Nd3 and Al2Nd phases and decrease in the content of Al solid soluted in Mg-Al matrix. The volume fractions of Al11Nd3 and Al2Nd phases are 3.64% and 0.34%, respectively. Compared with Mg-6Al alloy, the ultimate strength, yielding strength, and elongation of Mg-6Al-6Nd alloy at room temperature and 175℃ are enhanced in some degrees. The strengthening mechanisms of Mg-6Al-6Nd alloy are mainly composed of solid solution strengthening of Al solid soluted in Mg-Al matrix and grain refmement strengthening, dispersion strengthening, and composite strengthening brought by the precipitation of Al11Nd3 phase. The composite strengthening includes the load transfer from the matrix to Al11Nd3 phase and the enhancement of dislocation density due to the geometrical mismatch and thermal mismatch between the matrix and Al11Nd3 phase.

Strengthening Mechanisms to Promote Tourists' Environmental Responsible Behaviors in the Context of All-for-One Tourism

The overall promotion of all-for-one tourism requires tourists' civilized behaviors. This paper focused on tourists' environmental responsible behaviors in the context of all-for-one tourism. On the basis of clarifying the connotation, dimension and driving factors of tourists' environmental responsible behaviors, this paper firstly analyzed the relationship between all-for-one tourism and tourists' environmental responsible behaviors, and proposed that the development of all-for-one tourism was an important path to optimize and upgrade China's tourism industry and a new concept of sustainable development of tourism industry. The emergence of all-for-one tourism has formed a new tourism trend, which will drive tourists to behave more civilly. Tourists' environmental responsible behaviors are a "subject and share" concept, and from tourists' point of view, all-for-one tourism development is the most basic requirement. Secondly, based on the theory of "value-belief-norm", this paper focused on the strengthening mechanism of "interaction effect" between all-for-one tourism and tourists' environmental responsible behaviors, and put forward the idea of "double internalization strengthening" to guide tourists' environmental responsible behaviors from being passive to active, from individuals to groups. Finally, this paper discussed the promotion of environmental responsible behaviors of tourist's specific strategies from two aspects, "external drive" and "internal drive" to enhance the level of all-for-one tourism construction and strengthen the environment responsibility of tourists. "External drives" were as follows:(1) to make a scientific development plan to implement the "host and guest sharing" mechanism;(2) to optimize the landscape, the environment, services, enhance the local attachment of visitors;(3) to improve the infrastructure system, pay attention to the details of visitors' experience;(4) to strengthen environment education and publicity, deepening the quality of tourists' civilization construction, and strengthen the environmental responsible behaviors of tourists. "Internal drives" were as follows:(1) to strengthen the sense of environmental responsibility of tourists;(2) to establish a model of environmental responsible behaviors;(3) to strengthen the supervision of environmental responsible behaviors.

Superior mechanical properties and strengthening mechanisms of lightweight AlxCrNbVMo refractory high-entropy alloys(x=0,0.5,1.0)fabricated by the powder metallurgy process

Light and strong AlxCrNbVMo(x=0,0.5,and 1.0)refractory high-entropy alloys(RHEAs)were designed and fabricated via a the powder metallurgical process.The microstructure of the AlxCrNbVMo alloys consisted of a single BCC crystalline structure with a sub-micron grain size of 2-3μm,and small amounts(<4 vol.%)of fine oxide dispersoids.This homogeneous microstructure,without chemical segregation or micropores was achieved via high-energy ball milling and spark-plasma sintering.The alloys exhibited superior mechanical properties at 25 and 1000℃compared to those of other RHEAs.Here,CrNbVMo alloy showed a yield strength of 2743 MPa at room temperature.Surprisingly,the yield strength of the CrNbVMo alloy at 1000℃was 1513 MPa.The specific yield strength of the CrNbVMo alloy was increased by 27%and 87%at 25 and 1000℃,respectively,compared to the AlMo_(0.5) NbTa_(0.5)TiZr RHEA,which exhibited so far the highest specific yield strength among the cast RHEAs.The addition of Al to CrNbVMo alloy was advantageous in reducing its reduce density to below 8.0 g/cm^(3),while the elastic modulus decreased due to the much lower elastic modulus of Al compared to that of the CrNbVMo alloy.Quantitative analysis of the strengthening contributions,showed that the solid solution strengthening,arising from a large misfit effect due to the size and modulus,and the high shear modulus of matrix,was revealed to predominant strengthening mechanism,accounting for over 50%of the yield strength of the AlxCrNbVMo RHEAs.

Coarsening kinetics and strengthening mechanisms of core-shell nanoscale precipitates in Al-Li-Yb-Er-Sc-Zr alloy

The tailored nanoparticles with a complex core/shell structure can satisfy a variety of demands, such as lattice misfit, shearability and coarsening resistance. In this research, core-shell nanoscale Al3(Yb, Er, Sc,Zr, Li) composite particles were precipitated in Al-2 Li-0.1 Yb-0.1 Er-0.1 Sc-0.1 Zr(wt%) alloy through the double-aging treatment, in which the core was(Yb, Er, Sc, Zr)-rich formed at 300°C and the shell was Li-rich formed at 150°C. The coarsening kinetics and precipitate size distributions(PSDs) of Al3(Yb, Er, Sc,Zr, Li) particles aged at 150°C previously aged at 300°C for 24 h showed a better fit to the relation of 2∝ kt and normal distribution, indicating that the coarsening of precipitates was controlled by interface reaction, not diffusion. The Orowan bypass strengthening was operative mechanism at 150°C.

Fundamental strengthening mechanisms of ordered gradient nanotwinned metals

Ordered structures with functional units offer the potential for enhanced performance in metallic materials.Among these structures,gradient nanotwinned(GNT)microstructures demonstrate excellent controllability.This paper provides a comprehensive review of the current state-of-the-art studies on GNT structures,encompassing various aspects such as design strategies,mechanical properties characterization,spatially gradient strain evolution analysis,and the significant role of geometrically necessary dislocations(GNDs).The primary objective is to systematically unravel the fundamental strengthening mechanisms by gaining an in-depth understanding of the deformation behavior of nanotwinned units.Through this work,we aim to contribute to the broader field of materials science by consolidating knowledge and providing insights for the development of novel metallic materials with enhanced properties and tailored performance characteristics.

Strengthening mechanisms in magnesium alloys containing ternary Ⅰ,W and LPSO phases

This study was aimed at identifying underlying strengthening mechanisms and predicting the yield strength of as-extruded Mg-Zn-Y alloys with varying amounts of yttrium （Y） element. The addition of Y resulted in the formation of ternary 1 （Mg3YZn6）, W （Mg3Y2Zn3） and LPSO （Mg12YZn） phases which subse- quently reinforced alloys ZM31 ＋ 0.3Y, ZM31 ＋ 3.2Y and ZM31 ＋ 6Y, where the value denoted the amount of Y element （in wt%）. Yield strength of the alloys was determined via uniaxial compression testing, and grain size and second-phase particles were characterized using OM and SEM. In-situ high-temperature XRD was performed to determine the coefficient of thermal expansion （CTE）, which was derived to be 1.38 x 10^-5 K^-1 and 2.35 x 10^-5 K^-1 for W and LPSO phases, respectively. The individual strengthening effects in each material were quantified for the first time, including grain refinement, Orowan looping, thermal mismatch, dislocation density, load-bearing, and particle shearing contributions. Grain refinement was one of the major strengthening mechanisms and it was present in all the alloys studied, irrespective of the second-phase particles. Orowan looping and crE mismatch were the predominant strengthening mechanisms in the ZM31＋0.3Y and ZM31 ＋ 3.2Y alloys containing I and W phases, respectively, while load-bearing and second-phase shearing were the salient mechanisms contributing largely to the superior yield strength of the LPSO-reinforced ZM31 ＋ 6Y alloy.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Microstructures and strengthening mechanisms of Mg-8.2Gd-4.6Y-1.5Zn-0.4Zr alloy containing LPSO,β’ and γ type phases

The microstructures and strengthening mechanisms of the Mg-8.2 Gd-4.6 Y-1.5 Zn-0.4 Zr(wt%) alloy with long-period stacking ordered(LPSO),β’ and γ type phases were systematically studied.The results show that the LPSO with lamellar and block structures forms near the grain boundaries.The grains are clearly refined,and the 18 R LPSO phase is oriented along the extrusion direction after extrusion.Some particles also precipitate from the Mg matrix dynamically.The extruded alloy exhibits a remarkable agehardening response,and mechanical properties,with a tensile strength(TS) of 449 MPa,yield strength(YS) of 362 MPa,and elongation of 7.9% obtained in the peak-aged alloy.The strengthening mechanisms of the alloy in different states are discussed.Grain boundary and precipitation strengthening are the main strengthening mechanisms for the peak-aged alloy.

Tailored microstructures and strengthening mechanisms in an additively manufactured dualphase high-entropy alloy via selective laser melting

Dual-phase high-entropy alloys(DP-HEAs)with excellent strength-ductility combinations have attracted scientific interests.In the present study,the microstructures of AlCrCuFeNi3.0DP-HEA fabricated via selective laser melting(SLM)are rationally adjusted and controlled.The mechanisms engendering the hierarchical microstructures are revealed.It is found that the AlCrCuFeNi3.0fabricated by SLM at the scanning speed of 400 mm s-1falls into the eutectic coupled zone,and increasing the scanning speed will make this composition deviate away from the eutectic coupled zone due to the increased cooling rate.The enrichment of Cr and Fe solutes with large growth restriction values ahead of the solid/liquid interface can develop a constitutional supercooling zone,thus facilitating the heterogeneous nucleation and nearequiaxed grain formation.The synergy of the near-eutectic DP nano-structures and near-equiaxed grains instead of columnar ones effectively suppresses cracking for the as-built DP-HEA.During the tensile deformation,the intergranular back stress hardening similar to the grain-boundary strengthening is discovered.Meanwhile,the near-eutectic microstructures comprised of soft face-centered cubic and hard ordered bodycentered cubic(B2)DP nano-structures lead to plastic strain incompatibility within grains,thus producing the intragranular back stress.The Cr-rich nano-precipitates inside the B2 phase are found to be sheared by dislocation gliding and can complement the back stress.Additionally,multiple strengthening mechanisms are physically evaluated,and the back stress strengthening contributes obviously to the high performances of the as-built DP-HEA.

Strengthening Mechanisms for Ti-and Nb-Ti-micro-alloyed High-strength Steels

The strengthening mechanisms of hot-rolled steels micro alloyed with Ti （ST-TQS00） and Nh Ti （NT TQ500） were investigated by examining the microstructures of steels using optical microscope （OM）, scanning elec tron microscope （SEM） and transmission electron microscope （TEM）. The results revealed ahnost no differences in the solute solution strengthening and fine grained strengthcning of the two steels, whereas the contributions of pre cipitation strengthening and dislocation strengthening were different for ST-TQ500 and NT-TQ500. The measured precipitation strengthening effect of ST-TQ500 was 88 MPa higher than that of NT-TQ500： this difference was pri marily attributed to the stronger precipitation effect of thc Ti-containing nanoscale particles. The dislocation strengthening effect of ST TQ500 was approximately 80 MPa lower than that of NT-TQ500. This is tbought to be related to differences in deformation behavior during the finishing rolling stage; the inhibition of dynamic recrystallization from Nb in NT-TQ500 （Nb-Ti） may lead to higher density of dislocations in the microstructure.

Strengthening Mechanisms of 15 vol.% Al_(2)O_(3) Nanoparticles Reinforced Aluminum Matrix Nanocomposite Fabricated by High Energy Ball Milling and Vacuum Hot Pressing

Increasing nanoparticle volume fraction has been proved to be effective in improving the strength of nanoparticle reinforced Al matrix nanocomposite. However, the underlying mechanisms for the ultrahigh strength of those nanocomposites with high volume fraction(> 10 vol.%) nanoparticles are short of experimental research. In this study, the strengthening mechanisms of high strength Al matrix nanocomposite reinforced with 15 vol.% Al_(2)O_(3) nanoparticles were investigated experimentally and analyzed theoretically. The results show that the thermal mismatch induced geometrically necessary dislocations exhibit a negligible strengthening effect, because of their low density in the nanocomposite that is contradiction to the conventional dislocation punch model. Orowan mechanism makes a major strengthening contribution in view of the deformation process dominated by nanoparticle-dislocation interactions due to the extreme pinning effect of nanoparticles on dislocation motion. In addition, the several mechanisms including grain boundary strengthening, load transfer strengthening, and elastic modulus mismatch induced dislocation strengthening contribute to the strength increase.

Strengthening mechanism of T8-aged Al-Cu-Li alloy with increased pre-deformation

The microstructure evolution and mechanical properties of a T8-aged Al-Cu-Li alloy with increased pre-deformation(0-15%) were investigated,revealing the microstructure-strength relationship and the intrinsic strengthening mechanism.The results show that increasing the pre-deformation levels remarkably improves the strength of the alloy but deteriorates its ductility.Dislocations introduced by pre-deformation effectively suppress the formation of Guinier-Preston(GP) zones and provide more nucleation sites for T1 precipitates.This leads to more intensive and finer T1 precipitates in the samples with higher pre-deformation levels.Simultaneously,the enhanced precipitation of T1 precipitates and inhibited formation of GP zones cause the decreases in number and sizes of θ′ precipitates.The quantitative descriptions of the strength contributions from different strengthening mechanisms reveal that strengthening contributions from T1 and θ′ precipitates decrease with increasing pre-deformation.The reduced diameters of T1 precipitates are primarily responsible for their weakened strengthening effects.Therefore,the improved strength of the T8-aged Al-Cu-Li alloy is mainly attributed to the stronger strain hardening from the increased pre-deformation levels.

Effects of the extrusion parameters on microstructure,texture and room temperature mechanical properties of extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy

Microstructure,texture,and mechanical properties of the extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy were investigated at different extrusion temperatures(260 and 320℃),extrusion ratios(10:1,15:1,and 30:1),and extrusion speeds(3 and 6 mm/s).The experimental results exhibited that the grain sizes after extrusion were much finer than that of the homogenized alloy,and the second phase showed streamline distribution along the extrusion direction(ED).With extrusion temperature increased from 260 to 320℃,the microstructure,texture,and mechanical properties of alloys changed slightly.The dynamic recrystallization(DRX)degree and grain sizes enhanced as the extrusion ratio increased from 10:1 to 30:1,and the strength gradually decreased but elongation(EL)increased.With the extrusion speed increased from 3 to 6 mm/s,the grain sizes and DRX degree increased significantly,and the samples presented the typical<2111>-<1123>rare-earth(RE)textures.The alloy extruded at 260℃ with extrusion ratio of 10:1 and extrusion speed of 3 mm/s showed the tensile yield strength(TYS)of 213 MPa and EL of 30.6%.After quantitatively analyzing the contribution of strengthening mechanisms,it was found that the grain boundary strengthening and dislocation strengthening played major roles among strengthening contributions.These results provide some guidelines for enlarging the industrial application of extruded Mg-RE alloy.

Formability and strengthening mechanism of AA6061 tubular components under solid granule medium internal high pressure forming

A new technological process of tube forming was developed, namely solution treatment → granule medium internal high pressure forming → artificial aging. During this process, the mechanical properties of AA6061 tube can be adjusted by heat treatment to satisfy the process requirements and the processing method can also be realized by granule medium internal high pressure forming technology with the features of convenient implementation, low requirement to equipment and flexible design in product. Results show that, at a solution temperature of 560 ℃ and time of 120 min, the elongation of AA6061 increases by 313%, but the strength and the hardness dramatically decrease. At an aging temperature of 180 ℃ and time of 360 min, the strength and hardness of AA6061 alloy are recovered to the values of the as-received alloy. The maximum expansion ratio（MER） of AA6061 tube increases by 25.5% and the material properties of formed tube reach the performances of raw material.

Strengthening contributions in ultra-high strength cryorolled Al-4%Cu-3%TiB_2 in situ composite

Ultra-high strength Al alloy system was developed by cryorolling and the contribution of various strengthening mechanisms to the overall yield strength of the system was evaluated. Cryorolling of Al-4%Cu-3%TiB2 in situ composite followed by short annealing at 175 ℃ and ageing at 125℃ resulted in an ultra-high yield strength of about 800 MPa with 9%total elongation. The strengthening contributions form solid solution strengthening, grain refinement, dislocation strengthening, precipitation hardening and dispersion strengthening were evaluated using standard equations. It was estimated that the maximum contribution was from grain refinement due to cryorolling followed by precipitation and dispersion strengthening.