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.

Enhanced mechanical properties of molybdenum alloy originating from composite strengthening of Re and CeO_(2)

To enhance the mechanical properties of molybdenum alloys at both room and high temperatures,Mo-14Re-1CeO_(2)alloy was synthesized using the powder metallurgy method,and the corresponding microstructure and mechanical properties were characterized.The results indicate that the ultimate tensile strength of Mo-14Re-1CeO_(2)reaches 657 MPa,with a total elongation of 35.2%,significantly higher than those of pure molybdenum(453 MPa,and 7.01%).Furthermore,the compression strength of Mo-14Re-1CeO_(2)at high temperature(1200℃)achieves 355 MPa,which is still larger than that of pure molybdenum(221 MPa).It is revealed that there is a coherent interface between CeO_(2)and the Mo-14Re matrix with CeO_(2)particles uniformly distributed in both intergranular and intragranular regions.The improvements in mechanical properties are primarily attributed to the formation of Mo-Re solid solution,grain refinement,and dispersion strengthening effect of CeO_(2).

Experimental investigation of engineered geopolymer composite for structural strengthening against blast loads

The recent increase in blast/bombing incidents all over the world has pushed the development of effective strengthening approaches to enhance the blast resistance of existing civil infrastructures.Engineered geopolymer composite(EGC)is a promising material featured by eco-friendly,fast-setting and strain-hardening characteristics for emergent strengthening and construction.However,the fiber optimization for preparing EGC and its protective effect on structural elements under blast scenarios are uncertain.In this study,laboratory tests were firstly conducted to evaluate the effects of fiber types on the properties of EGC in terms of workability,dry shrinkage,and mechanical properties in compression,tension and flexure.The experimental results showed that EGC containing PE fiber exhibited suitable workability,acceptable dry shrinkage and superior mechanical properties compared with other types of fibers.After that,a series of field tests were carried out to evaluate the effectiveness of EGC retrofitting layer on the enhancement of blast performance of typical elements.The tests include autoclaved aerated concrete(AAC)masonry walls subjected to vented gas explosion,reinforced AAC panels subjected to TNT explosion and plain concrete slabs subjected to contact explosion.It was found that EGC could effectively enhance the blast resistance of structural elements in different scenarios.For AAC masonry walls and panels,with the existence of EGC,the integrity of specimens could be maintained,and their deflections and damage were significantly reduced.For plain concrete slabs,the EGC overlay could reduce the diameter and depth of the crater and spallation of specimens.

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.

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.

Strengthening effects of BFRP on reinforced concrete beams

Reinforced concrete （RC） beams externally bonded with basalt fiber reinforced polymer （BFRP） are experimentally investigated by using different numbers of bonding plies, transverse anchorages as well as the initial conditions of strengthened beams. The performances of the BFRP strengthening are compared with those of the carbon fiber reinforced polymer （CFRP） and the glass fiber reinforced polymer （GFRP） under the same experimental condition. Experimental results indicate that the strength and ductility of the strengthened beam with two plies of the BFRP are improved remarkably than those with one ply. The strengthening effects of the BFRP lie between those of the CFRP and the GFRP. The BFRP strengthening is little influenced by pre-cracks of concrete. Most failures are caused by interfaciai debonding induced by flexural cracks in the experiment. Clamping of Uwraps along the whole beam is less efficient than endpoint anchorage for increasing the ultimate load of the strengthened beam. Finally, the models suggested by the five guidelines for predicting the debonding strain of the CFRP are extended to the BFRP and the conservative estimates of the debonding strain of the BFRP are given as well.

Investigation on sintering and deformation strengthening of Mo-Cu alloy

Mo-Swt%Cu nanocomposite powders were fabricated by mechanical alloying, and full density alloy was obtained via liquid-phase sintering and post-treatment process. The microstructure of Mo-8wt%Cu alloy was investigated by scanning elec-tron microscope （SEM） , and the effects of process parameters on relative density, tensile strength and elongation were stud-ied. The results indicate that the relative density of Mo-Cu alloy is 98. 6% after sintering at 1 250℃ for 30 min, and its micro-structure is composite network The full density of Mo-Cu alloy can be obtained when specimens are treated through deforma-tion strengthening process of rotating forging and hydrostatic extrusion The tensile strength and elongation rate are 576 MPa and 5. 8% ,respectively, when hydrostatic extrusion deformation degree is 40%.

Study on Interactive Computer Program for Seismic Appraisal and Seismic Strengthening of R.C. Frame Structures

In many cases, seismic appraisal of existing structures is carried out by the two step method and seismic strengthening of R.C. frame structures is solved just based on rough calculation or experience of designers, which may lead to either lack of safety or too conservative in design. According to some related criteria and experts experience, a computer program is developed specially for seismic appraisal and seismic strengthening of R.C. frames (not more than 10 storeys) in this paper. Because the progra...

Study on Forming and Strengthening of Lishui Tourist Transport Center

The current development of Lishui tourist transport center is introduced,its forming reason and current problems are analyzed,finally its strengthening strategy is proposed.

MECHANICAL PROPERTIES OF CONTINUOUS CARBON FIBER SHEET USED IN STRENGTHENING REINFORCED CONCRETE MEMBERS IN BENDING

This paper discusses the method of determining the tensile strength and modulus of elasticity of continuous carbon fiber sheet.Five simply supported beams strengthened by CFS were tested.Based on th experiment,a computer analysis program is developed and a simplified formula is proposed.The calculated results agree well with the experimental results.The design method suggested by this paper can meet the engineering requirements.

Strengthening International Cooperation in the New Millennium

Attheturnofthenewmillennium,IwouldliketoextendthebestwishestoallChineseandforeignfriendsofeconomicandtradecircles,onbehalfofChinaCouncilforthePromotionofInternationalTrade(CCPIT)andChinaChamberofInternationalCommerce(CCOIC).CCPIThasbeendedicatedtopro...

Effect of large cold deformation after solution treatment on precipitation characteristic and deformation strengthening of 2024 and 7A04 aluminum alloys

The effects of large cold deformation after solution treatment on the precipitation characteristic and deformation strength of 2024 and 7A04 Al alloys were investigated.The tensile property tests indicate that the ageing response of the 2024 aluminum alloy is accelerated when treated by cold deformation after solution treatment,and the tensile strength is increased to about 140 MPa while the elongation still keeps above 8%.However,compared with the 2024 alloys,although the aging response of the cold deformed 7A04 aluminum alloy is accelerated,the tensile strength has not changed obviously and the elongation is even decreased drastically.The results of TEM observation show that the S’ phase inside the dislocation cells and at the boundaries of the dislocation cells of the 2024 aluminum alloy has a uniform distribution.But in 7A04 aluminum alloy the club-shaped η’ phase forms at the boundaries of dislocation cells even on dislocation lines,while there still exists small spheric G.P zone in the region with less dislocation.In addition the precipitates in 7A04 alloy with cold rolling present more obvious tendency of growth and coarsening than those without cold rolling.It is indicated that the coherent strain energy of the cylinder formed G.P zones with matrix in the 2024 alloy is smaller than that of the spherical G.P zone in 7A04 alloy,for which a different distribution of precipitates and different effect of strengthening are caused in artificial ageing after resolution treatment and large cold deformation.

Machining the Integral Impeller and Blisk of Aero-Engines:A Review of Surface Finishing and Strengthening Technologies

The integral impeller and blisk of an aero-engine are high performance parts with complex structure and made of difficult-to-cut materials. The blade surfaces of the integral impeller and blisk are functional surfaces for power transmission, and their surface integrity has signif- icant effects on the aerodynamic efficiency and service life of an aero-engine. Thus, it is indispensable to finish and strengthen the blades before use. This paper presents a comprehensive literature review of studies on finishing and strengthening technologies for the impeller and blisk of aero-engines. The review includes independent and inte- grated finishing and strengthening technologies and dis- cusses advanced rotational abrasive flow machining with back-pressure used for finishing the integral impeller and blisk. A brief assessment of future research problems and directions is also presented.

Lightweight Design of Automobile Drive Shaft Based on the Characteristics of Low Amplitude Load Strengthening

There are two kinds of internationally recognized approaches in terms of lightweight design.One is based on fatigue accumulated damage theory to achieve better reliability by optimal structural design; another is to use high performance lightweight materials.The former method takes very few considerations on the structural strengthening effects caused by the massive small loads in service.In order to ensure safety,the design is usually conservative,but the strength potential of the component is not fully exerted.In the latter method,cost is the biggest obstacle to lightweight materials in automotive applications.For the purpose of light weighting design on a fuel cell vehicle,the new design method is applied on drive shafts.The method is based on the low amplitude load strengthening characteristics of the material,and allows the stress,corresponding to test load,to enter into the strengthened range of the material.Under this condition,the light weighting design should assure that the reliability of the shaft is not impaired,even maximizes the strength potential of machine part in order to achieve the weight reduction and eventually to reduce the cost.At last,the feasibility of the design is verified by means of strength analysis and modal analysis based on the CAD model of light weighted shaft.The design applies to the load case of half shaft in independent axle,also provides technological reference for the structural lightweight design of vehicles and other machineries.

Aging Strengthening in Rapidly Solidified Cu-Cr-Sn- Zn Alloy

It is known that the strength of alloys can be successfully improved by rapid solidification. The paper presents a process where Cu-Cr-Sn-Zn lead frame alloy is produced by rapid solidification and aging. The microcrystalline structure of rapidly solidified Cu-Cr-Sn-Zn alloy is smaller grain structure examined by optical metallography. The effects of aging processes on the microstructure and properties of the lead frame alloy were investigated. Aged at 500℃ for 15 min the fine coherent precipitates Cr distribute in Cu matrix observed by transmission electron microscopy and the properties of hardness and electrical conductivity properties can reach 178HV and 61%IACS, respectively.

Improved mechanical properties and strengthening mechanism with the altered precipitate orientation in magnesium alloys

Aging prior to twinning deformation was proposed to alter the precipitate orientation of the plate-shapedβ-MgAlfrom(0002)basal planes(named basal plates)to■prismatic planes(named prismatic plates)in AZ31 Mg alloy.The experimental results showed that the compressive yield strength(CYS)of the sample containing prismatic plates increased 40 MPa and the compression ratio raised by 22%compared to that containing basal plates.The underlying strengthening mechanism was analyzed via a yield strengthen(YS)model with a function of grain size,precipitate characters(size,oritention,fraction)and Schmid factor(SF).It revealed that the improvement of CYS was mainly attributed to the altered precipitate orientation and refined grain size produced by twinning deformation.Particularly,the prismatic plates always have a stronger hardening effect on basal slip than basal plates under the same varites of precipitate diameter and SF.Besides,the decreased CRSS ratio of prismatic slip to basal slip revealed that the activity of non-basal slip in Mg alloy might be enhanced.More activated slip systems provided more mobile dislocations,contributing to the large compression ratio of the Mg rolled sheet with prismatic plates.

Co-strengthening contribution of δ′and T_1 precipitates in Al-Li alloys 2090 and 2090+Ce

The microstructure of dislocation in two kinds of ahiminum-lithium alloys 2090 and 2090 ＋ Ce was observed by means of TEM technology. The contributions of δ＇ and T1 precipitates to strength were separately calculated by using the results of quantitative metallography and analysis of micro-deformation behavior; the co-strengthening effect of δ＇ and T1 precipitates was studied. The results show that the adding relationship of co-strengthening of δ＇ and T1 is in accordance with q = 1.4 form at the near peak-aged condition, i.e., △τ^1.4δ = △τ^1.4δ＋ △τ^1.4T1, but the adding relationship is approximately a linear relation （q = 1） at the under-aged condition and becomes the parabola form when over-aged （q = 2）. The adding relationship of co-strengthening contribution of δ＇ and T1 is obviously dependent on aging time.

Fatigue Life Prediction under Service Load Considering Strengthening Effect of Loads below Fatigue Limit

Lightweight design requires an accurate life prediction for structures and components under service loading histories. However, predicted life with the existing methods seems too conservative in some cases, leading to a heavy structure. Because these methods are established on the basis that load cycles would only cause fatigue damage, ignore the strengthening effect of loads. Based on Palmgren-Miner Rule （PMR）, this paper introduces a new method for fatigue life prediction under service loadings by taking into account the strengthening effect of loads below the fatigue limit. In this method, the service loadings are classified into three categories： damaging load, strengthening load and none-effect load, and the process for fatigue life prediction is divided into two stages： stage I and stage II, according to the best strengthening number of cycles. During stage I, fatigue damage is calculated considering both the strengthening and damaging effect of load cycles. While during stage II, only the damaging effect is considered. To validate this method, fatigue lives of automobile half shaft and torsion beam rear axle are calculated based on the new method and traditional methods, such as PMR and Modified Miner Rule （MMR）, and fatigue tests of the two components are conducted under service loading histories. The tests results show that the percentage errors of the predicted life with the new method to mean life of tests for the two components are –3.78% and –1.76% separately, much lesser than that with PMR and MMR. By considering the strengthening effect of loads below the fatigue limit, the new method can significantly improve the accuracy for fatigue life prediction. Thus lightweight design can be fully realized in the design stage.

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.

Nanoscale precipitates and comprehensive strengthening mechanism in AISI H13 steel

The effects of heat treatment on the precipitates and strengthening mechanism in AISI H13 steel were investigated. The results showed that the presence of nanoscale precipitates favorably affected grain refinement and improved the yield strength. The volume fraction of precipitates increased from 1.05% to 2.85% during tempering, whereas the average precipitate size first decreased then increased during tempering. Contributions to the yield strength arising from the various mechanisms were calculated quantificationally, and the results demonstrated that grain refinement and dislocation density most strongly influenced the yield strength. In addition, under the interaction of average size and volume fraction, precipitates＇ contribution to the yield strength ranged from 247.9 to 378.5 MPa. Finally, a root-mean-square summation law of σ = σg ＋ σs ＋（σd^2 ＋ σp^2）^1/2, where σg, σs, σd, and σp represent the contributions of fine-grain strengthening, solid-solution strengthening, dislocation strengthening, and precipitation strengthening, respectively, was confirmed as the most applicable for AISI H13 steel, which indicates a strong link between precipitates and dislocations in AISI H13 steel.