Designing new low alloyed Mg-RE alloys with high strength and ductility via high-speed extrusion

Two new low-alloyed Mg-2RE-0.8Mn-0.6Ca-0.5Zn(wt%,RE=Sm or Y)alloys are developed,which can be produced on an in-dustrial scale via relatively high-speed extrusion.These two alloys are not only comparable to commercial AZ31 alloy in extrudability,but also have superior mechanical properties,especially in terms of yield strength(YS).The excellent extrudability is related to less coarse second-phase particles and high initial melting point of the two as-cast alloys.The high strength-ductility mainly comes from the formation of fine grains,nano-spaced submicron/nano precipitates,and weak texture.Moreover,it is worth noting that the YS of the two alloys can maintain above 160 MPa at elevated temperature of 250°C,significantly higher than that of AZ31 alloy(YS:45 MPa).The Zn/Ca solute segregation at grain boundaries,the improved heat resistance of matrix due to addition of RE,and the high melting points of strengthening particles(Mn,MgZn_(2),and Mg-Zn-RE/Mg-Zn-RE-Ca)are mainly responsible for the excellent high-temperature strength.

Recent developments of the high strength and high ductility nanostructured materials

This talk will summarize the recent work related to a kind of new nanomaterials produced by the SMAT (surface mechanical attrition treatment).The concept of surface nanocrystallization of materials will be presented.In terms of the grain refinement mechanism induced by plastic deformation,a novel surface mechanical attrition(SMA) technique was developed for synthesizing a nanostructured surface layer on metallic materials in order to upgrade the overall properties and performance.The grain refinement mechanism of the surface layer during the SMA treatment will be analyzed in terms of the nanostructure observations in several typical materials.Very high yield stress(5 times of the base material) on the surface layer of the material obtained by the SMAT has been observed.The effect of surface nanostructures on the mechanical behavior and on the failure mechanism of metallic material shows the possibility to develop a new strength gradient composite using co-rolling and nitriding.The role of residual stress induced during the treatment will be investigated and discussed.The developed materials are also porosity free materials which can be used as reference material for the local mechanical behavior investigation technique such as the nanoindentation.A general concept for obtaining high strength and high ductility nanostructured materials will be presented.The exceptional high strength and high ductility steels have developed.The simulation of the mechanisms for improving ductility of high strength nanostructured materials will be presented.The potential applications for the land transportation vehicles(car,bus,train) and wind energy have been investigated.Some examples of concept design for the integration of the advanced nanostructured steels will be presented.

Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10^-4s^-1to 1 s^-1, the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions（n2and n1） and the transition strain（εL） decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.

Development of a high-strength Mg alloy with superior ductility through a unique texture modification from equal channel angular pressing

In the current study,a homogenous ultra-fine grained microstructure with average grain size of 1.0μm is achieved in the Mg-Zn-Ca-Mn alloy through the reduplicative equal channel angular pressing(ECAP)at 300℃,and the mechanical properties are remarkably improved,with room-temperature yield strength of 269.6 MPa and elongation of 22.7%.The twinning deformation results in a discontinuous recrystallization behavior in the initial stage of ECAP.With further deformation,the continuously dynamic recrystallization contributes to an obvious grain refinement effect.The activation of non-basal slip system leads to the formation of a unique basal texture,which is related to the elevated ECAP temperature and the decreased grain size.Both grain refinement and texture modification derived from ECAP process result in the increase of yield strength,while the cracked secondary phase particles are beneficial to the enhanced ductility,through reducing the stress concentration and hindering premature failure.

Mechanism of improving ductility of high strength concrete T-section beam confined by CFRP sheet subjected to flexural loading

For the purpose of inventing a new seismic retrofitting method for the reinforced high strength concrete （HSC） T-section beam using carbon fiber reinforced polymer （CFRP） sheet, three series, a total of twelve T-section beams with nine specimens confined by CFRP sheet in the plastic zone and three control beams were conducted up to failure under four-point bending test. The effectiveness of confining CFRP sheet on improving the flexural ductility of tmstrengthened T-section beams was studied. The parameters such as the width and the thickness of CFRP sheet and the type of T-section were analyzed. The experimental results show that ductility and rotation capacity of plastic hinge can be improved by the confinement of CFRP sheet, and the ductility indices increase with the increment of width and thickness of CFRP sheet. A plastic rotation model considering the width of CFRP sheet and the effect of flange of T-section beam is proposed on the basis of the model of BAKER, and the test results show a good agreement with the perdicted results. The relevant construction suggestions for seismic retrofitting design of beam-slabs system in cast-in-place framework structure are presented.

Toward the development of Mg alloys with simultaneously improved strength and ductility by refining grain size via the deformation process

Magnesium(Mg) alloys, as the lightest metal engineering materials, have broad application prospects.However, the strength and ductility of traditional Mg alloys are still relativity low and difficult to improve simultaneously.Refining grain size via the deformation process based on the grain boundary strengthening and the transition of deformation mechanisms is one of the feasible strategies to prepare Mg alloys with high strength and high ductility.In this review, the effects of grain size on the strength and ductility of Mg alloys are summarized, and fine-grained Mg alloys with high strength and high ductility developed by various severe plastic deformation technologies and improved traditional deformation technologies are introduced.Although some achievements have been made, the effects of grain size on various Mg alloys are rarely discussed systematically and some key mechanisms are unclear or lack direct microscopic evidence.This review can be used as a reference for further development of high-performance fine-grained Mg alloys.

Seismic performance of steel reinforced ultra high-strength concrete composite frame joints

To investigate the seismic performance of a composite frame comprised of steel reinforced ultra high-strength concrete （SRUHSC） columns and steel reinforced concrete （SRC） beams, six interior frame joint specimens were designed and tested under low cyclically lateral load. The effects of the axial load ratio and volumetric stirrup ratio were studied on the characteristics of the frame joint performance including crack pattern, failure mode, ductility, energy dissipation capacity, strength degradation and rigidity degradation. It was found that all joint specimens behaved in a ductile manner with flexural-shear failure in the joint core region while plastic hinges appeared at the beam ends. The ductility and energy absorption capacity of joints increased as the axial load ratio decreased and the volumetric stirIup ratio increased. The displacement ductility coefficient and equivalent damping coefficient of the joints fell between the corresponding coefficients of the steel reinforced concrete （SRC） frame joint and RC frame joint. The axial load ratio and volumetric stirrup ratio have less influence on the strength degradation and more influence on the stiffness degradation. The stiffness of the joint degrades more significantly for a low volumetric stirrup ratio and high axial load ratio. The characteristics obtained from the SRUHSC composite frame joint specimens with better seismic performance may be a useful reference in future engineering applications.

Experimental investigation of axially loaded steel fiber reinforced high strength concrete-filled steel tube columns

An experimental study on the compressive behavior of steel fiber reinforced concrete-filled steel tube columns is presented. Specimens were tested to investigate the effects of the concrete strength, the thickness of steel tube and the steel fiber volume fraction on the ultimate strength and the ductility. The experimental results indicate that the addition of steel fibers in concrete can significantly improve the ductility and the energy dissipation capacity of the concrete-filled steel tube columns and delay the local buckling of the steel tube, but has no obvious effect on the failure mode. It has also been found that the addition of steel fibers is a more effective method than using thicker steel tube in enhancing the ductility, and more advantageous in the case of higher strength concrete. An analytical model to estimate the load capacity is proposed for steel tube columns filled with both plain concrete and steel fiber reinforced concrete. The predicted results are in good agreement with the experimental ones obtained in this work and literatures.

Flexural Behaviour of High－Strength Steel Fiber－Reinforced Concrete Beams

This paper presents the research results of twelve high strength concrete beams reinforced with steel fibers and bars. Fiber type I and II reduce the deflection by more than 25% and increase the ultimate load by about 10% compared to high strength concr

Experimental Study on Flexural Behavior of Post-Tensioning Bonded Partially Prestressed Ultra-High Strength Concrete Beams

This paper presents the results of four partially prestressed ultra-high strength concrete beams in flexure. The test results are used to evaluate the effects of prestressing tendon depth and area on flexure behavior of specimen beams. The test results indicate that: the cracking load,yielding load,peak load and stiffness postcracking of specimen beams are enhanced by reducing prestressing tendon depth or increasing prestressing tendon area, and the flexural ductility is improved by increasing prestressing tendon depth or reducing prestressing tendon area. The effect of complex reinforcement index considering the strength of the equivalence principle and the reinforcement position on loading levels under serviceability limit state,flexural strength and displacement ductility factor are studied. The influence coefficient of prestressing tendon kpis introduced in the complex reinforcement index. As the complex reinforcement index increases, the loading levels under serviceability limit state and flexural strength increases linearly,and the displacement ductility factor decreases linearly. The test results also verify the conventional beam flexural theory based on the plane cross-section assumption for predicting ultimate flexural strength of partially prestressed ultra-high strength concrete beams is valid. After the introduction of the coefficient kp,the calculation method of cracks in code for design of concrete structure in china are appropriated for the specimen beams.

High performance nano-structured stainless steel sheet

High-strength steels have been attracting more and more attention of people,Unfortunately.deterioration of ductility limited their applications.To solve this problem,a nano-structured stainless steel sheet is developed to combine high strength and high ductility.Processing of the surface mechanical attrition treatment(SMAT) was introduced to obtain a nano-grain layer on the double surface of the stainless steel sheet.The microstructure of the nanostructured steel sheet is characterized by an alternate distribution of coarse grained layer and nanocrystalline layer.Then the dual surface nano-crystallized stainless steel sheets were co-warm rolled at 500℃.The experimental results reveal that the mechanical properties of the nanostructured steel exhibit high yield strength in the range of 700 -950 MPa and tensi le strength higher than 930 MPa.Moreover,elongation to fracture reaches to 15%-48%, together with a uniform elongation stabilized to 13%-45%.

High strength and ductility of 34CrNiMo6 steel produced by laser solid forming

Because of the excellent mechanical properties of 34 CrNiMo6 steel, it is widely used in high-value components. Many conventional approaches to strengthening-steels typically involve the loss of useful ductility.In this study, 34 CrNiMo6 Steel having high strength and ductility is produced by laser solid forming(LSF)with a quenching-tempering(QT) treatment. Tempering of bainite is mainly by solid phase transformation in the previous LSF layers during the LSF process. The stable microstructure of LSF consists of ferrite and fine carbides. The microstructure transfers to tempered sorbite after heat-treatment. The tensile properties of the LSF steel meet those of the wrought standard. The UTS and elongation of LSF sample at 858 MPa, 19.2%, respectively, are greater than those of the wrought. The QT treatment enhanced the ultimate tensile strength and yield strength of the LSF sample. The ultimate tensile strength, yield strength, reduction in area, and elongation of the LSF+QT sample at 980 MPa, 916 MPa, 58.9%, and 13.9%,respectively, are greater than those of the wrought standard. The yield strength of the LSF+QT sample is approximately 1.27 times that of the wrought. The LSF samples failed in a ductile fracture mode, while the LSF+QT samples showed mixed-mode failure. The defects have only a small effect on the tensile properties owing to the excellent ductility of the LSF sample.

Achieving high strength and ductility in high-entropy alloys via spinodal decomposition-induced compositional heterogeneity

The compositional heterogeneity in high-entropy alloys(HEAs)has been reported to be an inherent en-tity,which significantly alters the mechanical properties of materials by tuning the variation of lattice resistance for dislocation motion.However,since the body-centered cubic(BCC)structure is not close-packed,the change of lattice resistance is less sensitive to the normal concentration wave compared to that in face-centered cubic(FCC)structured materials.In this work,we selected a refractory bcc HEAs TiZrNbTa for the matrix and added a small amount of Al to facilitate the special spinodal decomposition structure.In particular,(TiZrNbTa)98.5 Al 1.5 displayed a typical basket-weave fabric morphology of spinodal decomposition structure with a characteristic periodicity of∼8 nm and had an optimal combination of strength and ductility(the yield strength of 1123±9 MPa and ductility of∼20.7%±0.6%).It was de-termined that by doing in situ TEM mechanical testing,the plastic deformation was dominated by the formation and operation of dislocation loops which provided both edge and screw components of dislo-cations.The synergetic effect of the remarkable chemical heterogeneity created by the spinodal decompo-sition and the spreading lattice distortion in high entropy alloys is quite effective in tuning the mobility of different types of dislocations and facilitates dislocation interactions,enabling the combination of high strength and ductility.

Development of Low Carbon Microalloyed High Strength Ultra-Thin Cast Strip Products Produced by the Castrip~ Process With Excellent Strength-Ductility Properties

The CASTRIP* process produces a range of high strength Ultra-thin Cast Strip (UCS) products (380-550 MPa) in thicknesses between 0.9 mm and 1.5 mm, which is very challenging to produce via conventional hot and cold rolled processing routes. The twin roll CASTRIP process fully exploits the hardenability and strengthening potential of Nb in a low C-Mn-Nb-V microalloyed steel type. Significant microstructural strengthening from solute Nb was obtained, even at low microalloying levels, as well as modest Mn additions, through enhancing the hardenability and further strengthening was obtained in coated products by exploiting age hardening during processing on a continuous hot dip galvanising line. Atom probe tomography and TEM determined that Nb was retained in solid solution and subsequent age hardening resulted from the formation of Nb and V rich nanosized particles. Age hardening was achieved without loss of ductility producing galvanised strip with an excellent strength-ductility combination (Y.S. 】600 MPa, T.E. 】10%).

Microstructure tailoring of Al_(0.5)CoCrFeMnNi to achieve high strength and high uniform strain using severe plastic deformation and an annealing treatment

Ultrafine-grained alloys fabricated by severe plastic deformation(SPD)have high strength but often poor uniform ductility.SPD via high-ratio differential speed rolling(HRDSR)followed by an annealing treatment was applied to Al_(0.5)CoCrFeMnNi to design the microstructure from which both high strength and high uniform strain can be achieved.The optimized microstructure was composed of an ultrafine-grained FCC matrix(1.7-2μm)with a high fraction of high-angle grain boundaries(61%-66%)and ultrafine BCC particles(with a size of 0.6-1μm and a volume fraction of8%-9.3%)distributed uniformly at the grain boundaries of the FCC matrix.In the severely plastically deformed microstructure,the nucleation kinetics of the BCC phase was accelerated.Continuous static recrystallization(CSRX)took place during the annealing process at 1273 K.Precipitation of the BCC phase particles occurring concurrently with CSRX effectively retarded the grain growth of the FCC grains.The precipitation of the hard and brittleσphase was,however,suppressed.The annealed sample processed by HRDSR with the optimized microstructure exhibited a high tensile strength of over 1 GPa with a good uniform elongation of 14%-20%.These tensile properties are comparable to those of transformation-induced plasticity steel.Strengthening mechanisms of the severely plastically deformed alloy before and after annealing were identified,and each strengthening mechanism contribution was estimated.The calculated results matched well with the experimental results.

Effect of aluminium element on microstructure and properties of weld metal of 960 MPa steel

A kind of self-protective flux cored wire has been developed for joining 960 MPa high strength steel. Weld metal containing different aluminium elements contents was obtained by changing the content of aluminum powder in the composition of the flux core. The strength and toughness of weld metal were tested by tensile test and impact test at different temperatures, and the influence mechanism of aluminium element on the microstructure and mechanical properties of weld metal was analyzed by means of metallographic microstructure observation and scanning electron microscope observation. The results show that aluminium element content on impact ductility of weld metal of 960 MPa high strength steel is great, but the influence on tensile strength and elongation of weld metal is little. With increasing aluminium element contents of weld metal, the impact energy of weld metal increases at first and then decreases, the best aluminium element content of weld metal is 0.2 wt.%. Aluminium oxide is easy to be formed in weld metal with low aluminium element contents, and the aluminium oxide can easily become nucleation particle for acicular ferrite. It is conducive to formation more acicular ferrite and will improve impact absorbing energy of weld metal. Aluminium nitride will easily formed in weld metal with high aluminium element content, and the coarse ferrite microstructure appears in weld metal and reduces impact energy of weld metal.

Development of Nb-V-Ti Hot-Rolled High Strength Steel With Fine Ferrite and Precipitation Strengthening

A hot-rolled steel with high yield strength of 700 MPa, good elongation of about 20% and low ductile-brittle transition temperature （DBTT） lower than -70℃ has been developed in laboratory. The results show that adopting finishing rolling temperature of around 800℃ is rational, and coiling temperature is between 400 and 500℃ The strength of developed 700 MPa hot-rolled high strength steel is derived from the cumulative contribution of fine grain size, dislocation hardening and precipitation hardening. The fine grain strengthening and precipitation hardening are the dominant factors responsible for such high strength, and good elongation and toughness are predominantly due to fine grain ferrite.

High Ductility and Toughness of a Micro-duplex Medium-Mn Steel in a Large Temperature Range from—196 ℃ to 200 ℃

A medium-Mn steel （0.2C5Mn） was processed by intercritical annealing at different temperatures （625 ℃ and 650 ℃ ）. An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was de- veloped by austenite reverse transformation （ART） during intercritical annealing after forging and hot rolling. Ultra- high ductility with a total elongation higher than 30% was achieved in the temperature range from -196 ℃ to 200 ℃, and high impact toughness no less than 200 J at -40 ℃ was obtained. Based on the analysis of microstructure and mechanical properties, it was found that the enhanced ductility was determined by the phase transformation effect of austenite （TRIP effect）, while the delayed ductile to brittle transition was controlled by austenite stability.

Tensile ductility improvement of AlSi9Cu1 alloy by chemical composition optimization

Diesel engines, characterized by higher breakout pressure and compression ratio in comparison with gasoline engines, require particularly elevated tensile properties for their engine parts. In order to maintain both high strength and high ductility in the cylinder head, i.e., to obtain higher percent elongation without further reducing the tensile strength, Al Si9Cu1 alloy was used to prepare the cylinder head in an aluminum diesel engine. At the same time, the effect of different modification elements, Na or Sr, and Fe content on the reduction of secondary dendrite arm spacing(SDAS) was discussed, and the design of T7 heat treatment parameters were analyzed in order to improve the tensile ductility. The result shows:(1) The SDAS is as small as 18±3 μm for the Sr modified alloy.(2) The percent elongation of the alloy with Sr modification increases by 66.7% and 42.9%, respectively, compared with the unmodified alloy and the alloy with Na modification.(3) Lower Fe content alloy(0.10%) gives good results in percent elongation compared to the alloy with higher Fe content(0.27%); in particular, after Sr modification and T7 heat treatment, the elongation of over 5% is obtained.

基于MMC模型的Q460C高强结构钢延性断裂性能研究

为研究Q460C高强结构钢的延性断裂性能,对处于不同应力状态的9个试件进行了单调加载断裂试验。采用修正加权平均(MWA)法获得了Q460C钢材直至断裂的全过程真实应力-真实应变曲线,通过编写MATLAB优化程序校准了VGM和MMC断裂预测模型的参数。借助VUMAT子程序将相关参数代入ABAQUS/Explicit求解器模拟各试件的断裂行为,评估了两种断裂模型对各试件断裂预测的相对精度。通过对已有文献中两个圆周缺口试件的断裂模拟,验证了该文校准的MMC模型参数对Q460钢断裂预测的普适性。研究结果表明:采用MMC模型进行数值模拟得到的荷载-位移响应与试验结果吻合度较好,尤其是对纯剪和剪拉试件的断裂模拟;带应力软化的MMC模型可准确且形象地再现各试件的起裂和裂缝扩展;该文校准的MMC断裂模型参数对Q460钢材断裂行为预测具有相对较好的适用性。