Tailoring the microstructural characteristics and enhancing creep properties of as-cast Mg-5Bi-5Sn alloy through Mn addition

The creep properties, microstructural characteristics and creep mechanisms of as-cast Mg-5Bi-5Sn(BT55) alloy without and with Mn(BTM550) addition were investigated via creep at 423, 448, and 473 K as well as stresses of 30, 50 and 75 MPa. The results indicate that adding Mn can result in the formation of primary and the dynamic precipitated α-Mn phases. In addition, the morphology of the precipitated Mg_(3)Bi_(2) phase and the orientation relationship between Mg_(2)Sn precipitates and α-Mg can be effectively modified. Tailoring the microstructural characteristics is responsible for the improved creep performance of BTM550 alloy. The dominant creep mechanisms in BT55 and BTM550 alloys are dislocation cross-slip and climb, respectively. Furthermore, twinning and pyramidal slip play an assisting part in both alloys during creep process.

Influence of isothermal bainitic processing on the mechanical properties and microstructure characterization of TRIP steel

The mechanical properties of transformation induced plasticity （TRIP） steel are strongly affected by the conditions of iso-thermal bainitic processing. The multiphase microstructure of TRIP steel under different conditions of isothermal bainitic processing was investigated using OM,SEM,XRD and TEM. The volume fraction of retained austenite and the carbon content in austenite were determined quantitatively using X-ray diffraction patterns. The relationship between mechanical properties and isothermal bainitic processing parameters was investigated. The stability of retained austenite was analyzed by the volume fraction of retained austenite and the carbon content in retained austenite. The experimental results show that the multiphase microstructure consists of ferrite,bainite and metastable retained austenite.To obtain good mechanical properties,the optimal conditions of isothermal bainitic temperature and holding time are 410-430°C and 180-240 s,respectively. After isothermal bainitic processing under the optimal conditions,the corresponding volume fraction of retained austenite is 5vol%-15vol%,which can provide enough retained austenite and plastic stability for austenite with high carbon content.

Sintering characteristics and microstructure of WC-Co-VC/Cr_3C_2 ultrafine cemented carbides

The sintering characteristics, microstructure, and mechanical properties of ultrafine WC-12%Co-0.2%VC/0.5%Cr3C2 cemented carbides were investigated. Dilatometric and differential thermal analyses （DTA） indicate that the compacts start to shrink at 600°C, the shrinkage rate peak is at 1190°C, and the liquid formation temperature is lower than the W-C-Co eutectic temperature （1330°C）. Microstructure analysis results show that the cemented carbides with fine and homogeneous microstructure were obtained when sintered at 1430°C. Continuous and discontinuous grain growth was suppressed due to the synergistic action of VC/Cr3C2. The transverse rupture strength （TRS） of the samples reaches 4286 MPa, with the hardness HRA 92.1. The fine and homogeneous microstructure, alloy strengthening, and different phase constitutions of binder in the cemented carbides result in high hardness and TRS. Continuous and discontinuous grain growth was observed in the cemented carbide sintered at 1450°C, which results in significant decreases of hardness and TRS. It indicates that VC/Cr3C2 additions in the cemented carbides can only suppress the grain growth at a certain temperature.

Microstructure and mechanical properties of F_(GBA)/B_G air cooling bainitic steels containing niobium

The effect of different Nb additions on the mechanical properties and microstrucmre evolution of grain boundary allotriomorphic ferrite （FGBA） / granular bainite （BG） air cooling bainitic steels was investigated. The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa, respectively with the addition of 0.02wt% Nb. The steel acquires superior strength and toughness with the addition of 0.06wt% Nb. The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increases the volume fraction of granular bainite in FGBA/BG steels. The refinement effect of granular bainite is improved with the increase of Nb content.

Utilization of Recycled Concrete Powder in Cement Composite:Strength,Microstructure and Hydration Characteristics

Recycled concrete powder(RCP)is used more and more in cement-based materials,but its influence on the hydration process is still unclear.Therefore,this paper studied the influence of recycled concrete powder(RCP)on the hydration process of cement and provides a theoretical basis for the hydration mechanism of cement composite materials.The hydration heat method was used to systematically analyze the thermal evolution process of cement paste with or without RCP.Hydration products were identified using X-ray diffraction(XRD)and thermal analysis(TG–DSC).The pore structure change of cement pastes was analyzed by mercury intrusion porosimetry(MIP)method.The mechanical properties of mortar were also evaluated.Four recycled concrete powder(RCP)dosages,such as 10%,20%,30%and 40%are considered.The results indicate that with the increase of RCP content,the hydration heat release rate and total heat release amount of paste decreased,but the second heat release peak of hydration reaction advanced;the proportion of harmful pores and more harmful pores increases,the total porosity and the most probable pore size also increase;the fluidity and mechanical strength of mortar decrease,but the crystal type of hydration products does not change.When the content of RCP is less than 20%,it has little effect on the mechanical strength of mortar.When fly ash and silica fume are mixed,the fluidity difference of mortar decreases,and when the content of fly ash is the highest,the fluidity of mortar is the highest,which is 15mm higher than that of the control group.When RCP content is 15%,fly ash and silica fume content is 15%(FA:SF=3:2),the hydration heat of the clean pulp is the highest among all the compounding ratios,and the hydration reaction is the most complete;the proportion of harmless pores increased by 9.672%,the proportion of harmful pores and more harmful pores decreased,and the compactness of material structure increased;the compressive strength and flexural strength of mortar reached 50.6 MPa and 9 MPa respectively,both exceeding those of control mortar.

Additive manufacturing of magnesium and its alloys: process-formabilitymicrostructure-performance relationship and underlying mechanism

Magnesium and its alloys,as a promising class of materials,is popular in lightweight application and biomedical implants due to their low density and good biocompatibility.Additive manufacturing(AM)of Mg and its alloys is of growing interest in academia and industry.The domain-by-domain localized forming characteristics of AM leads to unique microstructures and performances of AM-process Mg and its alloys,which are different from those of traditionally manufactured counterparts.However,the intrinsic mechanisms still remain unclear and need to be in-depth explored.Therefore,this work aims to discuss and analyze the possible underlying mechanisms regarding defect appearance and elimination,microstructure formation and evolution,and performance improvement,based on presenting a comprehensive and systematic review on the relationship between process parameters,forming quality,microstructure characteristics and resultant performances.Lastly,some key perspectives requiring focus for further progression are highlighted to promote development of AM-processed Mg and its alloys and accelerate their industrialization.

Microstructure and mechanical properties of rheo-diecasting AZ91D Mg alloy

Taking AZ91D magnesium alloy as experimental material,the rheo-diecasting process was implemented by combining the self-developed taper barrel rheomoulding(TBR) machine with high pressure die casting(HPDC) machine.Microstructura characteristics of the rheo-diecasting components were investigated in different processing parameters.Microstructural evolution and solidification behavior of the semisolid slurry during the rheo-diecasting process were discussed,and tensile properties of the components were studied as well.The results show that,with the rotation speed of the internal taper barrel increasing,the microstructure of the components becomes fine with solid particles nearly spherical and uniformly distributed on the matrix.When the rotation speed is 700 r/min,the primary α-Mg particles have an average size of about 45 μm and a shape factor of about 0.81;the primary α-Mg particles become round and homogeneous with shearing temperature increasing,but the average size is slightly larger The solidification of the alloy melt during the rheo-diecasting process is composed of two distinct stages:the primary solidification and the secondary solidification.Compared with the conventional die-casting process,the rheo-diecasting process would improve the tensile properties of the components,especially the elongation by 80%.

Microstructure and mechanical properties of railway wheels manufactured with low-medium carbon Si-Mn-Mo-V steel

The suitability of carbide-free bainite steel as railway wheel materials was investigated. The low-medium carbon Si-Mn- Mo-V steel was designed to make railway wheels by forging and rolling. The slack quenching with water was conducted on the tread of rim section by programmed control to simulate isothermal heat treatment after being austenitized. Microstructures and mechanical properties have been studied. The results indicate that the microstructure of the rim is mainly carbide-free bainite, and the mixed microstructure of bainitic ferrite and granular bainite is observed in web and hub. The mechanical properties are superior to both the standard requirements and the commercial production, such as CL60 plain carbon. The Charpy impact energy is relatively high at room and/or subzero temperatures. The force-displacement curves and fractographies reveal the excellent ability of resistance to crack initiation and propagation.

Effect of cooling parameters on the microstructure and properties of Mo-bearing and Cr-bearing steels

To develop low-cost low carbon bainitic steel,Mo-bearing and Cr-bearing steels were melted in a vacuum induction furnace and were researched by thermal simulation and hot rolling at the laboratory.As the cooling rate increases from 0.2 to 50°C/s,the transformation temperatures of two steels lie between 650 and 400°C,and the final microstructures of them change from quasi-polygonal ferrite and granular bainite to lath bainite.Compared with cooling in air or by interrupted cooling,Mo-bearing and Cr-bearing steel plates cooled by sprayed water boast higher strength and superior toughness,for large-size islands are responsible for the poor mechanical properties.Compared to Mo,Cr is effective to isolate the bainitic reaction in low carbon steel,and the bainitic microstructure can also be obtained in Cr-bearing steel cooled at a wide range of cooling rate.

Role of Ag addition on microstructure,mechanical properties,corrosion behavior and biocompatibility of porous Ti-30 at%Ta shape memory alloys

In the present study,the thermal,mechanical,and biological properties of xAg/Ti-30Ta(x=0,0.41,0.82 and 2.48 at%)shape memory alloys(SMAs)were investigated.The study was conducted using optical and scanning electron microscopy(SEM),X-ray diffractometry(XRD),compression test,and shape memory testing.The xAg/Ti-Ta was made using a powder metallurgy technique and microwave-sintering process.The results revealed that the addition of Ag has a significant effect on the pore size and shape,whereas the smallest pore size of 11μm was found with the addition of 0.41 at%along with a relative density of 72%.The fracture stress and strain increased with the addition of Ag,reaching the minimum values around 0.41 at%Ag.Therefore,this composition showed the maximum stress and strain at fracture region.Moreover,0.82 Ag/Ti-Ta shows more excellent corrosion resistance and biocompatibility than other percentages,obtaining almost the same behaviour of the pure Ti and Ti-6Al-4V alloys,which can be recommended for their promising and potential response for biomaterial applications.

Microstructures and Electrochemical Properties of Cobalt-Free LaNi_(4.0)Al_(0.2)Fe_(0.4)Cu_(0.4-x)Sn_x (x=0～0.4) Electrode Alloys Prepared by Casting

The microstructure, hydriding performance, and electrochemical properties of LaNi4.0Ai0.2Fe0.4Cu0.4-x Snx（x = 0- 0.4） hydrogen storage alloys prepared by casting were investigated using XRD, SEM, pressure-composition isotherms, and electrochemical measurements. Substitution of Sn for Cu leads to the precipitation of LaNiSn phase. With increasing amount of tin substitution, cell volume, plateau pressures, and discharge capacities of the alloys decrease, whereas the cycle life of the alloys improves.

Study on microstructures and properties of preheated flash butt welded SW400 steel joints

Under the spirit of sustainable development, ‘lightweight’ has been gradually included into the vehicle design criterion by many manufacturers and used in automobile production. Following this trend, domestic wheel suppliers also begin to study the technology of lightweight wheel. One way to achieve this goal is improving strength grade of the steel and optimizing the structure design in the field of steel wheels. But there are a few problems in flash butt welding process in the application of high strength steel, leading to high rejection rates. SW400 steel is a special high strength wheel steel developed by Benxi Steel. Taking SW400 steel as the research material, this article studys the feasibility of improving the properties of rim flash butt welded joints by adding preheating process.

Processing and characterization of the microstructure and mechanical properties of Al6061–TiB_(2) composite

In the present research,aluminum metal matrix composites were processed by the stir casting technique.The effects of TiB2 reinforcement particles,severe plastic deformation through accumulative roll bonding(ARB),and aging treatment on the microstructural characteristics and mechanical properties were also evaluated.Uniaxial tensile tests and microhardness measurements were conducted,and the microstructural characteristics were investigated.Notably,the important problems associated with cast samples,including nonuniformity of the reinforcement particles and high porosity content,were solved through the ARB process.At the initial stage,particle-free zones,as well as particle clusters,were observed on the microstructure of the composite.However,after the ARB process,fracturing phenomena occurred in brittle ceramic particles,followed by breaking down of the fragments into fine particles as the number of rolling cycles increased.Subsequently,composites with a uniform distribution of particles were produced.Moreover,the tensile strength and microhardness of the ARB-processed composites increased with the increase in the reinforcement mass fraction.However,their ductility exhibited a different trend.With post-deformation aging treatment(T6),the mechanical properties of composites were improved because of the formation of fine Mg2Si precipitates.

FATIGUE PROPERTIES OF META-BAINITE IN STEEL 40CrMnSiMoVA

The microstructure,strength,toughness and fatigue properties of an ultra-high strength steel 40CrMnSiMoVA have been investigated.The so-called meta-bainite,composed of thin re- tained austenite films within or between the bainitic ferrite lathes was found in the steel after isothermally quenched at 300℃ for 1h.In comparison with the martensite structure obtained by isothermally quenching in martensite range,the meta-bainite has more excellent strength and plasticity,lower notch sensitivity,stronger strain harden ability,higher fatigue strength, longer strain or impact fatigue life,slower crack propagation rate and more remarkable overload effect on increasing fatigue life.

Tuning heterogeneous microstructures to enhance mechanical properties of nano-TiN particle reinforced Haynes 230 composites by laser powder bed fusion

Laser powder bed fusion(LPBF)is considered to be one of the most promising additive manufacturing technologies for producing components with geometries and high geometrical precision that are unattainable by traditional technologies.The superalloy exhibits exceptional mechanical and high-temperature performances,rendering it a prime candidate for advanced aero-engine applications.Despite the high demand for LPBF-manufactured superalloys,the superalloy is one of the materials manufactured difficultly by LPBF due to their large laser absorptivity fluctuation,poor molten pool stability and sharp temperature gradient.Hence,superalloys are characterized by severe pores,undesirable coarse columnar grains and poor mechanical properties.In this work,the effect of nano-TiN particles on defects,molten pool characteristics and microstructure and performance of the composites were investigated.The 4.5 wt%TiN/Haynes230 samples exhibited exceptional nanohardness and elastic modulus with maximum values reaching 5.53 GPa and 240.03 GPa,respectively.These superior mechanical properties were attributed to the combined effects of spatter and gas pore inhibition,grain refinement and duplex nanophases strengthening.Moreover,the stability of molten pool was enhanced,and spatter was effectively suppressed by adding nano-TiN particles,while grain refinement and columnar to equiaxed transitions were promoted.Furthermore,the matrix exhibited a high dislocation density due to a significant hindrance of dislocation movement caused by massive nano-phases(e.g.,TiN and M_(23)C_(6)),resulting in the formation of extensive dislocation tangles and rings.This work offers novel insights into the role of nanoparticles reinforced superalloy composites by LPBF.

Microstructure and strength features of warm and ice-rich frozen soil treated with high-performance cements

Warm and ice-rich frozen soil(WIRFS) exhibits lower shear strength due to the weak binding forces between soil particles and ice crystals. To enhance the strength of WIRFS, frozen soil was treated separately with Portland, Phosphate, Sulphoaluminate, Portland-Phosphate and PortlandSulphoaluminate cements. After the samples were cured under -1.0°C for 7 days, the microscopic pore distribution characteristics and the macro-mechanical properties of WIRFS were investigated using mercury intrusion porosimetry(MIP), scanning electron microscopy(SEM) and unconfined compressive strength(UCS) tests. To quantitatively analyze the laws of pore-size transformation and the variation of Hausdorff volumetric fractal dimensions for pre-and post-treated WIRFS, the CURVEEXTRACT and Image-Pro Plus(IPP) image analysis system has been developed for analysing SEM images of the soil samples. Statistics of the pore-area dimension and pore-volume dimension were calculated. The results reveal that the cement-based treatment of WIRFS can improve the cementation fill of soil pores and the bond forces between soil particles. There is an evident correlation between the microstructure characteristics and the mechanical properties of the treated WIRFS. As the fractal dimensions of pore-area decrease, the unconfined compressive strength of cement-treated WIRFS increases significantly. In contrast, as the fractal dimensions of pore-volume increases, the unconfined compressive strength decreases remarkably.

Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel

The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910℃ for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ℃ and the remaining samples were quenched in salt bath at 425 ℃ for various holding times. All samples were tempered at 200 ℃ for 2 h. Microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch （CVN） impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.

Mechanical Properties of Steels Treated by Q-P-T Process Incorporating Carbide-free-bainite/martensite Multiphase Microstructure

Quenching partitioning tempering （QP-T） process were applied to a quenched carbide-free bainite/mar- tensite （CFB/M） multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q P T treatment can he increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation （PSE） of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa ·%, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstrueture introduced in the quenching process during the Q-P-T treat- ment.

Rietveld refinement,microstructure,mechanical properties and oxidation characteristics of Fe-28Mn-xAl-1C(x=10 and 12 wt.%)low-density steels

The quantitative relationship between microstructure and properties of austenitic Fe-28Mn-xAl-1C（x=10 and 12 wt.%）low-density steels was evaluated using Rietveld method to refine X-ray diffraction（XRD）patterns.The results showed that a typical three-phase austenitic steel was obtained in the forged Mn28Al10（i.e.Fe-28Mn-10Al-1C）steel,which included about 92.85 wt.% γ-Fe（Mn,Al,C）（austenite）,5.28 wt.%（Fe,Mn）_3AlC_（0.5）（κ-carbide）,and 1.87 wt.% α-Fe（Al,Mn）（ferrite）.For the forged Mn28Al12（i.e.Fe-28Mn-12Al-1C）steel,nevertheless,only about 76.64 wt.% austenite,9.63 wt.%κ-carbide,9.14 wt.%ferrite and 4.59 wt.% Fe_3Al（DO_3）could be obtained.Nanometerκ-carbide and DO_3 were mainly distributed in austenite grains and at the interface between austenite and ferrite,respectively.The forged Mn28Al10 steel had a better combination of strength,ductility and specific strength as compared with the forged Mn28Al12 steel.The ductility of the forged Mn28Al12 steel was far lower than that of the forged Mn28Al10 steel.The oxidation kinetics of Mn28Al10 steel oxidized at 1323 Kfor 5-25 h had two-stage linear rate laws,and the oxidation rate of the second stage was faster than that of the first stage.Although the oxidation kinetics of Mn28Al12 steel under this condition also had two-stage linear rate laws,the oxidation rate of the second stage was slower than that of the first stage.When the oxidation temperature increased to 1373K,the oxidation kinetics of the two steels at 5-25 hhad only onestage linear rate law,and the oxidation rates of the two steels were far faster than those at 1323K for5-25 h.The oxidation resistance of Mn28Al12 steel was much better than that of Mn28Al10 steel.Ferrite layer formed between the austenite matrix and the oxidation layer of the two Fe-Mn-Al-C steels oxidized at high temperature.

W–Cu梯度复合材料的制备、组织与性能

采用不同配比的W粉和Cu粉为原料,经过混粉、放电等离子烧结后制备了三层的W–Cu梯度复合材料,研究了不同烧结温度下W–Cu梯度复合材料的微观组织、界面特征、物理性能、力学性能及抗热震性能。结果表明:900℃烧结制备W–Cu梯度复合材料相对密度高(95%)且保持了单层的原始设计成分;每个梯度层中W、Cu分布较均匀,W、Cu之间界面结合良好,未发生扩散。W–Cu梯度复合材料的力学性能呈梯度分布,W–40Cu层的显微硬度最高为HV 136。在压缩过程中,W–40Cu层首先发生断裂,最高压缩屈服强度为332 MPa。900℃烧结制备的W–Cu梯度复合材料的热导率为202 W·m^(-1)·K^(-1)。经抗热震实验后,梯度复合材料界面处无裂纹,具有良好的抗热震性能。