Fabrication and Mechanical Properties of TiC／TiAl Composites

TiC/ TiAl composites with different TiC content were fabricated by rapid heating technique of spark plasma sintering ( SPS ). The effect of TiC purticles on microstructure and mechanical properties of TiAl matrix was imestigated. The results indicate that grain sizes of TiAl matrix decrease and mechanical properties are improved because of the addition of TiC particles. The composites display a 26.8% increase in bending strength when 10wt% TiC is added and 43.8% improvement in fracture toughness when 5ut% TiC is added compared to values of TiC-free materials. Grain-refinement and dispersion-strengthening were the main strengthening mechanism. The improvement of fracture toughness was due to the deflexion of TiC particles to the crack.     

Crack propagation mechanism of γ-TiAl alloy with pre-existing twin boundary

The deformation and failure mechanisms of γ-TiAl alloy with pre-existing crack and twin boundary are investigated by using molecular dynamics simulation. The effects of the crack position on the deformation and failure mechanisms of γ-TiAl specimen are analysed through the snapshots of crack propagation, microstructure of crack tip and stress-strain curves. The simulation results show that the dislocation motion is impeded, the good ductility can be maintained and the strength would be improved simultaneously by the twin boundary. The microstructure evolution of crack tip would change with crack positions. Essentially,the deformation behaviour mainly results from the reaction of dislocation-dislocation, dislocation-twin and twin-twin. Besides,the hierarchical twin is a main plastic deformation mechanism leading to strength of γ-TiAl specimen enhancement with noncompromising ductility and strain hardening. Based on stress-strain curves, it can be concluded that the yield strength varies with crack positions. They are the determinant factors for variation of the yield strength with different crack positions such as dislocation behaviour, stacking fault and hierarchical twin. The ductile-brittle transition associated with the dislocation motion and the decohesion failure of crack tip atom can be observed from the lower boundary crack and the center crack models. The crack propagation caused by the coalescent of the void and the crack tip is the main failure mechanism of γ-TiAl specimen. In addition, the results reveal that the mechanism of crack propagation would be influenced by pre-existing twin boundary which can prevent the crack propagation and improve the fracture toughness.     

Effect of cold working and annealing on microstructure and properties of powder metallurgy high entropy alloy

An equiatomic CoCrFeNiMn high entropy alloy(HEA) was produced by powder metallurgy method. Cold rolling followed by subsequent annealing was conducted to further optimize the microstructure and mechanical properties. The results show that the SPSed CoCrFeNiMn HEA has an equiaxed single fee phase microstructrue. Cold rolling results in extensive dislocation pile-up and twinning within the grains. The 80% cold-rolled alloy shows very high yield strength of 1292 MPa, but a limited elongation of 3%. Subsequent annealing produces recrystallization and precipitation of fine σ particles with particle size of 30-100 nm. The annealed alloy has a yield strength of 540 MPa, which is about two to three times of the cast CoCrFeNiMn HEA, while still maintains a high tensile ductility of 41%. The improvement of the tensile properties is caused by the grain boundary strengthening,solid solution strengthening, and precipitation strengthening.     

Properties and Structure of Magnesium Matrix Composite Reinforced with CNTs

By using high pure Magnesium (99.9 wt% ) as matrix and multi-walled bended carbon nanotubes ( CNTs ) as reinforced phase, carbon nanotubes/magnesium matrix composite was prepared to the foundry method under the argon gas protection, and its mechanical properties were tested. The interface structure and component of plating and un-plating carbon nanotubes were analyzed by TEM and EDS , and the action mechanism was discussed. The experiment results show that the CNTs con strengthen mechanical properties of the nano-tube-reinforced Mg matrix composite, the tensile strength and elongation ratio are greatly improved. Furthermore, the plating CNTs are better than un-plating CNTs in strengthening effects. The tensile strength is inereased by 150% and the elongation ratio is increased by 30% than that of matrix when content of CNTs is 0.67 wt%.     

Microstructure and mechanical properties of submicron-grained NiAl-Al2O3 composite prepared by pulse current auxiliary sintering

Dense and submicron-grained NiAl-Al2O3 composite was fabricated by pulse current auxiliary sintering(PCAS).Its microstructure was analyzed by XRD,SEM and TEM,and its mechanical behavior was evaluated through compression test and fracture toughness test.The average grain sizes of NiAl and Al2O3 are about 200 nm and 100 nm respectively.The Al2O3 particles dispersed in NiAl matrix,forming intergranular structure and intragranular structure.During sintering,Al2O3 particles were remarkably spherized due to the unique sintering mechanism of PCAS,which is beneficial to the improvement of toughness.The NiAl-Al2O3 composite exhibits high compressive yield strength,whether at room temperature or elevated temperature.Its room-temperature(23 ℃) and elevated-temperature(1 200 ℃) compressive yield strength are up to 2 050 MPa and 140 MPa,respectively.Meanwhile,its fracture toughness is significantly enhanced,which is up to 8.2 MPa?m1/2.It is suggested that the main strengthening-toughening mechanisms are grain refinement strengthening and Al2O3 dispersion strengthening.The fracture of larger NiAl grain is the transgranular cleavage and this is induced by crack tip deflection and grain boundary weakening which are caused by intergranular and intragranular Al2O3 particles,respectively.     

Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr

An ultra-high strength aluminum alloy was produced by casting and then extruded to rods.The effect of heat treatment on the microstructure and mechanical properties of the alloy was investigated.After single ageing(120℃,24 h),the tensile strength was 812.4 MPa and the elongation was 6.2%.After retrogression reaging(RRA),the tensile strength was 751.2 MPa and the elongation was 6.4%.The strengthening mechanism is considered as fine grain strengthening,substructure strengthening and dispersion strengthening by Al_3(Sc,Zr).     

Microstructure and mechanical property of 2024／3003 gradient aluminum alloy

2024/3003 gradient aluminum alloy was prepared by semi-continuous casting using double-stream-pou-ring technique. The microstructures of the as-cast, pressed and heat-treated alloys were analyzed by scanning elec-tron microscope and transmission electron microscope. And the mechanical properties of the alloy in pressed and heat-treated states were studied. The results show that the ingots with diameter of 65 mm and external thickness about 5.5 mm are obtained when the temperatures of the melt in the internal and external ladles are 1 023 and 1 003 K, respectively, and the nozzle diameter is 2.0 mm. The microstructures of the as-cast alloy consist of α(AI) θ(CuAl2)q S(Al2CuMg) in the internal region and α(Al) MnAl6 in the external region. The phases found in the internal and external layers coexist in the transition zone. The transition layer is maintained after plastic deformation and heat treatment of the alloy. The tensile strength, yield strength and elongation of the alloy are 300 MPa,132 MPa and 16.0%, respectively, after T6 treatment. The tensile and yield strength are increased by 150.0% and 94.1%, respectively, compared with that of 3003 aluminum alloy. The maximum hardness in the internal region of 2024/3003 gradient aluminum alloy can be increased from HRF 55 in the pressed state to HRF 70 in the heat-treated state.     

Microstructures and Mechanical Properties of TiC Particle Reinforced TiAl Composites by Spark Plasma Sintering

Using spark plasma sintering(SPS)technique,TiC particle reinforcedγ-TiAl composites were prepared with varying weight fraction of TiC powders.The effects of the TiC fractions and distributions on the properties of the composites were investigated.The composite containing 7wt%TiC had the optimum three-point bending strength of 842 MPa,which was 200 MPa greater than that of the unreinforcedγ-TiAl intermetallic.The degradation of the bending strength occurred in the composites containing more than 7wt% TiC and this was believed to be attributed to agglomerated particles of TiC,which acted as crack initiation and propagation sites.The increase of strength in TiC reinforced IMCs came from the grain refinement and the interaction of dislocations with the reinforcing particles.The bending strength of the IMC containing 7wt% TiC was theoretically estimated to increase by 85 MPa and 200 MPa,respectively,by the grain refinement and dislocation strengthening,the total of which was almost in accordance with the improvement in that of the unreinforcedγ-TiAl intermetallic when considering normal estimation errors.     

Microstruetures and Mechanical Properties of TiC Particle Reinforced TiAl Composites by Spark Plasma Sintering

Using spark plasma sintering（SPS） technique, TiC particle reinforced γ-TiAl composites were prepared with varying weight fraction of TiC powders. The effects of the TiC fractions and distributions on the properties of the composites were investigated. The composite containing 7wt% TiC had the optimum three-point bending strength of 842 MPa,which was 200 MPa greater than that of the unreinforced γ-TiAl intermetallic. The degradation of the bending strength occurred in the composites containing more than 7wt% TiC and this was believed to be attributed to agglomerated particles of TiC, which acted as crack initiation and propagation sites. The increase of strength in TiC reinforced IMCs came from the grain refinement and the interaction of dislocations with the reinforcing particles. The bending strength of the IMC containing 7wt% TiC was theoretically estimated to increase by 85 MPa and 200 MPa, respectively, by the grain refinement and dislocation strengthening, the total of which was ahnost in accordance with the improvement in that of the unreinforced γ-TiAl intermetallic when considering normal estimation errors.     

Effect of lanthanum-praseodymium-cerium mischmetal on mechanical properties and microstructure of Mg-Al alloys

The effects of lanthanum-praseodymium-cerium mischmetal (LPC) on the microstructure and mechanical properties of Mg-Al alloy were investigated. With the addition of LPC, an additional rod-like Al11La3 phase was deposited in the alloy. LPC greatly improves the tensile strength of cast Mg-Al alloys but negatively affects the elongation of cast alloys above 473 K. Cast alloys are strengthened by both precipitation strengthening and dispersion strengthening at ambient temperature. When the temperature exceeds 473 K, only the dispersion operates as a strengthening mechanism.     

23.8%Mn TRIP/TWIP钢的组织性能及强化机制

研究了锰含量（质量分数）为23.8%的低碳高锰钢的力学行为和组织演变,并对其强化机制进行了探讨.结果表明：23.8%Mn TRIP/TWIP钢的屈服强度约为300 MPa,抗拉强度可达610 MPa,断裂延伸率可达到63%.实验钢拉伸变形呈连续屈服,其应变硬化指数n值约为0.48.该钢在变形初期的强化机制以应变诱发孪生为主,变形后期出现应变诱发马氏体相变.位错与形变孪晶、马氏体之间的相互作用也对强度的增加做出贡献.     

镁合金强韧化复合机制与方法的探索

由于单独的“固溶-时效强化”应用于镁合金时，其强韧化效果较低.为此，针对镁合金Hall Petch系数较大的特点，将“晶粒细化”和“时效强化”两种机制耦合或复合在一起，设计了“激冷固溶时效”、“固溶形变时效”两种方法，由此显著地提高了AZ91镁合金的强韧性水平.在试验条件下，经“激冷固溶时效”后的镁合金AZ91的压缩断裂强度和屈服强度可分别达到335.3 MPa和225.91 MPa；经“固溶形变时效”后的镁合金AZ91的抗拉强度、屈服强度和延伸率可分别达到350 MPa、300 MPa和10%以上.     

残余奥氏体及其机械稳定性与钢强韧性关系的研究

钢中残余奥氏体发生应变诱发马氏体相变的百分量与拉伸应变量的对数呈线性关系,用直线斜率的倒数Ks值可表示残余奥氏体机械稳定性的大小。提高钢中残余奥氏体量及其机械稳定性是改善钢强韧性的有效途径之一。残余奥氏体发生应变诱发马氏体相变吸收能量是提高钢韧性的主要原因。在低碳贝氏体钢中,以M—A岛形式存在的残余奥氏体因受岛中马氏体的强化作用使屈服强度保持较高水平,而残余奥氏体在拉伸的均匀塑性变形阶段因应变诱发相变形成的马氏体使钢的抗拉强度提高。     

几种不同原始硬度钢的喷丸强化行为与表征

材料硬度对喷丸强化具有很大的影响。目前表征喷丸强化常用的指标有显微硬度和X射线衍射半高宽,但从二指标出发会得出一些矛盾的结果。用X射线应力分析技术可以测得材料表面的应力,如此可得出表面屈服强度。通过改变热处理条件或化学成分得到的具有不同硬度材料的试验结果,喷丸强化处理可使软状态材料表面的半高宽值显著增大,同时也使其表面屈服强度有较大幅度的提高。喷丸使硬状态材料表面的半高宽大幅度减小,却使其表面屈服强度有不同程度的提高。对于中硬度表面,喷丸对半高宽影响不大,但使多数材料的表面屈服强度下降。这说明,用当前人们经常使用的半高宽参量来推断表面的强度可能得到错误的信息。表面屈服强度指标比通常应用的半高宽指标更能反映材料表面的力学特性。     

Effect of Nb on the Microstructure and Properties of Cold Rolled and Annealed Mo Microalloyed Bainitic Steel

Two kinds of low carbon bainitic steels,Nb-free Mo bearing and Nb + Mo addition steels,were cold rolled and annealed to investigate the effect of micro-alloying element Nb on the microstructure and properties of Mo microalloyed low carbon high strength bainitic steel. No precipitates were observed in Nb-free Mo bearing steel,whereas,two types of precipitates,i.e.,Nb( C,N) and composite( Nb,Mo)( C,N),were observed in the Nb + Mo microalloyed steel,resulting in precipitation strengthening. The strength of Mo bearing steel was improved by addition of Nb under the same annealing conditions. The grain size of Nb addition steel was almost the same as Nb-free steel. Unlike the obvious grain refinement and precipitation strengthening in hot rolling,the increase in yield strength of Nb addition steels in cold rolling and annealing mainly results from the precipitation strengthening,while the effect of grain refinement strengthening can be almost ignored.     

奥氏体─贝氏体钢组织及性能特点

通过新的组织设计及强韧化处理，研制了一类高碳低合金超高强度钢──奥氏体──贝氏体钢（简称奥──贝钢）。研究了其组织结构、力学性能和接触疲劳特性，论述了组织形成和强韧化机理。