Microstructure evolution and strengthening mechanism of high -performance powder metallurgy TA15 titanium alloy by hot rolling

Hot deformation of sintered billets by powder metallurgy(PM)is an effective preparation technique for titanium alloys,which is more significant for high-alloying alloys.In this study,Ti–6.5Al–2Zr–Mo–V(TA15)titanium alloy plates were prepared by cold press-ing sintering combined with high-temperature hot rolling.The microstructure and mechanical properties under different process paramet-ers were investigated.Optical microscope,electron backscatter diffraction,and others were applied to characterize the microstructure evolution and mechanical properties strengthening mechanism.The results showed that the chemical compositions were uniformly dif-fused without segregation during sintering,and the closing of the matrix craters was accelerated by increasing the sintering temperature.The block was hot rolled at 1200℃ with an 80%reduction under only two passes without annealing.The strength and elongation of the plate at 20–25℃ after solution and aging were 1247 MPa and 14.0%,respectively,which were increased by 24.5%and 40.0%,respect-ively,compared with the as-sintered alloy at 1300℃.The microstructure was significantly refined by continuous dynamic recrystalliza-tion,which was completed by the rotation and dislocation absorption of the substructure surrounded by low-angle grain boundaries.After hot rolling combined with heat treatment,the strength and plasticity of PM-TA15 were significantly improved,which resulted from the dense,uniform,and fine recrystallization structure and the synergistic effect of multiple slip systems.

High-Strength and High-Plasticity TWIP Steel for Modern Vehicle

In this paper new high-strength and high-plasticity twinning induced plasticity （TWlP） steel for modern automobile body was investigated. Some basic experimental results were given. The results indicate the TWlP steel has excellent properties. It exhibits high ultimate tensile strength （600,--1100 MPa） and extremely large elongation of 60% to 90%.In the future it would be capable of satisfying the requirements of new generation of vehicle.

Mg-Cu-Zn-Y-Zr bulk metallic glassy composite with high strength and plasticity

Mg65Cu20Zn5Y9Zr1 bulk metallic glass matrix composite with a diameter of 2 mm was produced by copper mold casting. Upon cooling the Mg65Cu20Zn5Y9Zr1 melt, Mg2Cu acicular crystalline phase precipitates uniformly with a size of about 20 μm long and 1 μm thick while the remaining melt undergoes glass transition. Room temperature compression tests revealed that the high fracture strength up to 830 MPa and the plastic strain of 2.4% before failure are obtained for the Mg-based bulk metallic glass matrix composite. The formation of the Mg2Cu phase was proposed to contribute to high strength and plastic deformation of the material.

Revisiting the Bjerrum's correction factor:Use of the liquidity index for assessing the effect of soil plasticity on undrained shear strength

The undrained shear strength （su） of fine-grained soils that can be measured in situ and in laboratory isone of the key geotechnical parameters. The unconfined compression test （UCT） is widely used in laboratoryto measure this parameter due to its simplicity; however, it is severely affected by sampledisturbance. The vane shear test （VST） technique that is less sensitive to sample disturbance involves acorrection factor against the soil plasticity, commonly known as the Bjerrum＇s correction factor, m. Thisstudy aims to reevaluate the Bjerrum＇s correction factor in consideration of a different approach and arelatively new method of testing. Atterberg limits test, miniature VST, and reverse extrusion test （RET）were conducted on 120 remolded samples. The effect of soil plasticity on undrained shear strength wasexamined using the liquidity index instead of Bjerrum＇s correction factor. In comparison with the resultobatined using the Bjerrum＇s correction factor, the undrained shear strength was better representedwhen su values were correlated with the liquidity index. The results were validated by the RET, whichwas proven to take into account soil plasticity with a reliable degree of accuracy. This study also showsthat the RET has strong promise as a new tool for testing undrained shear strength of fine-grained soils.

Effect of Plastic Deformation and H_2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel（C110 steel） were investigated. The studied casing specimens are as follows： original casing, plastic deformation（PD） casing and PD casing after being immersed in NACE A solution saturated with H2S（PD＋H2S）. Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness（JId） was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows： the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

Influence of pre- plastic deformation on thermal residual stress and yield strength of SiC_w/Al composites

With a micro mechanical model, the feasibility of modification of thermal residual stress of the composites treated by tensile pre plastic deformation was analyzed. The relationship between pre plastic strain and variation of thermal residual stress was established. By using the method of tensile pre plastic deformation, the thermal residual stress in 20%SiC w/6061Al composites was modified. The results show that, with increasing tensile pre plastic strain, the tensile residual stress in the matrix was decreased to zero gradually, and then it was turned into compressive stress. By comparison, it was found that the changing tendency of the test results is similar to that of theoretical analysis. In addition, due to pre plastic deformation, the dislocation density in the matrix was increased, and the yield strength of the composites was improved. The increasing yield strength is mainly due to the decreasing tensile residual stress and the changing of distribution of dislocation in the matrix.

Effect of Flaky Plastic Particle Size and Volume Used as Partial Replacement of Gravel on Compressive Strength and Density of Concrete Mix

Common ways of disposing waste plastic such as incineration and landfilling have negative impacts on the environment. Partial replacement of natural aggregate in concrete with waste plastic including polyethylene terephthalate (PET) is more environmental friendly and sustainable. The effect of adding 5% to 20% waste plastic by volume of natural coarse aggregate (“gravel”) and plastic particle size (3 to 7 mm) on the density and compressive strength of plastic-concrete mix after 28 days of curing was studied. The results showed that density of the concrete decreased from 2406.7 to 2286.7 kg/m3 as waste plastic increased from 5% to 20% v/v compared with 2443.3 kg/m3 recorded by concrete without waste plastic. Change in particle size from 3 to 7 mm has no significant effect on the density of the plastic-concrete mix. The compressive strength decreased as the volume and particle size of waste plastic increased. When waste plastic volume changed from 5% to 20% v/v, the compressive strength decreased from 20.5 to 15 MPa, 18.6 to 14.3 MPa and 17.2 to 13.8 MPa for 3, 5 and 7 mm waste plastic particle size respectively while the concrete without plastic has 21.33 MPa. Therefore, the addition of 5% (v/v gravel) of flaky waste plastic in the concrete produces a lightweight concrete which could offer economic benefit without substantially reducing the compressive strength of the plastic-concrete mix.

Effect of Fibre Packing on Random Variability of Compressive Strength of Unidirectional Carbon Fibre Reinforced Plastic

Compression tests on twenty unidirectional(UD) carbon fibre reinforced plastic(CFRP) specimens are conducted, the statistics on the measured compressive strength is calculated, and the fracture surface is characterized. Two types of different fracture surface are experimentally observed, and they are corresponding to very different values on the compressive strength. A finite element(FE) analysis is conducted to investigate the influence of random fibre packing on the compressive strength. And a riks method(provided in ABAQUS software) is applied in FE model to analyze fibre buckling behaviour in the vicinity of compressive failure. The FE analysis agrees well with the experimental observation on the two types of buckling modes and also the partition of compressive strength. It is clearly shown that the random fibre packing lays a significant influence on the random variability of compressive strength of CFRP.

巷道蝶形破坏强度准则低敏感性研究及工程应用

巷道围岩的塑性区形态对巷道的破坏形式及破坏程度有重要影响。为探究三向应力场下塑性区形态演化过程,基于弹性力学推导了轴向应力表达式,并依据蝶形塑性区边界方程求解思路,确定了三维强度准则下三向塑性区近似解的求解方法。通过等球应力p、等偏应力q以及不同Lode角θσ来确定围岩应力加载方案,对不同三维强度准则下的围岩塑性区形态演化规律进行深入研究,论证了蝶形破坏的准则低敏感性。基于蝶形破坏理论对羊场湾160206回风巷道的非对称变形破坏机制及控制技术进行深入分析。研究结果表明:(1)在相同p、q及不同θσ的应力加载条件下,5种强度准则下的塑性区形态均呈现圆形、类椭圆及蝶形的演化规律,且每种强度准则在相同θσ的情况下围岩的塑性区形态基本一致。(2)相同应力大小、不同应力方向的加载方案下,围岩的塑性区形态大不相同。圆巷围岩的塑性区形态很大程度上由水平侧压比决定,轴向侧压对围岩的塑性区尺寸影响较大,对塑性区形态影响较小。(3)羊场湾160206回风巷道在叠加采动影响下顶板呈现非对称大变形破坏,基于蝶形塑性区支护思路,应用非对称锚杆索+超前单元支架+钻孔卸压的协同支护技术,取得了良好的支护效果。

Elasto-plastic analysis of the surrounding rock mass in circular tunnel based on the generalized nonlinear unified strength theory

The present paper aims to establish a versatile strength theory suitable for elasto-plastic analysis of underground tunnel surrounding rock. In order to analyze the effects of intermediate principal stress and the rock properties on its deformation and failure of rock mass, the generalized nonlinear unified strength theory and elasto-plastic mechanics are used to deduce analytic solution of the radius and stress of tunnel plastic zone and the periphery displacement of tunnel under uniform ground stress field. The results show that: intermediate principal stress coefficient b has significant effect on the plastic range,the magnitude of stress and surrounding rock pressure. Then, the results are compared with the unified strength criterion solution and Mohr–Coulomb criterion solution, and concluded that the generalized nonlinear unified strength criterion is more applicable to elasto-plastic analysis of underground tunnel surrounding rock.

超高压应力增塑镦粗变形坯料外形轮廓理论模型预测

对超高压应力增塑镦粗成形工艺下坯料的成形过程展开研究,从理论角度构建了坯料在超高压应力下的外形轮廓模型,可实现超高压应力下坯料的外形轮廓预测。利用ABAQUS软件建立了超高压应力增塑镦粗有限元模型,分析了摩擦力和超高压应力对理论模型预测精度的影响。通过实验得到的坯料外形轮廓对仿真模型进行了验证,进一步探讨了超高压应力对坯料变形均匀性的影响机理。研究结果表明,理论模型能够较好地预测不同摩擦力和超高压应力下坯料的外形轮廓变化,仿真和实验得到的外形轮廓吻合较好。与传统镦粗相比,在超高压应力作用下坯料的塑性和变形均匀性均得到显著提高。

不同围压及加载速率下的塑性混凝土动力特性研究

为研究塑性混凝土在三向应力状态下的动力特性与破坏规律,设计符合工程实践的配合比方案,借助动三轴试验仪开展了其在不同围压和不同加载速率条件下的强度、应力及应变规律研究。最后依据试验成果建立了基于Bresler-Pister三参数的强度准则。结果表明:塑性混凝土抗压强度与加载速率呈近线性正相关,材料抗压强度小于5.0MPa时对低加载速率更敏感,抗压强度大于5.0MPa时对高加载速率更敏感;不同加载速率下材料应力-应变规律基本一致,当应变达到1.47%~1.90%后其应力出现峰值,随后大幅度降低;不同围压条件下塑性混凝土抗压强度随围压的增加同样呈线性增长趋势,随着围压的提高,塑性混凝土峰值应力维持时间越长,材料抗裂性能越好;基于Bresler-Pister的强度准则公式其计算剪应力与三轴实测误差在1.61%~4.35%之间,基本反映了材料的变形破坏规律。

圆形截面钢管混凝土受弯构件塑性发展系数的双因素计算模型研究

为了综合考虑截面几何和材料特性对圆形截面钢管混凝土(CFST)受弯构件抗弯强度的影响,选择截面厚径比和材料强度比作为自变量,研究建立了圆CFST受弯构件截面塑性发展系数的二元函数计算表达,并建立了截面抗弯强度计算模型。首先在确定厚径比和强度比取值范围的基础上,设计1 656个圆CFST受弯构件的数值模型,利用纤维模型法计算确定每个构件的截面抗弯强度和塑性发展系数。然后选择截面钢管壁厚与外径之比、钢管强度与核心混凝土强度之比作为自变量,通过回归分析建立圆CFST构件的截面塑性发展系数的二元函数表达式,据此提出了圆CFST构件截面抗弯强度的计算模型。最后通过与试验数据、现行设计规范和研究文献中不同计算模型进行对比分析,验证了建立的圆形截面钢管混凝土受弯构件的截面塑性发展系数和抗弯强度计算模型具有更高的精度和适用性。

基于统一强度理论的扰损围岩巷道脆塑性解答

钻爆法施工常造成岩质巷道扰损区围岩性能劣化。针对扰损围岩、扰损外围岩的弹塑性状态下不同的扰损围岩巷道力学模型,采用统一强度理论、弹-脆-塑性模型和非关联流动法则,建立扰损围岩巷道有关应力、位移和塑性区半径的脆塑性解答,探讨巷道力学模型的转换路径及判定方法,并将结果进行对比、验证,获得各因素的影响规律。研究表明:提出的巷道脆塑性解答合理反映了中间主应力、围岩扰损范围和峰后强度、剪胀特性的综合影响,通过参数组合变换可退化为多种解答并得到正确性验证,具有重要的理论意义和应用前景;巷道力学模型判定与中间主应力、围岩扰损范围密切相关,显著影响巷道应力-位移分布和围岩特征曲线;支护压力和巷道稳定变形随着围岩峰后强度下降、剪胀特性参数增加而明显增大。

碳纤维层合板胶接接头在湿热环境下的性能演变

为了研究碳纤维复合材料(CFRP)胶接接头对汽车涂装过程中的高温高湿环境的适应性,对单搭接CFRP胶接接头进行了烘烤和加速吸湿实验,对经历不同湿热环境的胶接接头进行了剪切拉伸和正向拉伸试验,对连接强度改变及接头的失效模式演变进行了讨论,对环氧树脂与聚氨酯两种胶粘剂连接的接头对烘烤的适应性进行了比较,并研究了在搭接区域边缘增加胶瘤对提高连接强度的效果。研究结果表明:单搭接CFRP胶接接头的剪切拉伸强度远高于正向拉伸强度。烘烤处理后CFRP层合板及CFRP胶接接头的强度不但没有下降,反而有所提高。DP420环氧树脂胶比DP6330聚氨酯胶具有更好的耐烘烤性。吸湿后,CFRP胶接接头的强度、刚度以及最大断裂位移出现了减小。接头的失效模式从胶层内聚失效逐渐演变为与CFRP层合板的层间失效。在搭接区域边缘增加胶瘤能有效提高CFRP胶接接头的剪切拉伸强度。

多重载荷下螺接脆性塑料零件的强度设计校核

针对脆性塑料零件采用螺纹联接安装时,在各种工况环境的静载荷和动载荷输入条件下,对安装孔区域出现裂纹的失效模式进行了分析。通过理论计算和结构仿真分析相结合,提出了强度计算校核方法和安装紧固力矩限值的计算方法,并通过力学环境试验进行了有效验证,给出了安装紧固的改进设计措施。

加工因素对HDPE基木塑复合材料静曲强度的影响

为了研究木塑复合材料加工因素对复合材料力学性能的影响,采用正交实验法以静曲强度为评价标准,利用极差分析和方差分析法分析了螺杆转速、造粒温度、偶联剂质量分数和木粉含量等加工因素对力学指标静曲强度影响的顺序及各因素的最佳水平,确定了木塑复合材料静曲强度的各加工因素的最佳参数组合;系统地分析了各加工参数对木塑复合材料力学性能指标静曲强度的影响。结果表明,影响木塑复合材料静曲强度的主要加工因素由大到小依次为偶联剂质量分数>木粉含量>转速>造粒温度;得到最佳静曲强度的加工因素参数组合为转速150 r/min、造粒温度145℃、偶联剂质量分数为5%、木粉含量为60%;木粉含量及偶联剂质量分数对材料的静曲强度具有显著性影响,转速和造粒温度对材料静曲强度影响的显著性较低,其中,随着偶联剂含量的增加,静曲强度呈明显上升的趋势;当木粉含量从30%增大至70%时,材料的静曲强度呈先增加后减小的趋势。

TC18钛合金大规格棒材热变形行为研究进展

近年来,随着自由锻造装备大型化的不断发展,航空航天关键结构件用锻件的大型化需求急剧增长,大规格棒材的制备成为了科研及工业化生产的重点工作。TC18钛合金作为一类高合金化、热变形参数敏感型高强高韧钛合金,已逐步成为航空航天关键装备的核心结构材料,因此,其棒材的大型化成为了近年来研究及生产的重点方向。本文梳理了近年来TC18钛合金大规格棒材(Φ300~Φ500 mm)生产技术概况,综述了TC18钛合金热变形数值模拟的相关研究,分析铸锭熔炼、锻造成型及热处理工艺的现状,总结大规格棒材现存问题及解决方法。最后,展望原材料制备过程今后一段时间的研究重点,结合使用单位对TC18钛合金棒材的使用需求,力争在未来打通原材料和成品锻件的全流程制造工艺,实现工艺个性化设计、组织性能定量调控。

泵送碱式硫酸镁水泥混凝土的制备和性能

碱式硫酸镁水泥混凝土是新开发出来不久的混凝土材料,具有高抗拉强度、抗冲击和耐腐蚀等优点,但由于其较小的塌落度,限制了其工程应用。本文试验研究了不同掺量萘系减水剂对泵送碱式硫酸镁水泥混凝土塌落度和强度的影响,确定强度等级C30、C40、C50和C60的萘系高效减水剂适宜掺量分别为2.0%、1.2%、1.5%和1.8%。分别研究了掺粉煤灰型和矿渣型泵送碱式硫酸镁水泥混凝土基本力学性能及在海水浸泡、淡水浸泡1280 d后的强度保留率并进行机理分析,发现粉煤灰型泵送碱式硫酸镁水泥混凝土抗海水腐蚀性较好,适合东部沿海地区应用;随着海水暴露时间的延长,泵送碱式硫酸镁水泥混凝土的Df值呈现幂函数的降低关系。本文还对大气中暴露2400 d的钢筋泵送碱式硫酸镁水泥混凝土小梁进行观测,未有锈涨现象。本文为碱式硫酸镁水泥混凝土在工程领域的应用具有重大意义。

硅灰热焊接改性塑料颗粒对砂浆抗压强度与微结构的影响

为解决塑料憎水表面所导致的塑料砂浆抗压强度较低的问题,本文利用废弃聚丙烯(PP)塑料的热塑性,通过热焊接工艺将硅灰焊接于PP表面,制备硅灰@塑料颗粒复合颗粒(SF@PP),并对复合颗粒的亲水性和火山灰活性进行测试。结果表明,硅灰热焊接改性显著改善了PP的亲水性并提高了其火山灰活性,促进了水化产物的界面沉积,界面密实度提升。相应地,塑料砂浆的孔隙率明显降低,抗压强度显著提升,提升幅度为26.07%~31.04%。研究结果有利于高效利用废弃塑料,并为高性能塑料砂浆的制备提供依据。