取向电工钢中碳化物在冷变形过程中的行为

用场发射扫描电子显微镜观察了取向电工钢在0-68%冷压加工过程中主要组成为碳化物的第二相粒子的分布状态,统计了不同尺寸粒子面密度的变化。结果表明,冷变形过程造成了碳化物粒子的碎化和回溶行为。粒子碎化到一定程度后其回溶可能是热力学自发过程。这种碎化和回溶有利于促进二次再结晶过程的顺利进行和锋锐Goss织构的生成,也有利于促进一次冷轧板的脱碳过程.     

取向电工钢中Goss晶粒生长的取向环境

将以MnS为主要抑制剂的普通取向电工钢作为实验材料,检测并分析脱碳样品的宏观织构及脱碳和随后加热至925℃时样品的微观织构,统计分析了Goss与周围晶粒的取向差分布。根据取向差原理计算了脱碳样品主要织构组分内各取向晶粒的取向差环境。结果表明,取向电工钢脱碳退火后Goss晶粒与周围晶粒的取向差分布呈现大角度特征,主要取向差分布范围为30°-45°,而非Goss晶粒与周围晶粒的取向差分布则呈现更多小角度特征。二次再结晶后,Goss晶粒与周围晶粒的取向差分布仍然以大角度特征为主。     

1420合金的拉伸断裂行为

研究了１４０２合金在不同热处理状态的拉伸性能和拉伸断口，并考察了合金的显微组织，分析了断裂机制，结果表明，合金在各种热处理状态呈稳定的穿晶韧性断裂，原因是δ′粒子细小，晶内强度低，而且晶界没有析出平衡相和无折出带，晶界强度没有降低。     

高温退火时间对含铌低温取向电工钢二次再结晶行为的影响

利用OM、SEM、EDS、EBSD等技术观察和分析二次再结晶中断抽出试样的组织和取向.结果表明:高温退火保温时间达到5 h时,部分晶粒发生了二次再结晶,随着二次再结晶持续时间的延长,发生二次再结晶晶粒的数量显著增多,达到25 h时基本形成了完全的二次再结晶组织.Goss织构随保温时间的延长逐渐增多,而{111}〈112〉织构和{411}〈148〉织构含量显著降低,Goss织构含量在退火保温25 h时达到91.1％,其强度达到284.771;抑制剂随高温退火时间的延长逐渐粗化,其对晶界的钉扎逐渐减弱直到完全失去抑制作用.     

工业纯钛低温拉伸和循环变形中的孪生行为

在-196℃下对钛进行了拉伸和低周循环变形,观察分析了变形后试样的微观组织.结果表明,工业纯钛在-196℃拉伸变形后,强度比在室温下拉伸变形有了明显的提高,塑性也有明显的增加;在-196℃下循环变形时,循环应力-应变曲线位于-196℃静拉伸应力-应变曲线的上方,显示出明显的循环硬化特征.微观组织观察表明,-196℃拉伸及循环变形试样中存在着大量的孪晶,且孪晶数量随着循环应变幅及循环周次的增加而增加.在工业纯钛-196℃下的拉伸及循环变形中孪生起重要作用.     

温度对Mg-10Gd-2Y-0.5Zr合金动态拉伸行为及断裂机理的影响

利用带有加热装置的Hopkinson Bar拉伸系统在423～798K、应变率处于103s-1范围内研究了两种挤压态的Mg-10Gd-2Y-0·5Zr合金的拉伸行为,并借助光学显微镜和扫描电镜对试样断口附近的显微组织和断口形貌进行了分析.结果表明:T≤673K时,材料的破坏机理主要表现为浅韧窝与准解理的混合型断裂;随着温度的进一步升高由于受晶界软化的影响材料的断裂机理逐渐转向以晶间断裂为主.T＜673K时材料的主要变形机制是基面滑移,随着温度的升高非基面滑移系将充分启动,材料的延伸率也相应地在735K时达到最大值.     

拉伸变形纯铜的显微组织特征

为了研究多晶体中晶粒取向对其形变显微组织的影响，采用透射电镜定量表征了拉伸变形无氧高纯铜(0FHC)的显微组织，研究了形变显微组织与晶粒取向之间的相关性．结果表明：在晶粒内部形变显微组织均匀一致，而在不同晶粒之间则存在明显差异；根据位错组态的不同可将形变组织分为3种类型，即I型组织、Ⅱ型组织、Ⅲ型组织；不同类型形变显微组织与晶粒取向间存在明显的相关性．     

Cu-0.54Al2O3弥散强化铜合金的拉伸变形和断裂行为

对Cu-0.54%Al2O3弥散强化复合材料的拉伸变形和断裂行为进行了研究.结果表明,经挤压比为30∶1 的热挤压后,复合材料的σb、σ0.2、δ和σ0.2/σb分别高达340 MPa、250 MPa、24%和74%.冷加工后合金的强度随退火温度的升高而逐渐下降,但σ 0.2/σb仍保持较高的值.挤压态合金的加工硬化指数n为0.256,明显低于纯铜.铜基体中均匀弥散分布着平均尺寸为18 nm,间距为100 nm的Al2O3纳米粒子,提高了铜基体的强度,阻碍了高温退火时再结晶的发生,降低了合金的加工硬化速率.挤压态合金的拉伸断口宏观上为杯锥状,微观上表现为明显的韧性断裂特征.     

电沉积制备纳米镍的拉伸变形行为

为了系统研究纳米材料的超塑性变形特点,用电沉积方法制备了平均晶粒尺寸为70 nm的纳米镍.采用单向拉伸实验研究了其在室温和高温时的力学性能,并用透射电子显微镜TEM、扫描电子显微镜SEM和X射线能谱仪EDS观察分析了纳米镍变形前后的显微组织.实验结果表明:制备的纳米镍在室温时表现出的延伸率很低,但强度可达1000 MPa以上.当温度升高至450℃,应变速率为1.67×10-3 s-1时单向拉伸实验得到380%的延伸率,说明制备的纳米镍具有低温超塑性性能.实验过程中,材料内部的晶粒发生明显的长大与拉长.拉伸过程中形成的氧化物夹杂成为裂纹源,断口表现为沿晶断裂.     

定向凝固Cu-Cr自生复合材料的拉伸性能和断口形貌特征

研究了Cu-Cr自生复合材料定向凝固试样的轴向拉伸性能和拉伸断裂机制,探讨了凝固速率对性能及断口形貌的影响.结果表明: Cu-Cr自生复合材料具有良好的常温拉伸性能,随着凝固速率增加,材料的轴向拉伸强度呈上升趋势.断口形貌观察表明:复合材料中的纤维状共晶体(α β)是应力的主要承载体,共晶体的断裂是Cu-Cr自生复合材料断裂的控制机制.     

铁素体/贝氏体双相钢的变形和断裂特性

对一种低碳硅锰钢进行TMCP实验,获得了不同铁素体形态的铁素体/贝氏体双相钢(FB钢),研究了FB钢在单轴拉伸下的变形行为及断裂特性,结果表明:在均匀塑性变形阶段,FB钢的瞬时加工硬化指数n*值与真应变ε的关系曲线可分为n*值较高、n*值随ε缓慢下降以及n*值随ε迅速下降三个阶段,与等轴铁素体/贝氏体双相钢相比,准多边形铁素体/贝氏体双相钢的强度和低应变区的n*值均比较高,FB钢拉伸试样颈缩区的孔洞或微裂纹产生在F-B相界面附近和铁素体内,有助于减弱裂纹尖端附近的局部应力集中,改善钢材的抗裂纹扩展性能.     

Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Abstract

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20?000?h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.     

Modelling ductile fracture behaviour from deformation parameters in HSLA steels

In this work, an attempt is made to model the ductile fracture behaviour of two Cu‐strengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.     

Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.     

Challenges of the study on precipitation behaviors of MnS in oriented electrical steels

The studies of the thermo-mechanic principles and the kinetic analysis of MnS precipitation behavior in oriented electrical steels conducted in the last dozen years have been summarized. The problems and insufficiency in the fields concerning nucleation theory of MnS precipitation, the relationship between the precipitation and dynamic recrystallization of the matrix as well as the influence of high-energy boundary precipitation on the pinning effect of the boundary migration were discussed. Suggestions were proposed to modify researches concerning the initial position density for MnS nucleation, the mutual interaction between the precipitation and the dynamic recrystallization as well as the kinetic principles of precipitation nucleation on grain boundaries with different boundary energy, which would complete the related theory of MnS precipitation behaviors.     

Brittle fracture of high-strength steels at very low temperatures

High-strength steels are used to increase the load carrying capacity of components. However, to guarantee a safe design, it is also necessary to combine high strength with adequate fracture toughness. In this paper, fracture toughness of three high-strength steels with yield strengths ranging from 460 to 890 MPa has been studied at very low temperatures. Taking into account experimental evidence, a new mechanism of cleavage at very low temperatures is proposed. This mechanism considers the possibility of reaching the ideal strength (the stress at which the lattice of a single crystal losses its stability) in the immediate vicinity of the fatigue crack tip. Moreover, a computational model able to calculate the external load needed to produce a catastrophic failure of these steels has been developed.     

低温变形Nb微合金钢铁素体相变的数学模型

以热力学和动力学理论为基础，研究了Nb微合金钢热变形过程中铁素体相的形核及长大过程，在形核速率计算中引入变形储能、晶界凸阶及微合金元素的作用，建立了低温变形诱导铁素体相变的动力学模型，分析了热变形参数和化学成分对相变分数和晶粒尺寸的影响．变形温度的降低和变形程度的增加，促进了α相变过程，相变体积分数增加，晶粒得到细化．C与Mn含量增加的效果则相反．模型应用于热轧带钢生产过程的模拟，计算结果和实测结果吻合良好．     

Isothermal tensile deformation behaviors and fracture mechanism of Ti-26Al-20Nb-0,5Mo alloy at different phase regions

In this paper, the microstructure evolution, the deformation behavior and the fracture mechanism of the Ti-26Al-20Nb-0.5Mo alloy have been systematically investigated during tensile test. Results show that increase in tensile temperature leads to decrease in strength and increase in ductility. Under the deformation temperature of room temperature and 600 °C, there were no change in the microstructure of the samples, and the movement of dislocations plays a very important role. With increase of deformation temperature, the spheroidization degrees of the lamellar orthorhombic phase increases and the volume fraction of that decreases. Particularly the deformation temperature rising to 950 °C, the morphology of some equiaxed α₂ phases had been flattened into an ellipsoidal shape and the lamellar orthorhombic phase had been spheroidized completely during the deformation process. The deformation mechanism was mainly based on merged of grains and grown at this moment. Moreover, the fracture changes from brittle fracture to ductile fracture with increase of deformation temperature. At the tensile temperatures of 750 °C and 950 °C, the dimples around the holes are very different, and the fractographs are wave feature and fish scale features, respectively. The reason for this feature is mainly related to the spheroidization degree of the lamellar during the tensile deformation process.     

Effects of temperature on tensile and impact behavior of dissimilar welds of rotor steels

Tensile and impact behavior of dissimilar weld joints of newly developed rotor steels 23CrMoNiWV88 and 26NiCrMoV145 were conducted at various temperatures below 350 °C. Inhomogeneous microstructures and asymmetrical micro-hardness along the dissimilar welding joint were observed. With the increase of temperature, strength decreased which was associated with the increased plasticity, and fracture location changed from weld metal (WM) to intermediate pressure (IP) base metal (BM) at around 300 °C. Compared to the homogeneous impact specimen with two fracture zones at fracture surface, a combined quasi-cleavage and ductile fracture mode with three zones is observed at the fracture surface of the dissimilar weld joint when the testing temperature is in the range of 0–40 °C. The occurrence of separated zones are mainly ascribed to the multi-layer welding process and thus improved the impact toughness of the welding joint.     

孪晶片层结构在室温轧制过程中的微观结构演变

研究了一种具有纳米孪晶片层结构的电解沉积铜的微观结构特征及其在室温轧制形变后的微观结构演变.结果表明,电解沉积制备的纯铜样品由柱状晶组成,柱状品内含有平行于样品沉积表面的纳米量级厚度的高密度孪晶片层结构,在孪晶界上缺陷很少,为共格孪晶界.形变后,孪晶片层的微观结构特征与片层厚度密切相关.粗大的孪品片层的形变行为以全位错运动为主,而细小的孪晶片层的形变行为以肖克莱(Shockley)位错在孪晶界上的滑移为主,从而导致几个纳米厚的超细孪晶片层消失.