Dynamic recrystallization behavior and mechanical properties of bimodal scale Al2O3 reinforced AZ31 composites by soild state synthesis

In this paper,(500 nm 1%+5μm 3%)bimodal scale Al2O3p/AZ31 composites was fabricated by solid state synthesis and the effect of bimodal scale Al2O3 particulates on its dynamic recrystallization behavior and mechanical properties was investigated.The optical microscopy,scanning electron microscopy,transmission electron microscopy and electron universal strength tester composites were used to characterize the composites.The results indicate that the grains size of the composites are significantly refined and the mechanical properties are obviously improved.Due to the presence of the bimodal scale Al2o3 particulates,the high-density dislocation zone is formed around nano-Al2o3p and the particle deformation zone is formed near micron-ABOap.These zones are ideal sites for the formation of recrystallization nucleus.Meanwhile,the addition of the bimodal scale Al2o3 particulates may delay or hinder the growth of matrix grain through the pining effect on the grain boundaries,resulting in significantly improving the yield strength and tensile strength of Al2O3p/AZ31 composites.

Modeling of microstructure evolution of AZ80 magnesium alloy during hot working process using a unified internal state variable method

In this paper, a unified internal state variable(ISV) model for predicting microstructure evolution during hot working process of AZ80 magnesium alloy was developed. A novel aspect of the proposed model is that the interactive effects of material hardening, recovery and dynamic recrystallization(DRX) on the characteristic deformation behavior were considered by incorporating the evolution laws of viscoplastic flow, dislocation activities, DRX nucleation and boundary migration in a coupled manner. The model parameters were calibrated based on the experimental data analysis and genetic algorithm(GA) based objective optimization. The predicted flow stress, DRX fraction and average grain size match well with experimental results. The proposed model was embedded in the finite element(FE) software DEFORM-3 D via user defined subroutine to simulate the hot compression and equal channel angular extrusion(ECAE) processes. The heterogeneous microstructure distributions at different deformation zones and the dislocation density evolution with competitive deformation mechanisms were captured.This study can provide a theoretical solution for the hot working problems of magnesium alloy.

Dependence of α-phase size on flow stress during dynamic recrystallization steady state in Ti60 alloy

The dependence of α-phase size on flow stress was characterized by a proposed kinetic model during dynamic recrystallization （DRX） steady state in Ti60 alloy. Accord- ing to the isothermal compression tests, the influence of deformation parameters on the steady-state flow stress was analyzed and the constitutive equation was established to predict the steady-state flow stress under different defor- mation temperatures and strain rates. A power-law rela- tionship between the DRX average grain size and steady- state flow stress with an exponent of -2 is obtained from the dynamic balance during DRX steady state. The effect of deformation parameters on at-phase size was observed through the microstructure after deformation, and the applicability of the model for Ti60 alloy was verified by the comparison between predicted and experimental data.

Unveiling the planar deformation mechanisms for improved formability in pre-twinned AZ31 Mg alloy sheet at warm temperature

To investigate the role of pre-twins in Mg alloy sheets during warm planar deformation,the stretch forming is conducted at 200℃.Results suggest the formability of the pre-twinned AZ31 Mg alloy sheet is enhanced to 11.30 mm.The mechanisms for the improved formability and the deformation behaviors during the planar stretch forming are systematically investigated based on the planar stress states.The Schmid factor for deformation mechanisms are calculated,the results reveal that planar stress states extremely affect the Schmid factor for{10-12}twinning.The detwinning is activated and the prismatic slip is enhanced in the pre-twinned sheet,especially under the planar extension stress state in the outer region.Consequently,the thickness-direction strain is accommodated better.The dynamic recrystallization(DRX)type is continuous DRX(CDRX)regardless of the planar stress state.However,the CDRX degree is greater under the planar extension stress state.Some twin lattices deviate from the perfect{10-12}twinning relation due to the planar compression stress state and the CDRX.The basal texture is weakened when the planar stress state tends to change the texture components.

New Type Austenite Dynamic Recrystallization of Microalloyed Forging Steels 38MnVS During Forging Process

Hot compression tests of microalloyed forging steels 38MnVS were carried out on the Gleeble-3800 thermo-mechanical simulator at the deformation temperatures from 950 to 1 150 ℃ with the strain rates ranging from 0.1 to 10 s^（-1）. The effects of the deformation temperature and strain rate on the austenite dynamic recrystallization and microstructural changes were researched. The experimental results show that the dynamic recrystallization accelerated with the increase of the deformation temperatures and the decrease of the strain rate. The activation energy of dynamic recrystallization was calculated, which was about 275.453 kJ/mol. The relation between the dynamic recrystallization and the Z-parameter was investigated, and the state chart of the dynamic recrystallization of the microalloyed forging steel 38MnVS was made according to the experimental data and the deformation parameters.

Thermal deformation behavior and microstructure evolution of modified IN617 alloy with different initial states

The thermal deformation behaviors of the as-cast and wrought modified IN617 nickel-based heat-resistant alloys at different temperatures(1000–1180℃)and strain rates(0.01–1 s^(−1))were studied.The constitutive equation was established to describe the relationship of the flow stress,temperature,and strain rate during thermal deformation.The effect of the thermal deformation conditions on the microstructure evolution of alloys was studied using electron backscatter diffraction.The results revealed that the thermal deformation activation energy of the as-cast alloy was greater than that of the wrought alloy.The dynamic recrystallization(DRX)process is slow at intermediate strain rate(0.1 s^(−1))due to the comprehensive influence of various factors,such as the critical strain of DRX nucleation and stored energy.The DRX volume fraction increases with the improvement of deformation temperature.The varied dynamic softening mechanisms induce the different thermal deformation behaviors of as-cast and wrought alloys.The dynamic recovery,discontinuous dynamic recrystallization(DDRX)and nucleation at slip zone caused by strain incompatibility in grains were observed during thermal deformation of as-cast alloys.In the process of thermal deformation of wrought alloys,DDRX was the primary dynamic crystallization mechanism.The continuous dynamic recrystallization was an auxiliary nucleation mechanism.

冷轧基板SPHE动态再结晶行为研究

通过热模拟实验研究了冷轧基板SPHE钢的动态再结晶行为。结果表明:在热变形过程中,变形温度越高,应变速率越小,越容易发生动态再结晶。SPHE钢发生动态再结晶的激活能为:Q_d=125.793kJ/mol;描述动态再结晶的动力学方程为:f_(dyn)=I-exp(-b(Z)t^(n(Z))),计算得到了其中的系数b(Z)、n(Z);描述动态再结晶体积百分数的方程为:X_(dyn)=1-exp[-k((e-ε_c)/ε_p)~m],通过非线性回归求出系数k、m;根据实验值和变形参数,作出SPHE钢的动态再结晶状态图,从而可优化热轧工艺,提高产品的质量。

钒含量对直接切削用非调质钢动态再结晶的影响

在Gleeble-3800热模拟实验机上进行热变形试验,研究不同钒含量的直接切削用非调质钢在变形温度为950~1 150℃,应变速率为0.1~10 s^(-1),变形量为60%的单道次压缩的奥氏体再结晶过程。计算得出,V1钢的动态再结晶激活能Q_d比V2钢的提高了79.617 kJ/mol,增加钒含量具有延迟奥氏体动态再结晶发生的作用。根据试验模拟结果并结合实际生产情况,确定了V1钢和V2钢的最佳热加工工艺参数。

固态金属剪切连接组织及界面演变研究

组织状态及界面形成过程对固态金属剪切连接后的连接强度及性能有重要影响,采用实验研究的方法,结合理论分析实施了不同压下量、空冷和水冷Q345钢剪切连接实验。通过微观组织分析、界面结合过程探究和连接强度测定,分析了组织演变和界面形成机制,得到了剪切连接过程的组织及界面演变规律。结果表明:随压下量增加,界面晶粒细化程度提高,组织得到明显细化,界面孔洞逐渐消失,形成共有晶粒界面;晶粒细化驱动为动态再结晶,界面形成为三阶段过程;压下量为125%空冷后,晶粒尺寸由原态40μm细化到10μm左右,连接界面消失,连接强度为500.56 MPa。

GH1016合金的动态再结晶行为

利用GLEEBLE-3500热模拟试验机完成了变形温度为1000~1150℃,应变速率为0. 1~10 s-1,变形量分别为20%、40%和60%的热压缩试验,获得了各试验条件下GH1016合金的真应力-真应变曲线,并利用数据拟合的手段求得了GH1016合金的临界变形条件方程。同时,得到了所有变形条件下的再结晶组织,分析了变形工艺对GH1016合金的动态再结晶过程的影响规律,最终获得了GH1016合金在试验条件下的动态再结晶状态图,可为生产现场的锻造工艺的合理制定提供理论参考依据,尤其适用于GH1016合金的精锻工艺的制定和优化。

2055铝锂合金热拉伸的显微组织与流变应力行为

用Gleeble-1500热模拟机对2055铝锂合金进行高温拉伸,研究合金在温度为480~540℃、应变速率为0.000 1~0.1 s^-1区间内的热变形行为,并通过光学显微镜(OM)、透射电镜(TEM)和扫描电镜(SEM)观察和分析合金的显微组织。结果表明:热拉伸过程分为3个阶段,各阶段的变形机制不同。热拉伸过程中表现出明显的流变稳态,合金的主要变形机制由动态回复向动态再结晶转变。在应变速率0.001 s^-1下,变形温度为480℃时,很难观察到再结晶晶核,变形温度为510℃和540℃时出现锯齿状晶界,且温度越高,锯齿状晶界越明显,该现象属于典型的几何动态再结晶;温度一定时,应变速率越低,合金的峰值应力越小,表明2055铝锂合金具有正应变速率敏感性。采用Sellars和Tegart提出的包含变形激活能Q和温度T的双曲线正弦函数本构方程来描述合金的热激活行为,热变形激活能为226.783 kJ/mol。

30% SiC_p/Al复合材料热变形及动态再结晶行为

采用Gleeble-1500D热模拟试验机研究30%SiCp/Al复合材料在温度为623~773 K、应变速率为0.01~10 s-1下的热变形及动态再结晶行为。结果表明：材料的高温流变应力-应变曲线主要以动态再结晶软化机制为特征,峰值应力随变形温度降低或应变速率升高而增大,材料热激活能为272.831 k J/mol。以试验数据为基础,建立q-s和？q/？s-s曲线,从而进一步获得动态再结晶的临界应变和稳态应变,通过试验数据的回归分析,建立动态再结晶的临界应变模型和稳态应变模型,并在此基础上,获得所需要材料的动态再结晶图。

13Cr油井专用钢的高温塑性变形行为研究

采用Gleeble-3800热模拟试验机,对石油油井管用钢13Cr马氏体不锈钢进行了高温轴向压缩试验,热变形温度为1000~1250℃,应变速率为0.1~20 s^-1。通过试验获得了试验钢的高温流变曲线,其流变曲线符合典型的动态再结晶特征。根据流变曲线计算出该钢的热变形激活能为405262 J·mol^-1,并建立了试验钢的热变形本构方程及Z参数公式,同时绘制了试验钢的动态再结晶状态图与热加工图,理论上确定了最佳热变形工艺范围是变形温度1075~1125℃和1175~1250℃,应变速率为0.1 s^-1,此时能量耗散效率达到峰值。

应力状态对AZ31镁合金最佳成形温度的影响

在20～250℃温度范围内,对AZ31镁合金薄板进行了单向拉伸、筒形件拉深以及胀形试验,并用金相显微镜观察了试验后试件的显微组织。分析了AZ31镁合金在不同工艺所对应的应力状态下塑性变形特点及其最佳成形温度。结果表明,变形过程中所受应力状态对AZ31镁合金最佳成形温度的影响很大,AZ31镁合金在成形过程中受单向拉应力时,其总延伸率随成形温度的升高而增加;应力状态主要为压应力时,最佳成形温度应在tr=1以下;而应力状态主要为双向拉应力时,其最佳成形温度应在tr=1以上。

挤压剪切与正挤压对AZ31镁合金塑性变形的影响

借鉴正挤压与多道次等通道挤压的特点提出了挤压(Extrusion)-剪切(Shear)复合挤压工艺(简称ES),制造了多副适合工业卧式挤压机的ES变形组合凹模,进行了ES挤压和普通挤压实验。构建了ES挤压和普通挤压的三维有限元热力耦合模型及数值模拟条件,对ES挤压过程的挤压力、累积应变演化进行了计算机模拟仿真。通过对坯料的应力状态进行了计算机模拟分析,发现ES挤压过程局部坯料受到四向压应力,ES挤压与普通正挤压相比可以显著提高镁合金变形过程的累积应变,因此可以更有效的细化晶粒。针对ES挤压和普通挤压棒料的不同位置进行了微观组织观察,发现在挤压温度为370℃、挤压比为12时ES挤压可以有效的细化晶粒,不仅可以细化棒材表层晶粒,心部也得到了细化。

挤压剪切对镁合金组织及物理场演变的影响机制

为了优化镁合金棒材的微观组织,借鉴正挤压与多道次等通道挤压的特点提出了挤压(Extrusion)-剪切(Shear)复合挤压工艺(简称ES),制造了多副适合工业卧式挤压机上的ES变形组合凹模,进行了ES挤压和普通挤压实验。构建了ES挤压和普通挤压的三维热力耦合的有限元模型及计算条件,模拟了ES挤压过程的挤压力、有效应力及应变演变规律,利用光学显微镜、透射及扫描电镜对微观组织演变进行了观察与分析。结果表明:根据对成形力演变过程和数值的分析,ES挤压过程可以分为四个阶段,其中有三个阶段挤压力变化较小,ES成形所需的挤压力大约是普通挤压所需挤压力的1.67倍;与普通挤压相比,ES挤压可大大提高镁合金的有效应变,最大有效应变可提高到3.8倍;ES成形可以改善镁合金中β相的形态与分布,不仅细化棒材表层晶粒,而且心部晶粒也得到了细化;动态再结晶晶粒及变形位错会越多,再结晶晶粒越细;带有转角的ES模具产生的剪切作用是镁合金发生多次动态再结晶的主要原因,在ES挤压过程中,第二相β相对基体有剪切和钉扎作用。

UML状态图形式化方法的分析与比较

UML缺乏精确的语义,难以对其所表示的系统进行形式化分析和一致性检验。为了使UML能够更精确地对系统模型进行描述,学者们提出了一些形式化的方法。论文对比分析了用Petri网、时序逻辑XYE/E和动态描述逻辑形式化UML状态图的方法,指出了它们各自的优缺点以及应用领域。

金属材料动态再结晶建模与模拟研究进展

金属及合金的动态再结晶过程一直以来是热加工领域的研究热点。文章综述了不连续动态再结晶、连续动态再结晶和几何动态再结晶等再结晶方式的物理演化机制和发生条件,并讨论了内变量法、元胞自动机法等模拟方法在金属材料再结晶领域的研究现状及发展前景。

UML和CPN在企业销售信息管理系统开发中的应用

UML是标准的统一建模语言,但缺乏合适的工具对其所建立的模型进行严格定量的分析。CPN(有色Petri网)是严格定义的数学对象,可以模拟和检验用UML描述的系统对象的行为属性。该文以企业销售信息管理系统为实例描述了通过UML进行建模的过程,并用CPN对所建立的模型进行了分析和检验。

低合金钢Q345E高温热变形行为及动态再结晶图

应用Gleeble-1500D热模拟试验机对低合金高强度结构钢(HLSA)Q345E进行高温单道次热压缩试验,研究了不同变形参数(变形温度T、变形速率ε和变形量ε)下Q345E钢的变形抗力,分析了各变形参数对该钢变形抗力和动态再结晶的影响。结果表明:随着应变速率的提高和变形温度的降低,Q345E钢的流变应力显著增大;在应变速率较低、高温时,易发生动态再结晶;在应变速率较高、低温时,不发生动态再结晶。建立了Q345E钢热态变形过程中的高温塑性本构方程和动态再结晶图,为科学设计和有效控制Q345E钢的成形工艺提供理论依据。