单晶/多晶镍拉伸力学性能的分子动力学模拟

为了提高航空发动机推重比,采用整体叶盘新技术却带来了盘叶连接区域高风险失效问题,采用分子动力学对盘叶连接区单晶/多晶镍(SPSNi)的拉伸力学性能进行模拟。首先对比了不同晶态镍拉伸力学性能,发现由于单晶/多晶界面的存在使得拉伸后界面处的非晶化程度加剧,易于萌生孔洞,加剧了SPSNi突然断裂的风险。最后重点研究了SPSNi的应变速率效应与温度效应。当应变速率1*10~8 s~(-1)e2*10~(10) s~(-1)时,SPSNi对加载应变速率几乎不敏感,抗拉强度σ_b小幅上升。超过2*10~(10) s~(-1)之后,抗拉强度σb随着应变速率的增加而迅速下降。这是因为在高应变速率下,SPSNi的fcc原子大规模且迅速转变为无序的非晶结构,导致了SPSNi承载能力迅速下降,可以将应变速率2*10~(10)s~(-1)作为SPSNi抗拉强度的阈值。SPSNi的抗拉强度σ_b随温度的升高而线性下降。这是由于在温度的影响下,塑性变形阶段SPSNi界面失配位错网络的初始镶嵌结构逐渐变得不规则,初始失配应力随着温度的升高而下降。     

Al0.1CoCrFeNi高熵合金力学性能的分子动力学研究

通过分子动力学方法研究了Al_0.1CoCrFeNi单晶高熵合金在室温（300 K）下沿轴向拉伸后的组织和力学性能变化。通过改变模拟应变速率和温度,分析了单晶高熵合金的拉伸性能;通过模拟室温拉伸实验,研究了含表面小裂纹单晶的显微组织和抗拉伸性能。结果表明,当应变速率在一定范围内时,抗拉伸强度随应变速率增大而增大;当应变速率为10¹⁰ s^-1时,杨氏模量和抗拉伸强度随温度降低而增大。表面有贯穿小裂纹的单晶高熵合金在拉伸一段时间后出现颈缩现象,随着大量滑移位错的快速发展,裂纹尖端出现应力集中,导致快速断裂。     

Ti60合金的热变形行为及工艺参数优化

在变形温度600～950℃,应变速率0.001～10s_^-1条件下,采用Thermecmaster-Z型热加工模拟试验机对Ti60合金进行等温恒应变速率压缩实验。通过分析流动应力行为,计算应变速率敏感指数m和应变硬化指数n,并综合考虑加工图和变形微观组织来研究该合金的热变形行为,得到优化的工艺参数范围。研究结果表明,Ti60合金的流动应力-应变曲线在不同热力参数条件下分别呈现流动稳态型和流动软化型。应变速率敏感指数m随着变形温度升高和应变速率降低而增大。应变硬化指数n随着变形温度升高而减小;随着应变速率的增加在低应变速率(0.001～0.1s_^-1)区间增大,在高应变速率(1～10s_^-1)区间减小;随着应变的增加在高温段(800～950℃)的低应变速率(0.001～0.1s_^-1)区间较缓慢地减小,在高温段(800～950℃)的高应变速率(1～10s_^-1)区间以及低温段(600～750℃)的所有应变速率(0.001～10s_^-1)区间较明显地减小。Ti60合金存在两个功率耗散效率峰值区域,其对应的热力参数窗口分别为温度725～875℃,应变速率≤0.003s_^-1和温度875～938℃,应变速率≤0.04s_^-1。从流动应力行为、应变速率敏感指数m、应变硬化指数n以及加工图综合考虑,Ti60合金的最佳热加工工艺参数为：温度800～875℃,应变速率0.001～0.003s_^-1,或温度875～938℃,应变速率0.001～0.04s_^-1。     

粉末TC4钛合金热压缩动态软化行为分析

通过热模拟压缩实验获得的应力应变曲线表明粉末TC4钛合金在温度为850~950℃,应变速率为0.1~10s_^-1范围内变形时具有加工硬化和连续的动态软化特性,建立了材料本构方程,很好的描述了粉末TC4钛合金的流变行为。进一步对动态软化行为进行了分析,并计算了各种因素对软化的影响程度。结果表明：变形温度越低,应变速率越小,流动软化程度越大;在应变速率为1s_^-1和10s_^-1时,主要是变形热导致流动软化;当应变速率为0.1s_^-1,温度为850℃和900℃时,有变形热、动态相变和α相形态演化三种软化因素,且温度越低,α相形态演化导致的软化占比越大,温度增加,动态相变软化所占比例增加;当应变速率为0.1s_^-1,变形温度为950℃时,有变形热和动态相变两种软化因素,变形量增加,动态相变软化所占比例增大。     

DEFORMATION BEHAVIOR OF SPRAY–DEPOSITED La–BASED BULK AMORPHOUS ALLOY IN SUPERCOOLED LIQUID REGION

研究了喷射成形大尺寸La₆₂Al_15.7(Cu, Ni)_22.3非晶合金在过冷液相区内的塑性变形行为. 结果表明, 随加热温度 的增加和应变速率的减小, 该非晶合金由非稳态变形向单一稳态变形行为转变. 当应变速率为5×10^-3 s^-1, 温度为443 K和挤压比为6.25时, 喷射成形La₆₂Al_15.7(Cu, Ni)_22.3非晶合金样品的密度由挤压前的5.723增加到挤压后的5.924 g/cm³, 达到了同成分吸铸态非晶合金密度 (6.134 g/cm³) 的96.6%. 挤压后非晶合金样品依然保持完全非晶态.     

新型钛合金Ti-555热变形行为及热加工图研究

本文通过高温热压缩试验研究Ti-555钛合金热变形过程中变形温度、应变速率对流变应力的影响,采用Arrhenius双曲正弦函数模型推导出Ti-555本构方程,并依据动态材料模型建立了ε=0.6时的热加工图。结果表明,Ti-555钛合金流变应力对应变速率和变形温度较为敏感,热变形时随变形温度升高或应变速率降低,流变应力下降。根据热加工图确定了两个热加工安全区参数为(1)变形温度为850～950 ℃、应变速率为0.6～10 s_^-1;(2)变形温度为950～1150 ℃、应变速率为0.36～0.9 s_^-1。     

GH2907合金热变形行为及热加工图

通过热模拟压缩实验研究了GH2907合金在变形温度为950~1100℃、应变速率为0.01~10s_^-1、变形量为60%条件下的热变形行为,流变应力随着变形温度的升高或应变速率的降低而显著降低;根据Arrhenius方程和Zener-Hollomon参数,计算了热变形激活能Q,建立了GH2907合金的热变形本构方程;根据动态材料模型,确定了GH2907合金在不同应变下的功率耗散图,功率耗散效率η较高的区域位于温度为1050~1100℃,应变速率为0.01~0.03s_^-1范围,在该变形区域内组织发生了明显的动态再结晶现象;基于Preased失稳判据,绘制了GH2907合金在不同应变下的热加工图,流变失稳区位于高温高应变速率区域,即温度为970~1100℃,应变速率为0.6~10s_^-1范围,在该变形区域内动态再结晶晶粒沿着绝热剪切带和局部流动分布。根据GH2907合金热加工图及微观组织分析得到适宜的加工区域是温度为1050~1100℃,应变速率为0.01~0.03s_^-1范围。     

新型Al-Mg-Zn合金不同应变率动态冲击性能与组织变化

通过Gleeble-1500、分离式Hopkinson压杆、金相、扫描和透射电镜探究了Al-Mg-Zn合金准静态及动态冲击过程中的力学性能和组织演化。Al-Mg-Zn合金在准静态下表现为整体应变硬化效应。合金在1300s_^-1~3800s_^-1对应变率敏感,在4800s_^-1时几乎无应变率敏感性。合金晶粒随应变率变化发生不同程度的变形,且随着应变率的提高,晶粒变形不均匀性加重;析出相粒子形态、密度、尺寸等在4800s_^-1动态冲击前后发生明显变化。     

粉末冶金态与铸态Ti-5553合金高温不连续屈服行为和绝热温升效应对比研究

本文系统地研究了粉末冶金态与铸态Ti-5553合金在温度为700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温不连续屈服行为和绝热温升效应,并对这两种同名义成分不同制备工艺的钛合金进行了对比研究。结果表明：两种合金不连续屈服的幅度均与应变速率呈正相关关系,并与温度呈近似负相关关系, 两种合金中出现的不连续屈服现象符合动态理论。在相同变形条件下,铸态合金中不连续屈服的幅度更大,其原因是相对于粉末冶金态合金,铸态合金中的起始位错密度低,这更有利于晶界处可动位错的突然增殖与扩展。两种合金在热变形中绝热温升的大小均随应变速率的升高而逐渐增大,并随着变形温度的升高而逐渐降低。在相同变形条件下,粉末冶金态合金的绝热温升效应相比与铸态合金较弱,这是因为粉末冶金态合金具有较低的变形抗力和较高的协调变形能力。     

不同Zn/Cu质量比对Mg-Zn-Cu显微组织和性能的影响

论文采用光学显微镜、X射线衍射仪、扫描电子显微镜及显微硬度测试、室温和高温拉伸性能测试、蠕变性能测试研究了Ce和不同的Zn /Cu质量比对Mg-Zn-Cu显微组织和室温及高温力学性能的变化规律、高温变形性能、强化机制和抗蠕变性能的影响。研究结果表明,室温下挤压态Mg-8Zn-8Cu-Ce的拉伸强度和屈服强度分别为320 MPa和291 MPa,在423K温度下,拉伸强度仍高于220MPa。合金具有优良的蠕变性能,稳态蠕变速率为1.21×10_-8 s_-1,蠕变量仅为0.562%。在相同的变形温度下,铸造Mg-7Zn-3Cu-Ce的真实应力随着应变速率的增大而增大,表明合金是应变速率敏感材料。相同的应变速率下,合金的真实应力随着温度的升高而减小,但没有明显的动态再结晶和软化现象。     

Tensile behavior of Al−4%Mg−0.4%Sc−0.5% misch metal alloy at room temperature

The effects of strain rate and pre-deformation in Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm (misch metal) alloy on tensile behavior and P-L effect have been investigated. Pre-deformation of Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy clearly enhances the yield strength and ultimate strength, though it decreases the fracture strain. The yield strength of pre-deformed Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy is higher than that of commercially used Al−Mg based alloys. The strength of Al−4 wt.%Mg−0.5 wt.%Sc−0.5 wt.%Mm alloy was changed slightly at a strain rate between 2×10⁻⁵s⁻¹ and 2×10⁻³s⁻¹, but changed significantly when predeformation was introduced. Tensile test results of as-cast Al-4 wt.%Mg-0.4 wt.%Sc-0.5 wt.%Mm alloy show a significant oscillation of serration during deformation at room temperature, and the critical strain (ε _c ), which is the strain at the start of serration, decreases with increasing strain rate. Pre-deformation of Al−4wt%Mg−0.4wt%Sc−0.5wt%Mm also affects the serration oscillation: it decreases the critical strain at lower strain rate and increases it at higher strain rate (>2×10⁻⁴s⁻¹).     

Tensile properties of hot rolled Mg_(97)Zn_1Y_2 alloy sheets at elevated temperatures

The elevated temperature tensile properties of Mg97Zn1Y2 magnesium alloy sheets, hot rolled at 390, 420 and 450 ℃ respectively, were tested in a temperature range from room temperature to 250 ℃ with a strain rate of 1.0×10-3 s-1. The results show that the variations in yield strength for Mg97Zn1Y2 magnesium alloy sheets hot rolled at 390 ℃ and 420 ℃ with temperature resemble each other due to their similar morphology of the chain-shaped strengthening phase. The yield strength maintains at a high level of 283 MPa before 200 ℃ and decreases significantly at 250 ℃. Despite of the fine lamellar structure of Mg97Zn1Y2 magnesium alloy sheet hot rolled at 450 ℃, its yield strength decreases linearly owing to occurrence of the coarse grain, and drops to 239 MPa at 250 ℃. The elongation for all hot rolled Mg97Zn1Y2 magnesium alloy sheets increases slightly with increasing testing temperature.     

Increase in the plasticity of molybdenum after high-rate deformation under pressure

The dependence of the plasticity of polycrystalline molybdenum on the strain rate has been studied after deformation performed under pressure and at an ambient temperature of 293 K. The samples were deformed in tensile tests. The strain rate of tensile deformation and the pressure in each experiment are constant. The range of the strain rate is 8.3 × 10^?7 to 8.3 m/s and the pressure is 0.1–500 MPa. At a pressure of ambient atmosphere, molybdenum at a strain rate less than 200 s^–1 in value (10^2.3 s^–1) fractures in a brittle manner with zero residual strain. The brittle fracture of the working part of a cylindrical sample occurs simultaneously in several places. The molybdenum plasticity decreases in the range of low strain rates regardless of the pressure and, on the contrary, plasticity under pressure increases in the range of high strain rates. The strain rate of tensile deformation at which the dependence of the plasticity on the strain rate under pressure changes is 8.3 × 10^–4 m/s (a strain rate of 0.25 s^–1). The high plasticity of molybdenum after deformation under pressure is observed at high strain rates.     

Tensile and Compressive Properties of Mg-3Al-2Zn-2Y Alloy at Different Strain Rates

The tensile and compressive properties of Mg-3Al-2Zn-2Y alloy at room temperature at strain rates in the range of 0.001-1400 s^?1 and 0.001-4800 s^?1, respectively, were investigated. The ultimate strength in tension tests has positive effect to strain rate, while that in compression tests has positive effect at strain rates in the range of 0.001-1800 s^?1 and negative effect when the strain rate increases to 4800 s^?1. The strain rate sensitivity of ultimate strength is different for tension and compression. To both tension and compression, the density of dislocation increases with increasing of strain rate and parallel dislocations come being at high strain rate. A large number of twins appear when the strain rate increases to certain degree. The fracture characteristics change from quasi-cleavage to ductile fracture as tensile strain rate increases, while the compression strain rate has little influence on the fracture characteristics.     

Superplastic Behavior of Copper-Modified 5083 Aluminum Alloy

An AA5083 aluminum alloy was modified with two different levels of Cu additions, cast by direct-chill method, and thermo-mechanically processed to sheet gauge. Copper additions reduced sheet grain size, decreased tensile flow stress and significantly increased tensile elongation under most elevated temperature test conditions. The high-Cu (0.8 wt.%) alloy had the finest grain size 5.3 μm, a peak strain-rate sensitivity of 0.6 at a strain-rate of 1 × 10⁻² s⁻¹, and tensile elongation values between 259 and 584% over the temperature range, 400-525 °C, and the strain rate range, 5 × 10⁻⁴ to 1 × 10⁻² s⁻¹, investigated. In biaxial pan forming tests, only the Cu-containing alloys successfully formed pans at the higher strain rate 10⁻² s⁻¹. The high-Cu alloy showed the least die-entry thinning. Comparison of ambient temperature mechanical properties in O-temper state showed the high-Cu alloy to have significantly higher yield strength, ultimate strength, and ductility compared to the base 5083 alloy. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

Microstructure and elevated-temperature tensile properties of differential pressure sand cast Mg-4Y-3Nd-0.5Zr alloy

The microstructures of an Mg-4Y-3Nd-0.5Zr alloy by differential pressure casting were investigated using scanning electron microscopy(SEM) and transmission electron microscopy(TEM), and its tensile deformation behavior was measured using a Gleeble1500 D themo-simulation machine in the temperature range of 200 to 400 °C at initial strain rates of 5×10-4 to 10-1 s-1. Results show that the as-cast microstructure consists of primary α-Mg phase and bone-shaped Mg5 RE eutectic phase distributed along the grain boundary. The eutectic phase is dissolved into the matrix after solution treatment and subsequently precipitates during peak aging. Tensile deformation tests show that the strain rate has little effect on stress under 300 °C. Tensile stress decreases with an increase in temperature and the higher strain rate leads to an increase in stress above 300 °C. The fracture mechanism exhibits a mixed quasi-cleavage fracture at 200 °C, while the fracture above 300 °C is a ductile fracture. The dimples are melted at 400 °C with the lowest strain rate of 10-4 s-1.     

Tensile behavior of a free-standing Pt-aluminide (PtAl) bond coat

The tensile behavior of a high activity stand-alone Pt-aluminide (PtAl) bond coat was evaluated by the micro-tensile test method at various temperatures (room temperature to 1100 °C) and strain rates (10^?5 s^?1–10^?1 s^?1). At all strain rates, the stress–strain behavior of the stand-alone coating was significantly affected by the variation in temperature. The stress–strain response was linear, indicating brittle behavior, at temperatures below the brittle–ductile transition temperature (BDTT). The coating exhibited appreciable ductility (up to 2%) above the BDTT. The strength (both yield stress and ultimate tensile strength) of the coating decreased and its ductility increased with increasing temperature above the BDTT. The tensile behavior of the coating was sensitive to strain rate in the ductile regime, with its strength increasing with increasing strain rate at any given temperature. The BDTT of the coating was found to increase with increasing with increasing strain rate. The coating exhibited two distinct mechanisms of deformation above the BDTT. The transition temperature for the change of deformation mechanism also increased with increasing strain rate.     

准静态加载下时效态Inconel 718薄板的各向异性屈服准则及硬化模型构建

当金属件的特征尺寸缩小到微尺度时,会产生尺寸效应,从而使对微成形的理解变得复杂。本文以0.1mm厚的时效态Inconel 718薄板为研究对象,对其进行了力学性能测试。基于力学测试数据,探究了时效态Inconel 718薄板在相同应变速率、不同拉伸方向上各向异性、延伸率、屈服强度及最大抗拉强度的变化规律,并建立了介微观尺度下各向异性及屈服强度的预测模型和考虑应变量及应变速率的准静态硬化模型。结果表明：时效态Inconel 718薄板具有明显的各向异性,其延伸率以45°为极值点呈现先增大后减小的变化规律,屈服强度和最大抗拉强度的变化规律与之相反。由于尺寸效应的存在需要两组不同的材料参数对各向异性及屈服强度进行预测。当应变速率大于0.1 s_^-1时,材料屈服强度表现出明显的应变速率敏感性,该硬化模型不再适用。     

Tensile ductility of an Fe–40Al–0.6C alloy

Binary Fe–40Al and ternary Fe–40Al–0.6C alloys were cast, hot-extruded into rods, annealed at low temperatures to reduce the non-equilibrium vacancy concentration and tested in uniaxial tension at room temperature in air, over a range of strain rates from 4.2×10⁻¹ s⁻¹ to 4.2×10⁻⁸ s⁻¹. Yield strength, fracture strength, tensile ductility and the work-hardening behavior in the 0.2–1.0% plastic deformation range were monitored. Resulting fracture surfaces were examined at low and high magnifications, and the change in the fraction transgranular cleavage as a function of test strain rate was correlated with the observed mechanical properties. Prior to testing, both alloys exhibited fairly coarse grain size (∼80–100 μm); whereas the binary alloy was single phase, the ternary alloy contained a dispersion of lath-shaped perovskite carbides (Fe₃AlC_0.5) in the grain interior and at grain boundaries. In the binary alloy, ductility decreases continuously with decreasing strain rate and this behavior has been previously attributed to an environmental effect. For a given strain rate, over the range of strain rates examined, the ternary alloy demonstrates improved ductility over the binary alloy; furthermore, at the extremely slow strain rates (<4×10⁻⁷ s⁻¹), the ductility of the ternary alloy increases with decreasing strain rate after reaching a minimum. Whereas in the binary alloy, fracture mode remains completely intergranular over the entire strain rate regime, in the ternary alloy, fracture mode is completely intergranular at the fastest strain rate but gradually transitions to a predominantly transgranular cleavage mode with decreasing strain rate. A maximum in the fraction transgranular cleavage is reached coincident with the ductility minimum, beyond which (i.e. lower strain rates) the fraction transgranular cleavage decreases sharply. These observations are discussed in terms of the possible role of these carbides as hydrogen traps and their consequential effects on mechanical properties.