LOW CYCLE FATIGUE BEHAVIORS AND MICROSTRUCTURES OF Ti–2Al–2.5Zr WITH FINE GRAIN AT RT AND 77 K

对细晶Ti--2Al--2.5Zr合金进行了室温/低温(77 K)疲劳实验及微观组织观察. 结果表明: 室温低应变幅Δε_t/2(=0.5%, 1.0%)下,合金表现为循环软化; 室温高应变幅(1.5%, 2.0%)下, 则表现为循环应力饱和; 77 K时, 不同应变幅下均表现为循环硬化, 且随应变幅升高, 循环硬化程度增强. 疲劳寿命测试结果表明: 低温疲劳寿命始终高于室温. 断口SEM观察表明, 室温和低温下, 疲劳裂纹扩展区均有明显的疲劳条纹,疲劳裂纹以穿晶方式扩展, 室温下伴随有大量二次裂纹, 低温下的二次裂纹数量明显减少. TEM观察表明: 低温下孪生是合金主要的变形方式, 包括{1011}和{1121}型孪晶. 疲劳变形位错组态为: 室温较低应变幅(0.5%, 1.0%)下, 形成位错线和局部位错缠结; 室温下应变幅提高到1.5%和2.0%时,\{1010}柱面和{1121}锥面滑移同时开动, 位错组态演化为亚晶和明显的位错胞. 77 K下, 应变幅2.0%时形成沿 柱面平行分布的位错带; 77 K下应变幅升高到4.5%时, 多滑移形成相互垂直的位错线. 低温诱发形变孪晶是Ti--2Al--2.5Zr低温疲劳寿命升高的原因.     

2205双相不锈钢的高温变形行为

利用Gleeble-3800热力模拟试验机在温度为1223-1523 K, 应变速率为0.01-10 s^-1的条件下进行了2205双相不锈钢热压缩变形实验, 测定了真应力-真应变曲线, 分析了变形组织. 结果表明: 奥氏体分布在随温度升高而含量增加的铁素体基体上, 升高温度和降低应变速率可促进奥氏体发生动态再结晶. 基于热变形方程计算得到了热变形激活能Q=451 kJ/mol, 表观应力指数n=4.026. 真应力-真应变曲线存在的“类屈服平台”效应与Z参数有关, 随着Z参数的减小而逐渐增强. 基于简化应力函数的ln Z与σ_p之间的线性关系在临界点(ln Z_c=38.18)发生偏移;峰值应力与温度及应变速率的关系可表示为: σ_p=20.6lnε+1118002/T-266.8(ln Z>38.18); σ_p=9.1lnε+493874/T-701.9(ln Z≦38.18)     

基于DMNR模型建立EB炉熔炼TC4钛合金的双重多元非线性回归本构方程

通过热模拟压缩实验研究EB炉熔炼TC4钛合金在应变速率为0.01 s_^-1-10 s_^-1,变形温度为800 ℃-1100℃条件下的热变形行为,计算合金在不同变形条件下的应变速率敏感性指数m,并基于DMNR模型建立EB炉熔炼TC4钛合金的双重多元非线性回归本构方程。结果表明：在变形开始阶段,加工硬化占主导作用,流动应力随着应变的增加而增加,当达到峰值应力时,软化作用比较明显,位错开始发生滑移和攀移,流动应力随着应变的增加而降低。在低温小应变速率下m值较大,在高温大应变速率下m值较小,m值越大,对应的显微组织越均匀。采用所建立的非线性回归本构方程能够较好地预测EB炉熔炼TC4钛合金的流动应力,预测值与实测值之间的平均绝对相对误差为5.83% ,相关系数为0.98。     

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。     

Low-Cycle Fatigue Behavior of a Nickel Base Single Crystal Superalloy at High Temperature

研究了2种高温条件下镍基单晶合金的低周疲劳行为。试验温度和总应变幅是影响合金低周疲劳寿命的2个主要因素,在相同温度下,低周疲劳寿命随应变幅的减小而增大;在同一应变幅下,870℃的疲劳寿命均小于760℃的疲劳寿命。二次细小y相有效阻碍了位错的滑移,提高了合金在760℃低周疲劳变形抗力,位错滑移带成为疲劳裂纹萌生及扩展的主要途径;870℃循环应力曲线前期出现短暂硬化和后期软化的现象,y’相逐渐粗化和高密度的位错缠结是循环软化的主要原因。局部应力集中与合金内微孔的交互作用是疲劳裂纹萌生的源头。     

一种镍基单晶合金高温低周疲劳行为研究(英文)

研究了2种高温条件下镍基单晶合金的低周疲劳行为。试验温度和总应变幅是影响合金低周疲劳寿命的2个主要因素,在相同温度下,低周疲劳寿命随应变幅的减小而增大;在同一应变幅下,870℃的疲劳寿命均小于760℃的疲劳寿命。二次细小y相有效阻碍了位错的滑移,提高了合金在760℃低周疲劳变形抗力,位错滑移带成为疲劳裂纹萌生及扩展的主要途径;870℃循环应力曲线前期出现短暂硬化和后期软化的现象,y'相逐渐粗化和高密度的位错缠结是循环软化的主要原因。局部应力集中与合金内微孔的交互作用是疲劳裂纹萌生的源头。     

600 ℃时钛合金高温低循环疲劳行为及其微观机理

研究近α型TG6钛合金盘锻件双态组织在600 ℃不同应变幅条件下的低循环疲劳行为,总应变幅△εt/2控制在±0.6%～±1.5%,应变比R＝-1,采用三角波方式加载.结果表明:600 ℃低循环疲劳条件下,随着总应变幅的增加,循环峰值应力smax提高,而疲劳寿命Nf下降;对于TG6钛合金盘锻件双态组织,存在一个循环软化/硬化的总应变幅临界值(△εt/2)c,约为±1.0%,当总应变幅高于此值,表现为循环硬化行为,而低于此值时,则表现为循环软化行为;在所有应变幅条件下,疲劳裂纹均为多源萌生模式,600 ℃低循环疲劳变形行为主要受α晶粒内位错的平面滑移所控制,位错平面滑移集中的结果是最早在α晶粒内萌生疲劳裂纹;随着疲劳测试温度的提高,不同滑移系上位错滑移的临界分切应力的差别缩小,促进位错交滑移的进行.     

DIMENSIONAL STABILITY OF 2D Cf/Mg--2.0Re--0.2Zr COMPOSITES

采用压力浸渗法制备碳纤维织物(2D C_f)及单向碳纤维(1D C_f)增强镁合金复合材料, 测试了两种复合材料在 50---350 ℃范围内的热膨胀行为. 结果表明, 2D C_f/镁合金复合材料(简称2D)平面内不同方向的平均热膨胀系数均随温度升高不断降低. 从50到350℃, 0°/90°方向的平均热膨胀系数由 4.03×10^-6 ℃^-1降至1.83×10^-6 ℃^-1; 45°方向的平均热膨胀系数由4.53×10^-6 ℃^-1降至2.31×10^-6 ℃^-1. 根据推导公式可以准确计算2D增强复合材料20---150 ℃范围内0°/90°方向的平均热膨胀系数. 20---150 ℃热循环测试结果表明, 2D复合材料具有较好的尺寸稳定性, 在热循环过程中存在应变滞后现象, 其残余塑性变形主要是基体合金在热应力作用下产生的塑性变形, 并且从第2次热循环起, 复合材料在热循环中产生的净应变不随热循环次数增加而变化.     

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.     

考虑加工硬化和动态软化的FeCrNiMn中熵合金本构模型

采用等温压缩分析了Fe_0.25Cr_0.25Ni_0.25Mn_0.25中熵合金在900~1050 ℃、0.001~1 s^-1应变速率范围内的流变行为。结果表明,热变形以动态再结晶为主,与其他低堆垛层错能的合金一样,流变曲线呈单峰形状。建立了本构模型来描述整个变形过程,分析了加工硬化行为和动态软化过程。利用Kocks-Mecking图发现,在加工硬化阶段,合金的硬化速率随应力呈线性降低,因此应力-应变行为可以用传统的位错密度模型来描述。同时,采用经典的JMAK方程描述由动态再结晶引起的软化过程。此外,对本构模型进行了进一步的修改,减少了参数的数量,简化了回归分析。所提出的半物理模型不仅可以准确地预测应变范围外的应力-应变行为,而且可用于其他低层错能合金。     

Mechanical properties and hardening mechanism of Fe–Al–Ni single crystals containing NiAl precipitates

The microstructures and mechanical properties of Fe–23.0 Al–6.0 Ni (at.%) single crystals containing NiAl precipitates were investigated and the hardening mechanism due to the precipitates was discussed, focusing on the activated slip systems. When these alloys were slowly cooled to room temperature after homogenization at 1373 K, the NiAl phase with the B2 structure precipitated in the body-centered cubic (bcc) Fe–Al matrix, satisfying the cube-on-cube relationship with a small misfit strain. The single crystals containing the NiAl precipitates exhibited a high yield stress above 1 GPa at room temperature. In addition, the activated slip system and deformation behavior depended strongly on the loading axis. For instance, 〈1 1 1〉 slip, which is the primary slip for the bcc matrix, occurred at 〈1 4 9〉 and 〈0 0 1〉 orientations and the NiAl precipitates were sheared by the slip. A critical resolved shear stress of 〈1 1 1〉 slip in the NiAl phase was known to be extremely high, which led to strong precipitation hardening. On the other hand, at 〈5 5 7〉 and 〈0 1 1〉 orientations, 〈0 0 1〉 slip, which is the primary slip system for the NiAl precipitates, forcibly sheared the bcc Fe–Al matrix, also leading to strong hardening. Thus, in the Fe–Al–Ni alloys, the difference in the primary slip system between the bcc Fe–Al matrix and the NiAl precipitates resulted in extreme hardening. This hardening mechanism caused by the NiAl precipitates effectively increased the yield stress even at high temperatures. In fact, the crystals exhibited a high yield stress at ～1 GPa up to 823 K.     

Improvement of the creep resistance of FeCo–2V alloy arising from a small addition of Cr

This work presents our recent findings that a small addition (0.5 at. %) of Cr to FeCo–2V alloy leads to a great improvement in creep resistance. High resolution electron microscopy was applied to study microstructural evolution of the Cr added alloy during the creep process performed at 600 °C under 200 MPa. At an initial step of the creep, there appear plate-like precipitates with bcc/fcc structure as well as some rod-like ones with hcp structure. A coherent relationship is identified between the precipitates and bcc FeCo matrix. With prolonging the creep, the rod-like hcp precipitates are revealed to remain in the bcc matrix, showing a good stability under the creep condition and in turn resulting in piling-up of dislocations to a great extent around the precipitates. In addition, the Cr added alloy is shown to have a large stress exponent of 8.4, indicating a strong interaction between dislocations and the hcp precipitates.     

Effects of CoAl precipitates on deformation behavior of Fe-Al-Co single crystals

Effects of the CoAl precipitates with the B2 structure on the deformation behavior of Fe-xAl-xCo (at.%) single crystals (x = 15, 18, 20) were examined focusing on loading axis, precipitate morphology and deformation temperature. Strong precipitation hardening occurred owing to the precipitation of the CoAl phase in the bcc matrix, and not only <111> slip but <001> slip took place, which resulted from the difference in primary slip systems between the bcc matrix and the CoAl precipitates. It is also noted that the activated slip system depended strongly on the size and volume fraction of the CoAl precipitates. For instance, the primary slip direction at x = 18 changed with increasing precipitate size in the following order: <111>, <001> and <111>. Higher volume fraction of the CoAl precipitates at higher x was favorable for the activation of <001> slip. The activated slip system also varied with deformation temperature, associated with the temperature dependence of the critical resolved shear stress of <111> and <001> slips.     

On the mechanism of nucleation of copper precipitates upon aging of Fe-Cu alloys

Structure of the Fe-5.5% Cu alloy has been studied after aging at 500°C to show that the lattice of copper precipitates changes with increasing time of holding at a temperature from bcc to 9R to fcc. The free energy of the solid solutions and the phase-separation diagrams of the bcc and fcc phases of copper have been calculated. Barriers for the nucleation of copper particles with the bcc and fcc lattices have been evaluated. Factors that favor the appearance of intermediate structural forms of copper, which precede the formation of the stable fcc phase, are discussed.     

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF β-NiA1 CONTAINING α-Fe PRECIPITATES

1. 'IntroductionThe BZ-structural intermetallic, NiAI, has received considerable attention because ofits potential for high temperature applicationsll)21. Its advalltages are the low density of5.95g/cm' (approximately two-thirds the density of nickel-base superalloys), excellent thermal conductivity (four to eight times that of nickeLbase superalloys) and good oxidationresistance at high temperature. However, before the compound can be exploited for thispurpose its low ductility at low tempe…     

Microstructure characterization of Cu-rich nanoprecipitates in a Fe–2.5 Cu–1.5 Mn–4.0 Ni–1.0 Al multicomponent ferritic alloy

The evolution of precipitates in a Fe–2.5 Cu–1.5 Mn–4.0 Ni–1.0 Al multicomponent ferritic alloy during annealing at 500 °C was systematically investigated by aberration-corrected scanning transmission electron microscopy. The atomic-scale structure and chemistry characterization reveal that primary precipitates with enriched Cu, Ni, Mn and Al originate from continuous growth of B2 ordered domains in the as-quenched alloy. The formation of a Cu-rich body-centered cubic (bcc) phase takes place by the decomposition of the B2 ordered primary phase, which forms a Cu-rich bcc core and ordered B2-Ni(Al,Mn) shell. The B2 shells serve as a buffer layer to moderate the coherent strain and to prohibit the inter-diffusion between the Cu-rich precipitates and bcc-Fe matrix, giving rise to a low coarsening rate of the precipitates. The Cu-rich precipitates experience a structural transformation from bcc to 9R at a critical size of ～6 nm during long time annealing, corresponding to obvious coarsening of the precipitates and dramatic loss in hardness of the alloy.     

Crystallographic laws governing the formation of the structure of pseudo-single crystals of some 3<Emphasis Type="Italic">d</Emphasis> metals and related alloys

Methods of metallography, X-ray diffraction, transmission electron microscopy, and dilatometry have been used to study the formation of the structure of pseudo-single crystals of nitrogen-bearing steel Kh18AG20 upon the δ → γ (bcc → fcc) transformation, in pseudo-single crystals of pure cobalt and binary alloy Co-29.7% Ni upon the β → α (fcc → hcp) transformation, and in pseudo-single crystals of zirconium upon the β → α (bcc → hcp) transformation. It has been established that the precipitation of austenite from ferrite during the δ → γ transformation in a single crystal of the Kh18AG20 steel occurs via a crystallographically ordered mechanism with the fulfillment of orientation relationships close to the Kurdjumov-Sachs orientation relationships. In the volume of the pseudo-single crystal there were realized six orientations of austenite with the retention at room temperature of a significant fraction of residual δ (α) ferrite. It has been shown that in the process of cooling of the β single crystals of cobalt, Co-29.7% Ni alloy, and zirconium to below the temperature of the β → α transition there formed several crystallographic orientations of the α phase that are grouped into packets. In each packet, there exist crystals of the α phase of one orientation. In accordance with the Wassermann orientation relationships, in the pseudo-single crystals of cobalt and Co-29.7% Ni alloy there are realized packets of four variants. In the pseudo-single crystal of zirconium, six variants of packets based on the Burgers orientation relationships are realized.     

Cyclic deformation mechanisms in precipitated NiTi shape memory alloys

Results are presented on the cyclic deformation of single crystal NiTi containing Ti₃Ni₄ precipitates of various sizes. Mechanical cycling experiments reveal that the cyclic degradation resistance of NiTi is strongly dependent on crystallographic orientation. Under compression, orientations approaching the [100] pole of the stereographic triangle possess the highest fatigue resistance. Orientations approaching the [111] pole of the stereographic triangle demonstrate the lowest fatigue resistance. Aging to produce small coherent Ti₃Ni₄ precipitates (10 nm) improves the fatigue resistance of NiTi compared to the other heat treatments (solutionized or overaged) for nearly all orientations. NiTi with 10 nm Ti₃Ni₄ precipitates consistently showed stabilized martensite due to mechanical cycling, and an absence of dislocation activity. Samples with large incoherent Ti₃Ni₄ precipitates (500 nm) consistently showed significant dislocation activity due to mechanical cycling in addition to stabilized martensite colonies. The first cycle stress–strain hysteresis was found to correlate to the fatigue resistance of the material. Samples demonstrating large inherent hysteresis, with different heat treatments and orientations, showed poor fatigue performance. Rational for the observed behaviors is discussed in terms of operant deformation mechanisms and ramifications on modeling the cyclic deformation of NiTi are presented.     

Orientation relations in aluminide coatings on single crystals of nickel superalloys

The crystallographic orientation of NiAl refractory coatings on the surface of single crystals of high-temperature nickel alloy ZhS32 is studied. The orientation relation between single-crystal substrates based on an fcc γ-phase and coatings based on a bcc β-phase is studied.     

单晶Nb拉伸,压缩及恒应变疲劳后的滑移系和滑移特征

研究了单滑移位向和多滑移位向Ｎｂ单晶在拉伸，压缩及循环变形时的滑移系，根据试样表面滑移线的方向，在极图上确定滑移面，单滑移位向（［３２１］）的单晶，在拉伸时的滑移系为（１０１）［１１１］，在压缩时是（１０１）［１１１］和（２１３）［１１１］．根据ｂｃｃ晶体变形特点，分析了拉伸，压缩和循环变形的滑移系之间的关系，用光学显微镜和透射电镜复型观察了试样的表面形貌，确定了多滑移位向（［１１０］位向）单晶滑