In situ observation on fatigue crack growth in SCS-6/Ti-15-3 composite at elevated temperatures

Direct observation on fatigue crack growth behavior in SiC (SCS-6) fiber-reinforced Ti-15-3 alloy matrix composite subjected to a constant tension–tension loading mode was performed by scanning electron microscope using a single edge-notched specimen in vacuum at room temperature and 550 °C. The fatigue crack growth rate at 550 °C was lower than that at room temperature, and the difference between the fatigue crack growth rates at room temperature and 550 °C increased with increasing fatigue cycles. The crack opening displacement at 550 °C was smaller than that at room temperature when the crack length exceeded a definite value, though the interface friction stress between the fiber and matrix at elevated temperature was much smaller than that at room temperature. The above results were explained qualitatively by a residual stress mechanism at the crack front and the crack closure behavior at crack wake, which could be produced by matrix creep asymmetry in tension and compression at elevated temperature during each fatigue cycle.     

The fatigue response and fracture behavior of a spray atomized and deposited aluminum-silicon alloy

In this article the results of a recent study designed to improve our understanding of the cyclic fatigue and fracture characteristics of a spray atomized and deposited hypereutectic aluminum-silicon alloy are presented. Specimens of the alloy were cyclically deformed to failure at ambient temperature under fully reversed total strain amplitude controlled tension-compression loading. The alloy exhibited low cyclic plasticity and fatigue life under total strain amplitude controlled deformation. Cyclic stress amplitude controlled high-cycle fatigue characteristics were established at an elevated temperature (150 °C). The cyclic stress response, high-cycle fatigue life and fracture characteristics of the alloy are compared with a conventional ingot metallurgy processed counterpart and discussed in light of intrinsic microstructural features, nature and magnitude of stress, and ductility of the material.     

High temperature high cycle fatigue behavior of new aluminum alloy strengthened by (Co,Ni)3Al4 particles

High cycle fatigue (HCF) behavior of a new heat-resistant aluminum alloy at elevated temperature was investigated. This alloy consists of an α-Al matrix, a small amount of precipitated Mg₂Si, and distributed (Co, Ni)₃Al₄ strengthening particles. HCF tests were conducted with a stress ratio of (R)=0 and a frequency of (F)=30 Hz at 130 °C. The fatigue limit (maximum stress) of this alloy was 120 MPa at 10⁷ cycles. This is a value superior to that of conventional heat-resistant aluminum alloys such as the A319 alloy. Furthermore, regardless of the stress conditions, the new heat-resistant Al alloy has an outstanding fatigue life at high temperatures. The results of fractography observation showed that second phases, especially (Co, Ni)₃Al₄ particles, were effective to the resistance of fatigue crack initiation and propagation. On the other hand, Mg₂Si particles were more easily fractured by the fatigue crack. This study also clarifies the micromechanism of fatigue deformation behavior at elevated temperature related to its microstructure.     

Effect of strain rate on the mechanical response of a Ti3Al-Nb-Mo alloy

In this study, the effect of strain rate and temperature on the substructure evolution and mechanical response of a Ti₃Al-Nb-Mo alloy (Ti-24.5Al-10.5Nb-l.5Mo) was investigated. The yield and flow stresses were found to decrease gradually with increasing temperature at the strain rate of 3000 s¹ and no flow stress/temperature anomaly was observed. The variation of hardening behavior of Ti-24.5Al-10.5Nb-1.5Mo alloy with temperature and strain rate was found to be negligible, implying that the dynamic recovery mechanism for this alloy is rather athermal. Dynamic recovery of Ti-24.5Al-10.5Nb-l.5Mo alloy is thought to be quite difficult and slow because of its complicated dislocation core structure. Ti-24.5Al-10.5Nb-l.5Mo alloy exhibited predominantly planar dislocation debris after deformation. Following the room temperature deformation at low strain rate the majority of the dislocations were a-dislocations lying on the basal and 1storder pyramidal slip planes and the 2nd-order pyramidala/2+c slip on {1211}. Increasing the rate of deformation at room temperature to 6000 s¹ resulted in the increaseda-slip on prism planes and the lessor amount of basal slip. At elevated temperatures, the substructure after high rate deformation was found to be similar to that following high rate deformation at room temperature except for an increased amount of basal slip and somewhat higher incidence of the 2nd-order pyramidal slip. The evidence of an increased amount of shearing of the β_o phase was observed at elevated temperature, suggesting the decreased strength of the β_o phase.     

含缺口TC21钛合金腐蚀疲劳性能分析

研究了TC21钛合金缺口试样在两种腐蚀环境（油箱积水、3.5%NaCl水溶液）与室温空气环境下的疲劳性能与断裂机理。并与光滑试样在室温空气环境下疲劳性能进行对比。结果表明,室温空气环境下,当两种试样疲劳寿命均达到5×105次循环时,缺口试样的循环应力值较光滑试样下降了52.7%;相同环境下随着应力水平降低,试样疲劳寿命增加;相同应力条件下,3.5% NaCl水溶液环境下试样疲劳寿命最低,油箱积水环境下次之,室温空气中TC21钛合金试样疲劳寿命最高;当应力较低时,差异更为显著。在腐蚀环境下,溶液中离子与金属原子发生电化学反应,加速了裂纹的萌生与扩展,3.5% NaCl水溶液中离子浓度较大,电化学反应更为剧烈     

Effects of Thermally Assisted Warm Laser Shock Processing on the Microstructure and Fatigue Property of IN718 Superalloy

The effects of laser shock processing (LSP) and warm laser shock processing (WLSP) on the microstructure of surface hardening layer and high-cycle fatigue performance at room temperature and high temperature (600 ℃) of IN718 alloy were investigated.It has been revealed that the grain refined hardening layer with greater residual compression stresses,higher fraction of coincidence site lattice (CSL) boundaries and dislocation densities was formed in WLSP-treated alloy than in LSP-treated alloys.Moreover,microtwins included γ" phase/high density dislocation complex was found in the surface of WLSP-treated alloy.These characters caused the significant enhancement of the medium value fatigue strength of WLSP-treated alloy at room temperature and elevated temperature.Apparently,the microtwins included y" phase/high density dislocation complex formed in the surface hardening layer of LSP-treated alloy has more complicated steric structure and more stable at elevated temperature than γ" phase/low density dislocation complex formed in LSP-treated alloy,leading to the slow recovery process.Therefore,the surface hardening layer in the WLSP-treated alloy remained more ideal strengthen-ing effect under high-cycle fatigue at elevated temperature than that in LSP-treated alloy.This resulted in the much longer fatigue crack initiation incubation and longer high-cycle life of WLSP-treated IN718 alloy under cycling load at 600 ℃.This discovery provides a new cognition of fatigue resistance by WLSP treatment of precipitation strengthening superalloy.     

TB6钛合金疲劳及裂纹扩展性能研究

本文对TB6钛合金锻件弦向和径向两种取样方向分别进行了室温和200℃下旋转弯曲高周疲劳、轴向低周疲劳和疲劳裂纹扩展性能试验研究。试验结果表明,弦向（C）和径向（R）两种取样方向对该合金锻件的旋转弯曲高周疲劳、轴向低周疲劳性能和疲劳裂纹扩展性能没有影响;温度升高可加速该合金锻件的疲劳裂纹萌生,但在裂纹扩展阶段,该合金高温下的韧性优势与屈服强度降低的劣势平衡的结果使其在室温～200℃温度范围内的疲劳性能基本不受温度的影响;在10—20mm的厚度范围内,厚度对该合金的疲劳裂纹扩展性能没有影响;在3．5％NaCl盐雾环境中。腐蚀介质对TB6钛合金的疲劳裂纹扩展速率在初始阶段有迟滞作用,但在应力强度因子范围大于14MPa m后有加速作用。     

Effect of pre-strain on fatigue crack growth of 2E 12 aluminum alloy

The influences of pre-strain on the mechanical property and fatigue crack growth of 2E12 aluminum alloy were evaluated by SEM, TEM, mechanical property and fatigue tests. The axial fatigue tests were conducted under a constant amplitude sinusoidal wave loading at stress ratio of 0.1 in laboratory air and salt fog at room temperature. The results show that the yield stress of pre strain material is higher than that of the material without undergoing pre-strain, but pre-strain can not make the increase of the growth rate of fatigue crack. Fatigue crack growth rates of the alloy in salt fog are higher than those in air. The increased fatigue crack growth of the alloy in a given environment and more brittle striations can be observed in salt fog.     

Influence of cubic boron nitride grinding on the fatigue strengths of carbon steels and a nickel-base superalloy

The influence of cubic boron nitride (CBN) grinding on fatigue strength was investigated on an annealed carbon steel, a quenched and tempered carbon steel at room temperature, and a nickel-base superalloy, Inconel 718, at room temperature and 500 °C. The results were discussed from several viewpoints, including surface roughness, residual stress, and work hardening or softening due to CBN grinding. The fatigue strength increased upon CBN grinding at room temperature, primarily because of the generation of compressive residual stress in the surface region. However, in the case of Inconel 718, this marked increase in the fatigue strength tended to disappear at the elevated temperature due to the release of compressive residual stress and the decrease of crack growth resistance at an elevated temperature.     

TC21合金应力控制和应变控制的低周疲劳行为

研究TC21合金应变控制和应力控制的低周疲劳行为.实验温度为室温,循环应变比和应力比均为0.1,载荷波形为三角波.结果表明,在应变疲劳的最初阶段,TC21合金循环拉应力时快速软化,循环压应力时快速硬化,随着循环进行软化和硬化速度降低.在整个循环阶段,软化速度与应变有关;背应力影响较小,摩擦应力一直在变化,循环应力的变化与摩擦应力有关.应力控制的低周疲劳结果表明,TC21合金循环蠕变明显,循环蠕变与应力大小有关,摩擦应力是影响循环蠕变的主要因素.     

压铸稀土镁合金高周疲劳试验研究

采用升降法对AZ91D、AZ91D 1?和AZ91D 2?压铸镁合金室温高周疲劳行为进行了试验研究。结果表明:适量稀土Ce的添加能够使压铸镁合金AZ91D的室温拉伸性能和疲劳强度得到提高;利用升降法计算出3种成分合金在应力比R=0.1、循环基数为107下的条件疲劳极限分别为96.7 MPa1、16.3 MPa和105.5 MPa,相当于其抗拉强度的46%左右;合金的疲劳断口呈现出准解理与韧窝断裂的混合特征。     

2A97铝锂合金的疲劳裂纹萌生及早期扩展行为(英文)

研究2A97铝锂合金的疲劳裂纹萌生及早期扩展行为。在室温条件下,采用光滑试样进行疲劳测试,其中最大应力为恒定值,应力比R为0.1,频率f为40 Hz。利用金相显微镜、透射电镜、扫描电镜及电子背散射衍射等手段对合金的微观组织进行分析,研究合金的疲劳裂纹萌生及早期扩展行为与其微观组织的关系。结果表明:2A97合金的疲劳裂纹主要萌生于试样表面的杂质相和粗大第二相处;其疲劳裂纹的早期扩展行为主要受晶粒结构与位错或滑移带共同作用的影响。当相邻晶粒的错配度接近于其晶内的最优滑移面的位向差时,大角度晶界强烈阻碍滑移带的运动,从而导致裂纹分叉和偏折。     

Tensile response of an Fe–40Al–0.7C–0.5B alloy

The uniaxial tensile properties of a B2 Fe–40Al based alloy containing 0.7 at.% carbon and 0.5 at.% boron have been obtained at room and elevated temperatures in air as a function of strain rate and compared to the response of a similar alloy without boron (i.e Fe–40Al–0.6C). Furthermore, tests were also conducted as a function of strain rate in oxygen at room temperature to elucidate the effect of environment on these properties. Both alloys contain a dispersion of a perovskite carbide within the grains and at grain boundaries. In addition, the quaternary alloy contains a fine dispersion of metastable borides and complex faults within the grains. Yield stress and tensile elongation appear strain rate insensitive (in the regime tested) for the boron-containing alloy at room temperature in oxygen, whereas some loss in elongation is encountered at the slowest rate in the ternary alloy; in all cases, fracture path is intergranular. In air, at room temperature, a strong strain-rate dependence of elongation is recognized in both alloys, their response being identical at the faster rates but diverging at the slow rates. A strong correlation is noted between the increase in percent transgranular cleavage and the drop in tensile elongation. The brittle-to-ductile transition temperature is sensitive to strain rate, shifting to higher temperatures rapidly with increasing strain rate. A variety of fracture paths is encountered depending on test temperature and strain rate.     

Structure and properties of ultra-fine grain Cu–Cr–Zr alloy produced by equal-channel angular pressing

The structure, thermal stability and properties are investigated of a Cu–Cr–Zr alloy with ultra fine grains (UFG) of 160 nm diameter produced by severe plastic deformation through equal-channel angular pressing (ECAP). Special attention is paid to optimization of multi-functional thermal, electrical and mechanical properties of this alloy by aging after ECAP. Fatigue life and cyclic response under strain-controlled experiments are investigated aiming at clarification of mechanisms of plastic deformation and fracture in the precipitation hardened ECAP materials. It is shown that the precipitation strengthened UFG structure remains stable both under elevated temperatures as high as 500°C and under cyclic loading at room temperature. Substantial improvement of fatigue life is evidenced in comparison with conventional coarse-grain materials. The appearance of cyclic softening is noticed and its nature is discussed in terms of dislocation–particle interaction and possible dissolution of precipitates during fatigue.     

温度对N18(NZ2)合金循环变形行为的影响

研究了200、300、400℃温度下N18(NZ2)合金的循环变形行为,同时研究了室温和400℃低周疲劳行为.结果表明:N18(NZ2)合金低周疲劳寿命Nf随着塑性应变范围△εp的增加而降低,并遵循Coffin-Manson关系:Nβf△εp=C;N18(NZ2)合金在400℃高温下,其循环滞后回线出现锯齿状波形,即出现Portevin-LeChatelier效应.且在200、300、400 ℃下,合金表现出与常温下不同的循环特性,呈现出一致循环硬化的现象;在高温,N18(NZ2)合金疲劳断口局部出现韧窝型断裂,并出现细小的二次裂纹.大量二次裂纹的存在是400℃疲劳断口的主要特征.     

ZK60镁合金板材成形性能试验

对ZK60镁合金板材在室温和高温条件下进行杯突试验。试验结果表明,室温下ZK60镁合金板材杯突值IE=4.5mm,在370℃条件下材料的杯突值IE=16.5mm,胀形高度增加了266.7%;ZK60镁合金板材高温胀形的破裂,是由冲头附近板材沿切向方向的裂纹扩展和沿厚度方向缩颈所产生的断裂造成。     

Superelastic cycling and room temperature recovery of Ti74Nb26 shape memory alloy

The superelastic cyclic response of Ti₇₄Nb₂₆ shape memory alloy (SMA), and the nature of cyclic evolution of its superelastic properties and their unexpected static recovery process after cycling, were investigated at room temperature. The critical stress for stress-induced martensitic transformation (σ_SIM) and stress hysteresis (Δσ) continuously decrease with increasing number of superelastic cycles. However, cumulative irrecoverable strain during cycling in samples of particular processing conditions increases only up to a certain number of cycles before decreasing with further cycling. Stress-free aging at room temperature after cycling was shown to increase σ_SIM and Δσ. The unexpected room temperature recovery is attributed to the recovery of retained martensite and point defects. Similar experiments conducted on conventional Ni-rich Ni–Ti SMAs also show static recovery at room temperature, indicating that the recovery process is not unique to Ti–Nb SMAs.     

Cold Deep Drawing of Commercial Magnesium Alloy Sheets

A cold deep drawing process for commercial AZ31 magnesium alloy sheets was developed. The commercial sheets were successfully formed into circular cups at room temperature by optimising the annealing temperature of the sheets, i.e. a limiting drawing ratio of 1.75 was attained for an annealing temperature of 500 °C. The increases in elongation, n-value and r-value, and the decrease in flow stress effective in the improvement of drawability were obtained for the annealing. The apparatus for cold deep drawing without heating becomes much simpler than that for the conventional warm deep drawing. The effects of the lubricant, the clearance between the die and the punch and the corner radius of the punch on the drawability were examined. The limiting drawing ratio was increased by applying force onto the edge of a blank through the die corner. In addition, cold deep drawing of magnesium alloy square cups was performed. It was found that comparatively shallow magnesium alloy cups are satisfactorily formed at room temperature without heating.     

选区激光熔化制备Ti-6Al-4V合金的热处理工艺及力学性能

采用选区激光熔化技术(SLM)制备Ti-6Al-4V合金圆棒试样,通过不同的热处理工艺改善材料的拉伸性能,并对SLM制备的Ti-6Al-4V合金试样开展了高周疲劳性能测试。通过微观组织和疲劳试样断口分析,揭示了显微组织结构与拉伸性能的关系,以及Ti-6Al-4V合金的疲劳裂纹起始源和裂纹扩展机理。结果表明,热处理工艺对SLM成型Ti-6Al-4V合金的力学性能有显著的影响,920 ℃×1 h水冷,随后800 ℃×2 h炉冷的固溶时效热处理制度可以获得较好的综合室温拉伸性能。其室温组织为晶界上分布的α相和晶粒内部片层状分布的α+β相。SLM成型Ti-6Al-4V合金显微组织中的晶界形成与扫描路径相关,热处理过程中α相会优先在扫描分区搭接处析出。与手册锻件的疲劳寿命曲线比较,在同样的最大应力水平下,增材试样的疲劳寿命比锻件的疲劳寿命低,这种降低的趋势随着应力水平的降低而逐步增大。在400 MPa的应力水平下(R=-1),锻件的疲劳寿命已经在2×10⁷水平,增材试样的疲劳寿命依然较低,约为锻件的1%。SLM成型Ti-6Al-4V合金的应力疲劳寿命偏低,是由于试样中存在未熔合缺陷造成。扫描分区搭接处易产生未熔合缺陷,而疲劳裂纹也会沿着这些缺陷扩展。     

Effect of Cu addition on overaging behaviour,room and high temperature tensile and fatigue properties of A357 alloy

The aims of the present work are to evaluate the overaging behaviour of the investigated Cu-enriched alloy and to assess its mechanical behaviour, in terms of the tensile and fatigue strength, at room temperature and at 200 °C, and to correlate the mechanical performance with its microstructure, in particular with the secondary dendrite arm spacing (SDAS). The mechanical tests carried out on the overaged alloy at 200 °C indicate that the addition of about 1.3 wt.% Cu to the A357 alloy enables to maintain ultimate tensile strength and yield strength values close to 210 and 200 MPa, respectively, and fatigue strength at about 100 MPa. Compared to the quaternary (Al−Si−Cu−Mg) alloy C355, the A357−Cu alloy has greater mechanical properties at room temperature and comparable mechanical behaviour in the overaged condition at 200 °C. The microstructural analyses highlight that SDAS affects the mechanical behaviour of the peak-aged A357−Cu alloy at room temperature, while its influence is negligible on the tensile and fatigue properties of the overaged alloy at 200 °C.