滚珠丝杠副的接触疲劳行为

通过显微硬度、化学成分、扫描电镜和能谱仪等方法对接触疲劳试验样品进行了分析。结果表明,试样经过一定周次的循环加载,滚道表面出现点蚀、微裂纹和剥落等现象。疲劳裂纹主要从滚道表面萌生,并与表面呈一定角度向内部扩展。材料内部的非金属夹杂物对裂纹的萌生和扩展发生影响。次表面疲劳裂纹优先在Al₂O₃·MgO颗粒周边萌生和扩展,并没有在TiN颗粒周围萌生。     

应力比对Mg-3Al-2Sc合金疲劳行为的影响

在不同应力比条件下对热挤压变形的Mg-3Al-2Sc合金进行了疲劳试验,分析了应力比对合金裂纹扩展行为和断裂机制的影响。结果表明:在应力幅Δσ=60 MPa时,随应力比的增加,Mg-3Al-2Sc合金疲劳寿命显著降低,裂纹扩展速率提高。在裂纹萌生区和扩展区疲劳断裂机制主要表现为解理断裂,而在瞬断区主要表现为准解理和沿晶断裂的复合断裂机制。     

TiC／Ti复合材料动态拉伸的裂纹形成及扩展机制

采用熔铸法制备的原位自生TiC／Ti复合材料，在SEM中静载动态拉伸，原位观察和研究了裂纹的萌生及扩展机制。结果表明，TiC颗粒表面及应力集中处最容易萌生微裂纹；在不同位置萌生的微裂纹中，处于有利位向的微裂纹不断扩展，并与周围的裂纹连接形成主裂纹；主裂纹扩展主要是通过自身扩展与周围的裂纹连接相结合的方式进行，当裂纹扩展受阻时，裂纹前方颗粒处形成新的裂纹或基体中形成塑性坑，并通过扩展相互连接；裂纹扩展到一定程度后，试样全面失稳而迅速断裂。根据试验观察与分析建立了裂纹萌生与扩展模型。     

TWIP钢的低周疲劳断裂机制

通过采用扫描电镜及透射电镜等手段,观察并研究了TWIP钢在低周单轴循环对称拉压载荷下的疲劳断裂后的显微组织。结果表明:TWIP钢矩形试样的疲劳裂纹一般萌生于角部,从表面萌生时可能表现为多个疲劳源。在低周疲劳变形过程中,TWIP钢不但产生了形变孪晶,还产生了大量的微条带,其实质为细微孪晶片层和驻留滑移带。疲劳裂纹主要萌生于微条带对晶界和孪晶界的撞击引发的孔洞。孔洞串连接起来成为裂纹,夹杂物促进了裂纹扩展。随着裂纹的扩展,试样的承载面积不断减小,最终发生快速的韧性断裂。     

Cr-Mn-Si-Ni高强度钢疲劳裂纹的萌生行为

通过扫描电镜原位疲劳试验,研究Cr-Mn-Si-Ni高强度钢疲劳裂纹的萌生行为,观察并记录从滑移带自然萌生的小裂纹和其发展到宏观裂纹的过程。研究结果表明：在低周疲劳情况下,不是所有萌生的裂纹都会持续扩展,大部分微裂纹在疲劳早期即已萌生,但是裂纹发展很慢,导致失效的裂纹快速扩展在疲劳晚期发生,微裂纹长度与循环数之间也有一定的关系。     

38CrMoAlA钢表面氮化多冲疲劳性能研究

研究了38CrMoAlA钢表面氮化多冲疲劳裂纹萌生寿命、裂纹断裂寿命及裂纹扩展速率。结果表明:在低冲击能量状态下氮化对裂纹萌生影响不显著;而高冲击能量状态下氮化显著提高裂纹的萌生寿命和断裂寿命。冲击能量和氮化对裂纹扩展速率影响不显著,在相同的冲击能量和应力强度因子范围ΔK下,氮化试样的裂纹扩展速率低于未氮化试样的裂纹扩展速率。     

复相化钻杆在循环应力下断裂行为研究

采用疲劳试验、扫描电镜动态拉伸装置和透射电镜,研究了复相化钻杆的疲劳强度和在循环应力作用下裂纹萌生、扩展的特点,并分析讨论了显微组织对钻杆疲劳断裂行为的影响.研究结果表明：下贝氏体组织或下贝氏体/马氏体界面可使钻杆的显微断裂模式由沿晶＋穿晶断裂转变为穿晶断裂,增加了裂纹转折和二次裂纹,提高了材料的裂纹扩展抗力.在钻杆材料中引入适当比例的下贝氏体＋马氏体复相组织,明显增长了裂纹萌生周期,可使钻杆在机械性能相近情况下具有优于回火索氏体组织的疲劳性能.     

驻留时间和加载速率对无铅焊点低周疲劳行为的影响

在25 ℃下利用单轴微力疲劳试验机对96.5Sn-3Ag-0.5Cu无铅焊点进行不同驻留时间(1~20 s)和不同应变速率(0.01~0.08 mm/s)条件下的低周疲劳试验. 结果表明,在25 ℃下1~20 s的驻留时间对焊点的疲劳寿命影响不大;随着应变速率的加快,焊点的疲劳寿命逐渐降低,断裂机制逐渐由延性断裂向脆性断裂转变. 不同应变速率条件下的疲劳裂纹主要在焊点边缘钎料与金属间化合物(IMC)之间的界面处萌生,并在近IMC层的钎料内扩展. 焊点断口主要分为:裂纹扩展区和最终断裂区.     

钢结硬质合金GJW50热疲劳开裂的探讨

对钢结硬质合金GJW50试样进行了冷热循环实验,观察了试样在热应力作用下热疲劳裂纹的萌生、扩展以及试样开裂的全过程。结果表明:热疲劳裂纹出现前,在试样的光滑缺口边缘上产生了明显的塑性变形,呈现出凹凸不平。在试样缺口顶端的凹坑内萌生首条裂纹,萌生地是小颗粒的WC粒子集聚区或大颗粒的WC"自裂纹"。萌生的首条裂纹沿着与热循环方向平行的方向扩展,最终成为主裂纹。其扩展途径主要为沿WC聚集区和钢基体相的界面扩展以及在大面积的WC粒子聚集区内扩展。主裂纹遇到钢基体相后受阻,裂纹尖端钝化、转向,寻找耗能少的区域扩展。主裂纹在扩展时形成二次裂纹,但未形成明显上的龟裂;最终,仍然是主裂纹导致试样断裂。     

喷射沉积Al-Si/SiCp制动盘材料的热疲劳微裂纹扩展行为

采用V形缺口试样,研究喷射沉积Al-Si/SiCp复合材料制动盘在25(450 ℃热循环下的热疲劳行为.通过金相显微镜和扫描电镜观察了复合材料的组织和热疲劳裂纹形貌,研究热疲劳裂纹形成与扩展机制.结果表明:热疲劳主裂纹主要从V形缺口处萌生;在同样的热循环次数下,热处理前的试样要比热处理后的试样先出现裂纹,且裂纹扩展的速率较快;裂纹绕过Si颗粒向前扩展以及裂纹穿过Si颗粒向前扩展是裂纹与Si颗粒相互作用的主要机制;SiC颗粒与热疲劳裂纹有明显的交互作用.因此,改善Si相的形态和分布以及加强Al/SiC颗粒间的界面结合有利于提高热疲劳裂纹扩展的抗力.     

加Ti方式对ZL101A合金疲劳裂纹扩展性能的影响

在工业电解槽上用不同的方式生产出电解Al-Si-Ti合金和电解低钛铝基合金,并以这两种合金分别制备ZL101A合金试样,在MTS810型材料试验机上测量疲劳裂纹扩展速率。采用拟合试验数据代表点的新方法处理数据,得到Paris公式中两个重要参数和疲劳裂纹扩展门槛值。对比分析表明,电解低钛铝基合金制备的ZL101A合金具有较好的抗疲劳裂纹扩展性能。微观分析说明,产生差异的原因在于电解Al-Si-Ti合金中含铁量较高。     

铜铝复合接触线用银铜合金疲劳试验

通过采用不同应变幅控制,对不同变形量冷拉银铜合金进行室温低周疲劳试验.结果表明,随着应变幅的增大,滞后环面积也随之增大;随着循环周次的增加,循环应力逐渐降低,从而产生疲劳软化;加工率为38%的银铜合金循环周期大于加工率为19%的银铜合金循环周期;试样断裂后存在三个明显疲劳特征区:裂纹源、裂纹扩展区、瞬断区;银铜合金的裂纹扩展具有穿晶和沿晶两种方式.     

原位自生TiB2/7050复合材料的高周疲劳断裂机制

TiB₂/7050铝基复合材料在航空发动机等领域具有重要的应用前景。本文研究了TiB₂颗粒质量分数为4%的原位自生7050铝基复合材料在T6热处理状态下的室温高周疲劳性能,利用扫描电子显微镜对复合材料的疲劳断裂机制进行分析。结果表明:在应力比R=?1、指定寿命为3×107周次时,TiB₂/7050铝基复合材料的疲劳强度为211.9 MPa,高于7050铝合金的疲劳强度;疲劳裂纹萌生源主要分布在近样品表面的夹杂、大尺寸的TiB₂颗粒及显微孔洞等区域;疲劳裂纹的扩展在遇到TiB₂颗粒带时,疲劳条带的宽度会明显减小,即TiB₂颗粒提高了复合材料的抗疲劳裂纹扩展能力,使得复合材料具有高的疲劳寿命。     

Influence of Sr on microstructure and mechanical properties of ZL114 cast alloy

In the present work,the effects of trace element Sr on the microstructure,tensile properties,fractography and morphology of the eutectic Si of ZL114A (Al-7Si) cast alloy were investigated.The cast ZL114A aluminum alloy was prepared using a metal mold,and then heat treated with the T6 technique.The test results of mechanical properties show that modification element Sr can improve significantly the ultimate tensile strength,elongation and hardness of the ZL114A-T6 aluminum alloy.By adding 0.04％Sr,the values of the tensile strength Rm,elongation A and hardness HB increase from 230 MPa,1％ and 65 to the maximum of 305 MPa,8％,and 100,respectively.SEM analysis indicates that the near tensile fracture surface on longitudinal section of the ZL114A-T6 alloy reveals a transgranular fracture mode without Sr modification,while the tensile fracture is an intergranular fracture mode after Sr modification.The morphology of the eutectic Si phase is changed from acicular to fine fibrous.     

Fatigue crack growth and wear behaviors in rheocast Al−Si−Mg alloys

Rheocast aluminum alloys, which consist of globular α-Al cells, refined grains and eutectic Si particle, were used to investigate fatigue crack growth and wear characteristics. The Si particles were systematically varied from coarse and acicular shapes to small and globular ones. At low ΔK fatigue crack growth rates decreased in samples consisting of acicular Si particles of large grain size, which induced a large amount of crack closure. Large and acicular Si particles were easily cracked and separated the particle/matrix interface, which promoted to fracture at smaller ΔK. On the other hand, small Si particles made fatigue crack grow, even at a high ΔK region, and increased the fracture toughness of the alloy. However, in the wear test, small eutectic Si particles were pulled out by friction force during sliding wear and the wear loss amount increased with increase in sliding distance.     

A356合金原位拉伸裂纹萌生与扩展行为分析

采用扫描电镜原位拉伸观察了电解低钛A356合金铸态下裂纹萌生与扩展特征。结果表明:铸态A356合金裂纹优先萌生于二次枝晶臂间薄弱的共晶体区、大块的共晶硅相处;裂纹的扩展主要是微裂纹易沿着共晶体与基体界面扩展,合金组织中的共晶硅相对裂纹的扩展有一定的阻碍作用,当裂纹与共晶硅颗粒相遇时,扩展方向发生偏离而转向裂纹尖端前沿处有共晶体区开裂、共晶硅相发生断裂、或与基体界面发生分离的更薄弱区。     

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.     

GH4169D合金电子束焊接接头显微组织和持久断裂特征

采用金相显微镜和扫描电镜分析了GH4169D合金电子束焊接接头的显微组织,利用显微硬度计测试了母材、热影响区和焊缝的显微硬度,采用体式显微镜和扫描电镜研究了焊接接头持久断裂特征。结果表明,GH4169D母材中的主要析出相为1~20μm长的片层状晶界η相、30~80 nm的颗粒状γ′相和少量的碳氮化物。热影响区中的主要析出相为10~20 nm的颗粒状γ′相,几乎没有晶界η相。焊缝中为枝晶组织,枝晶间存在含有共晶组织的白色析出相,析出相尺寸为2~6μm,枝晶杆中含有10 nm以下的细小颗粒状γ′相。母材的显微硬度低于热影响区和焊缝,不同区域的显微硬度主要受γ′相尺寸的影响。焊接接头的持久断裂过程包括蠕变裂纹扩展、快速扩展和瞬断3个阶段,蠕变裂纹扩展区中为沿晶断裂,快速扩展区中为沿晶和穿晶混合断裂,瞬断区为穿晶断裂。蠕变裂纹扩展起始于试样表面的热影响区,热影响区中晶界η相含量低和裂纹尖端晶界氧化是导致焊接接头持久性能低于母材的主要原因。     

Corrosion Fatigue Behavior of 7A85 Aluminum Alloy Thick Plate in NaCl Solution

The S–N curves of 7A85-T7452 aluminum alloy in laboratory air and in neutral 3.5 wt% NaCl solution were obtained by axial fatigue tests. Results show that the detrimental effect of the aggressive solution was not noticeable at high-cyclic-stress regions, but the effect was significant at low-stress region. Corrosion fatigue mechanism was discussed by corrosion morphology analysis, fracture surface analysis and microstructure characterization. It was found that the corrosion fatigue crack commonly initialed at the localized intergranular corrosion site. TEM analysis showed that the microstructures of 7A85-T7452 aluminum alloy were characterized by fine and homogeneously distributed matrix precipitates, as well as continually distributed anodic grain boundary precipitates. The types of microstructures are the reason for its intergranular corrosion susceptibility. The corrosion fatigue process of 7A85 aluminum alloy in 3.5 wt% NaCl solution can be divided into four stages: the crack initiation stage, the stable growth stage with low and high growth rate and the final rupture stage. The sodium chloride solution mainly affected the crack initiation stage and the stable growth stage with low growth rate, and when the crack growth rate reached a threshold, the effect was reduced.     

铸造ZL101A／SiCp复合材料的研究

采用真空搅拌复合工艺制备了铸造ZL101A／SiC复合材料,研究了变质和细化处理对复合材料组织的影响。结果表明：变质和细化处理铸造 ZL101A／SiC复合材料制备工艺的重要处理措施,可明显改善复合材料的组织。利用透射电镜对AL／SiC界面特征及界面反应进行分析,同时对该复合材料的铸造性能（熔体合金流动性能、线收缩、体收缩和热裂倾向）以及力学和物理性能进行了测试。