2024-T3和2524-T34铝合金疲劳裂纹的萌生机制

通过2024-T3和新型2524-T34铝合金的疲劳实验和对试样表面及疲劳断口的观测,研究了材料的微观结构和疲劳裂纹萌生机制.实验在室温下完成,应力比为0.1、加载频率为15 Hz.结果表明:实验材料呈现了再结晶的层状晶粒结构,晶粒沿着轧制方向被拉长,并较为平坦.2024铝合金中二相粒子的分布更为密集无序,且粗大、不规...     

粉末冶金铝合金的疲劳裂纹扩展行为

疲劳性能是粉末冶金铝合金一项重要的使用性能,研究疲劳裂纹扩展行为是研究疲劳性能的一种重要的方法。总结了影响粉末冶金铝合金疲劳裂纹扩展速率的各种外部因素和内部因素,外部因素主要包括应力比、温度、制备方法等,内部因素有晶粒尺寸、夹杂物、铝基复合材料中的增强相颗粒等,并详细阐述了这些因素的影响机制。     

WE94镁合金长寿命疲劳小裂纹萌生和扩展行为研究

镁合金作为结构材料在汽车、航空、航天等领域有广泛的应用前景,在实际服役过程中存在高频振动荷载的长期作用。为解决镁合金结构件在长寿命服役条件下的安全与可靠性问题,基于旋转弯曲和超声振动加载两种疲劳实验方法,研究WE94镁合金在高周与超高周区间（104～109周次）的疲劳强度与失效机制。结果表明,疲劳裂纹萌生于沿基面的滑移带并在断裂面上形成解理状“小平面”形貌;而在疲劳小裂纹扩展初期,裂纹面存在大量细小平行状条带,该条带于裂纹尖端与孪晶带的交互作用过程中形成。因此,镁合金疲劳小裂纹的萌生与扩展过程对材料局部微结构形态与变形机制表现出密切的相关性。     

LD10铝合金疲劳裂纹扩展速率的研究

用三点弯曲多试样方法研究了LD10铝合金的疲劳裂纹扩展速率规律,从而为装备的安全可靠性提供数据依据,利用这些数据可以评估在役装备的剩余寿命。     

Ni3Al合金热/机械疲劳裂纹扩展行为研究

对ＩＣ１０定向凝固高温合金进行了４５０￣９９０℃热／机械疲劳裂纹扩展行为的试验研究。分别研究了相位角、保持时间、温度、频率对ＩＣ１０合金裂纹扩展行为的影响,并建立了热／机械疲劳裂纹扩展速率预测模型。研究发现：温度的升高和频率的降低均会加速裂纹扩展;同相位热／机械疲劳裂纹扩展速率大于反相位热／机械疲劳裂纹扩展速率,而且它们两者介于最大温度和最小温度的等温度裂纹扩展速率之间;载荷保持加速裂纹扩展。     

低碳马氏体／贝氏体复合组织的疲劳裂纹萌生行为EI

本文用四点弯曲加载法测定了２０ＣｒＭｎＭｏ钢矩形缺口试样的条件疲劳裂纹萌生寿命，并用微观分析研究了疲劳裂纹的萌生行为。试验结果表明，通过短时等温处理，在低碳马氏体组织中引入２０～３０％贝氏体，可显著提高缺口试样的疲劳萌生寿命。复合组织中的贝氏体虽易于滑移变形引发裂纹，但在存在较大应力集中条件下（ｋｔ≥２），因可同时形成较多的非扩展裂纹，能有效缓和应力集中，从而有利于延长扩展裂纹的形成寿命。     

热交变载荷下10%Cr耐热钢蠕变疲劳裂纹萌生特征

为调节新能源间歇式发电模式所带来的弊端,需要使用火力发电厂调峰稳定电网波动。调峰过程中机组频繁启停,加剧高温部件的疲劳蠕变损伤。通过分析现有寿命模型描述温度交变载荷下汽轮机转子钢性能的不足,提出预载荷实验方案。且以预载荷实验数据为基础,优化现有寿命预测模型。通过模拟交变温度下的临近工况实验,对比应力应变关系和疲劳蠕变寿命,对所优化的寿命模型进行了评估。     

腐蚀环境下2E12航空铝合金疲劳裂纹扩展行为研究

采用SEM,TEM及疲劳性能测试等分析测试手段,系统地研究了2E12铝合金在室温空气、潮湿气体及盐雾环境下的疲劳裂纹扩展速率。结果发现2E12合金具有良好的疲劳性能,疲劳裂纹扩展速率优于国外的2524铝合金。利用修正的Paris公式分析腐蚀介质对合金的疲劳裂纹扩展性能影响轻重程度依次为;室温空气〈潮湿气体〈盐雾环境。不同环境条件下合金的疲劳裂纹形貌均表现微观裂纹扩展的晶界小平面、宏观裂纹扩展的疲劳条纹以及剪切撕裂的微观结构。在腐蚀环境下,宏观裂纹扩展表现出更多的脆性裂纹扩展特征,氢脆导致塑性区脆化及腐蚀诱导的阳极溶解是导致裂纹扩展性能降低的原因。     

颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和扩展行为的影响

通过原位扫描电子显微镜(SEM)研究了粉末冶金制备的Cu/WCp复合材料的疲劳裂纹萌生和扩展行为,分析了颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和早期扩展行为的影响。结果表明:疲劳微裂纹萌生于WCp颗粒和基体Cu的界面;微裂纹之间相互连接并形成主裂纹,当主裂纹和颗粒相遇时裂纹沿着颗粒界面扩展。在低应力强度因子幅ΔK区域疲劳小裂纹具有明显的"异常现象",并占据了全寿命的71%左右。疲劳小裂纹的早期扩展阶段易受局部微观结构和颗粒WCp的影响,扩展速率波动性较大,随机性较强;当小裂纹长度超过150μm时,裂纹扩展加快直至试样快速断裂。裂纹偏折、分叉和塑性尾迹降低了疲劳裂纹扩展速率,而颗粒界面脱粘则提高了复合材料的疲劳裂纹扩展速率。通过数值模拟也可以发现颗粒脱粘增大了材料的疲劳扩展驱动力,从而提高了疲劳裂纹扩展速率。     

基于损伤力学的疲劳裂纹萌生及扩展规律研究

针对金属构件疲劳裂纹的萌生、扩展寿命及其规律等问题,应用损伤力学理论与有限元相结合的方法,建立了计算疲劳裂纹全寿命的统一模型。引入附加载荷法,通过MATLAB编程计算,实现了对刚度矩阵的连续计算,并给出了编程的流程。通过对单个单元的损伤计算,得到了单元从无损到破坏过程中等效应力的变化;通过计算各个构件损伤单元寿命,进而给出了金属构件总体疲劳寿命。分析得到了微裂纹萌生及扩展寿命占总体疲劳寿命的80%以上,并应用有限元软件ANSYS模拟给出了缺口件裂纹萌生及扩展过程。理论计算的结果与试验数据对比基本一致,验证了本工作方法的准确性。     

Influence of the annealing and defects on the VHCF behavior of an SLM AlSi10Mg alloy

In the paper, the influence of the annealing temperature on the Very High Cycle Fatigue (VHCF) of AlSi10Mg specimens produced through selective laser melting (SLM) is experimentally assessed. VHCF tests at 20 kHz are carried out on Gaussian specimens subjected to a heat treatment suggested by the system supplier (heating for 2 hours to 320°C and air cooling) and to a heat treatment proposed by the authors (heating for 2 hours to 244°C and air cooling). The defects originating failure and the conditional P‐S‐N curves are compared. Experimental results show that an annealing temperature of 320 ° C induces the spheroidization of the Si network, which enhances the ductility but has a negative effect on the VHCF response. On the contrary, by reducing the heating temperature to 244 ° C , the original as‐built microstructure is not altered and the minimization of the residual stresses permits to enhance the VHCF response.     

Very-high-cycle fatigue crack initiation and propagation behaviours of magnesium alloy ZK60

The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.     

选区激光熔化AlSi10Mg合金不同温度原位拉伸变形行为及断裂机理研究

为了提高选区激光熔化AlSi10Mg合金在航空航天领域的应用,基于自主研发的原位SEM高温拉伸台,本文对比分析了原位拉伸非校准样品的选区激光熔化AlSi10Mg合金在室温、200、300 ℃条件下的力学性能与显微组织动态演化,并总结了断裂机理。结果表明,选区激光熔化AlSi10Mg合金的显微结构由α-Al基体、共晶Si和大量的气孔组成,且共晶Si呈连续网格状均匀分布在α-Al基体上。随着温度的升高,选区激光熔化AlSi10Mg合金的强度降低。屈服强度从室温的207 MPa降低到300 ℃时的52 MPa,极限抗拉强度从室温的304 MPa降低到300 ℃时的71 MPa,延伸率则随温度的升高而增大,从室温的7.4%增大到300 ℃时的59.5%。室温拉伸过程中试样并未出现明显的颈缩现象,而是随着温度的升高,试样的颈缩现象逐渐明显,表明试样经历了更加充分的塑性变形, 并且随着温度的升高,试样的断裂位置越来越偏离标距段中心。通过对试样变形行为的研究发现,200 ℃时,变形主要集中在晶内,发生晶内滑移;而300 ℃时滑移主要集中在晶界,导致晶界滑移。由于试样表面及内部存在大量缺陷,因此,室温下选区激光熔化AlSi10Mg合金的断裂机理为熔池边界的组织突变结合孔洞连通造成的准解理断裂。随着温度的升高,由于初始孔洞边缘的应力集中产生新的孔洞形核,新形核的孔洞与相邻孔洞相连通,导致试样的最终断裂。     

Crack initiation and propagation characteristics of a dual-phase Mg–Li alloy under high-cycle and very-high-cycle fatigue regimes

Ultralight Mg–Li alloys are promising aerospace materials as they are the lightest structural alloys at present; however, their fatigue behaviors remain to be explored. This work focuses on the fatigue strength and crack initiation behavior of an extruded dual-phase Mg–Li alloy (LZ91) under high-cycle and very-high-cycle fatigue regimes. The fatigue limit of LZ91 alloy at 10⁹ cycles was determined to be 78 MPa, and the fatigue ratio is 0.46. Microstructural characterization demonstrates that fatigue cracks tend to initiate at β-Li phase-enriched regions. The α-Mg phase presents a < > fiber texture with a basal plane that has low deformation in the extrusion direction and acts as an enhanced phase in relation to the β-Li phase. Deformation discrepancies cause localized cyclic plasticity at the Li phase that leads to fatigue crack initiation.     

退火态激光选区熔化成形AlSi10Mg合金组织与力学性能

AlSi10Mg合金具有高比强度、高耐磨性等优良特点。由于其成分接近共晶点,成形性能良好,被广泛应用于激光选区熔化技术。然而其热处理制度仍然沿用传统铸态合金的热处理规范,影响了其性能的充分发挥。本工作采用激光选区熔化技术制备了AlSi10Mg合金,并研究了沉积态和后续热处理过程中组织演化规律及其对室温力学性能的影响机制。研究发现：沉积态组织由沿沉积方向生长的α-Al柱状枝晶及枝晶间网状Al-Si共晶组成,具有强烈的〈100〉方向织构,沉积层由三部分组成,分别是细晶区、粗晶区及热影响区,抗拉强度389.5 MPa,伸长率4%。退火过程中,共晶Si破碎、球化,基体中过饱和Si不断析出长大。当退火温度从200 ℃提高到500 ℃时,Si颗粒发生Ostwald熟化,平均尺寸增长了23倍。经过300 ℃和500 ℃退火处理后,试样抗拉强度分别为287.0 MPa和268.0 MPa,但伸长率分别提高到10.3%和17.2%。     

Solute cluster size effect on the fatigue crack propagation resistance of an underaged Al–Cu–Mg alloy

The effect of Cu–Mg cluster size and number density on the fatigue fracture behavior of Al–Cu–Mg alloy with various aging conditions was investigated by means of transmission electron microscopy (TEM), atom probe tomography (APT), scanning electron microscopy (SEM) and fatigue testing. Results showed that the fatigue crack propagation (FCP) resistances of 170 °C/1 h and 170 °C/8 h samples were higher than that of 170 °C/0.5 h sample due to increased number density of great size Cu–Mg co-clusters (>50 atoms). These large clusters were harder to dissolve during cycle deformation, thus reduced the cyclic softening effect and enhanced the FCP resistance. Moreover, as aging prolonged, the critical shear stress (τ_m) of co-clusters by modulus hardening increased from 10.2 (MPa) in 170 °C/0.5 h sample to 12.4 in 170 °C/1 h sample and 12.1 in 170 °C/8 h sample. Thus the force required for the movement of dislocations impeded by co-clusters, as well as the resistance of FCP caused by co-clusters, in 170 °C/1 h and 170 °C/8 h sample was higher than that in 170 °C/0.5 h sample. The 170 °C/8 h sample possessed the lower FCP resistance than 170 °C/1 h sample because of the existence of S′ phase. S′ phase was a kind of semi-coherent unshearable precipitate and hence reduced the planar-reversible slip.     

In situ characterization of humidity effect on the fatigue damage evolution of a cast aluminium alloy

In this paper, the feedback signal of ultrasonic fatigue system was used to deduce the accumulated fatigue damage in situ using the ultrasonic nonlinearity parameter. It was observed that, compared with the decrease in resonant frequency, the ultrasonic nonlinearity parameter shows a greater sensitivity to fatigue damage evolution (i.e. crack initiation and propagation). Ultrasonic nonlinearity parameters obtained from tests conducted under various environmental humidity levels were monitored and analysed. Through changes in the ultrasonic nonlinearity parameter, it was concluded that both of the fatigue crack initiation life and crack propagation life were reduced by increasing the humidity levels.     

Microstructural evolution and thermal fatigue resistance of grey cast iron

In the present work, thermal fatigue in grey cast iron has been investigated by means of a numerical and an experimental approach. Temperature gradients were generated within the material by means of a testing rig specifically designed for the experiments. The temperature gradients were responsible for the formation of severe stress fields that led to the failure of the specimens after a fairly low number of cycles. Crack growth was monitored during the tests, and the microstructure and hardness of samples were analysed after failure and compared with those of untested alloy. The repeated thermal cycles at peak temperatures of 600, 700, and 800°C led to important microstructural alterations of cast iron and to a drop in material hardness. The pearlite lamellae lost their original shape and became more fragmented. Oxygen‐rich regions surrounding the graphite flakes were produced by microgalvanic corrosion mechanism.     

The influence of modified intermetallics and Si particles on fatigue crack paths in a cast A356 Al alloy

Mechanical fatigue tests were conducted on uniaxial specimens machined from a cast A356-T6 aluminium alloy plate at total strain amplitudes ranging from 0.1 to 0.8% ( R = − 1). The cast alloy contains strontium-modified silicon particles (vol. fract. ~6%) within an Al–Si eutectic, dispersed α intermetallic particles, Al₁₅ (Fe,Mn)₃ Si₂ (vol. fract. ~1%), and an extremely low overall volume fraction of porosity (0.01%). During the initial stages of the fatigue process, we observed that a small semicircular fatigue crack propagated almost exclusively through the Al–1% Si dendrite cells. The small crack avoided the modified silicon particles in the Al–Si eutectic and only propagated along the α intermetallics if they were directly in line with the crack plane. These growth characteristics were observed up to a maximum stress intensity factor of ~ K trmax = 7.0 MPa m^1/2 (maximum plastic zone size of 96 μm). When the fatigue crack propagated with a maximum crack tip driving force above 7.0 MPa m^1/2 the larger fatigue crack tip process zone fractured an increased number of silicon particles and α intermetallics ahead of the crack tip, and the crack subsequently propagated preferentially through the damaged regions. As the crack tip driving force further increased, the area fraction of damaged α intermetallics and silicon particles on the fatigue fracture surfaces also increased. The final stage of failure (fast fracture) was observed to occur almost exclusively through the Al–Si eutectic regions and the α intermetallics.     

Al—Si—Cu—Mg系合金W（AlxSi4Mg5Cu4）相与Al2Cu相的显示与鉴别

研究了Ａｌ－Ｓｉ－Ｃｕ－Ｍｇ系中富铜的Ｗ（ＡｌｘＳｉ４Ｍｇ５Ｃｕ４）相与Ａｌ２Ｃｕ相的金相显示鉴别法，并用化学染色试剂彩色显示区分铸造铝硅铜镁系合金（α＋Ｓｉ＋Ａｌ２Ｃｕ＋Ｗ）四元共晶中的Ｗ相及Ａｌ２Ｃｕ相。