渗注磷酸盐对AZ31镁合金疲劳裂纹扩展的影响

交变载荷作用下在AZ31镁合金疲劳裂纹尖端渗注磷酸盐转化液,研究磷酸盐的沉积行为及其对AZ31镁合金疲劳裂纹扩展速率的影响。采用扫描电镜(SEM)、能谱仪(EDS)、X射线衍射仪(XRD)观察分析裂纹尖端的形貌和物相组成,并采用贴应变片方法确定渗注磷酸盐转化液前后应力强度因子的变化。结果表明:渗注的磷酸盐转化液在AZ31镁合金疲劳裂纹尖端形成Zn3(PO4).4H2O及MgZnP2O7复合覆层,能改变疲劳裂纹尖端的应力大小和分布状态,使应力强度因子降低约30%,从而有效地增强疲劳裂纹闭合效应,降低或延滞AZ31镁合金疲劳裂纹的扩展。     

AZ31镁合金裂纹扩展各向异性声发射特征分析

利用声发射技术对AZ31镁合金轧制方向和横向的疲劳裂纹扩展行为进行了研究。结果表明,镁合金疲劳裂纹扩展过程中产生的声信号撞击数与循环载荷的关系,以及撞击数上升率和应力强度因子幅的关系d C/d N-ΔK,分别与常规疲劳裂纹扩展试验结果相一致,裂纹失稳扩展临界应力强度因子幅与常规试验结果的误差分别为2.86%(裂纹沿轧制方向)和3.00%(裂纹沿横向);载荷一定时,裂纹沿横向扩展总是比沿着轧制方向扩展更慢一些,进入失稳扩展阶段更迟。微观组织显示,裂纹沿横向扩展边缘处的孪晶明显增加,断口处也表现出更明显的塑性,这对裂纹的扩展产生了阻碍,即材料在横向的抗裂性能要优于轧制方向,同时证明了利用声发射监测裂纹扩展行为的可靠性。     

AZ31B镁合金及其焊接接头的疲劳断裂机理

对AZ31B镁合金进行疲劳实验,在2×106循环次数下,母材、对接接头、横向十字接头和侧面连接接头的疲劳强度分别为66.72,39.00,24.38和24.40MPa。采用光学显微镜对裂纹扩展特征进行分析,结果表明,AZ31B母材的疲劳裂纹宏观扩展路径平滑,但微观观察发现疲劳裂纹扩展方向曲弯,有些裂纹分成两岔;裂纹尖端扩展均为沿晶扩展。焊接接头裂纹均在焊趾部位起裂,对接接头和横线十字接头的裂纹沿着热影响区扩展;侧面连接接头的裂纹起裂位于焊脚部位。采用扫描电子显微镜对疲劳断裂机理进行分析。疲劳断口由准解理或解理台阶组成,均为脆性断裂,断口中存在二次裂纹,对接接头中存在疲劳条纹,其间距约为5μm。     

AISI321不锈钢单周过载疲劳裂纹扩展的延迟效应

在恒△Ｋ控制条件下研究了ＡＩＳＩ３２１不锈钢在空气中不同过载比下的单周过载疲劳裂纹扩展延迟效应。结果表明：过载比越大，延迟效应越显著，延迟效应作用距离远大过载塑性区尺寸。讨论了裂纹过载延迟效应机理，认为ＡＩＳＩ３２１不锈钢过载后的疲劳裂纹扩展延迟行为主由塑性诱发裂纹闭合引起，并对其进行了定量描术。     

AZ31镁合金搅拌摩擦焊接头腐蚀疲劳裂纹扩展行为

文中研究了AZ31镁合金搅拌摩擦焊接头的腐蚀疲劳裂纹扩展行为。结果表明,在1% NaCl溶液(质量分数)中,AZ31镁合金接头各区域的腐蚀疲劳裂纹扩展门槛值和强度均低于在空气环境中的结果,但裂纹扩展速度较高。在空气环境下,在裂纹扩展起始阶段,母材区域门槛值最低,裂纹最容易扩展,而在裂纹稳定扩展区域,焊核区域的裂纹扩展速率最快,为da/dN = 7.80×10-6(ΔK)2.78,前进侧热力影响区的裂纹扩展速率最慢,为da/dN = 1.94×10-5(ΔK)1.73。在腐蚀疲劳环境下,母材区域的门槛值最小,最容易发生扩展行为,进入稳定扩展阶段时,后退侧热力影响区扩展最快,为da/dN =3.12×10-4(ΔK)1.71,焊核区域扩展最慢,为da/dN = 2.78×10-4(ΔK)1.50。AZ31镁合金搅拌摩擦焊接头各区域的腐蚀疲劳裂纹扩展机理主要是裂纹尖端的阳极溶解和氢脆机理。裂纹扩展路径曲折复杂,裂纹中部发现裂纹“闭环”和分叉形貌,裂纹尾部细小,出现裂纹“跳跃”情况,裂纹尖端附近的施密特因子分布不均匀,裂纹尖端附近的组织整体呈现较软的取向,裂纹总是沿着易于滑移的软取向组织扩展。     

环境对AISI321不锈钢疲劳裂纹扩展过载效应的影响

研究了3．5％NaCl溶液对恒应力强度因子范围△K控制下AISI321不锈钢疲劳裂纹扩展过载行为的影响．与在空气中的结果比较表明，两种条件下存在相似的过载延缓效应，但是环境极大地削弱了过载对疲劳裂纹扩展的延缓效应，削弱程度随过载比不同而变化，以“损失循环数”表征约为40％-70％，以“过载影响区尺寸”表征则为30％-40％．分析表明，用“损失循环数”表征过载对裂纹扩展的延缓效应不够充分，无法区分环境对裂纹闭合的削弱和增强作用，“过载影响区尺寸”可以区分环境对过载塑性形变增强裂纹闭合的综合影响．对断口的SEM分析发现，过载造成的塑性形变在溶液中比在空气中受到更多的磨损，表明环境对过载延缓效应的削弱除了对裂尖的影响外，还包括对过载导致的塑性诱发裂纹闭合的影响．     

AZ91HP镁合金疲劳裂纹扩展行为与断裂机理

试验分析了AZ91HP镁合金在压铸态(F)、固溶状态(T₄)和固溶-时效状态(T₆)的裂纹扩展行为,并探讨了压铸态镁合金疲劳断口组织特征与裂纹扩展机制。结果表明:压铸态AZ91HP-F疲劳裂纹扩展行为与ΔK值的大小有关,ΔK值越小,断口越平滑,ΔK值越大,断口越粗糙;裂纹扩展过程存在塑性诱发裂纹闭合效应,这一效应使得裂纹扩展的K_op值增加,ΔK_eff值减小,并使疲劳裂纹扩展速率与合金塑性有关,由于AZ91HP的塑性依T₆、F、T₄状态依次增加,导致裂纹扩展速率依次降低。     

疲劳裂纹扩展的非比例过载效应SCIEI

本工作研究了比例(Ⅰ型)过载及非比例(复合型)过载对常幅Ⅰ型疲劳裂纹扩展的影响。结果表明:Ⅰ型过载所引起的裂纹滞后扩展效应远较复合型为强。因此,在估算疲劳寿命时将零、构件实际承受的过载(大都为非比例过载)简单地当作比例过载处理,可能会过高地估计寿命,带来不安全因素。文章从Ⅰ型及复合型过载所引起的裂纹钝化、裂端塑性区的形状、尺寸及裂纹闭合等方面讨论了它们对裂纹扩展滞后效应的影响。     

AZ31B镁合金焊接接头的疲劳裂纹扩展行为(英文)

对AZ31B 镁合金焊接接头和热影响区的疲劳裂纹扩展行为进行研究,分析了焊接接头[L-T(W)]和热影响区的紧凑拉伸试验[C(T)],其中热影响区的C(T)试验包括焊缝平行于挤压方向[T-L(H)] 和垂直于挤压方向 [L-T(H)]两种。结果表明:对于L-T(W) 试样,裂纹沿挤压方向扩展,裂纹扩展经历先快后慢的扩展过程;T-L(H) 试样裂纹平行于缺口方向扩展,L-T(H)试样裂纹为平行于缺口方向和与缺口成一定角度两种扩展方向,裂纹扩展经历先慢后快的扩展过程。裂纹尖端扩展为穿晶和沿晶的混合模式,疲劳断口为准解理特征的脆性断口。     

淬火速率对砂铸Al-7Si-0.4Mg合金拉伸性能及疲劳裂纹扩展行为的影响

研究了淬火速率对峰值时效状态下的砂铸Al-7Si-0.4Mg合金拉伸性能及疲劳裂纹扩展行为的影响.结果表明,随着淬火速率的提高,合金的抗拉强度、屈服强度以及α(Al)相的显微硬度均有提高,抗拉强度和屈服强度与淬火速率的对数值近似成线性关系.淬水速率对合金的延伸率影响较小,但对合金的室温疲劳裂纹扩展速率有明显影响,较高的淬火速率可以明显提高合金的疲劳裂纹扩展阻力.     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

Fatigue crack propagation behavior of AZ31 Mg alloy in ultra-high vacuum

The fatigue crack propagation (FCP) behavior of extruded AZ31-F was examined in conjunction with crack closure phenomena. FCP experiments were carried out in an ultra-high vacuum system using a fatigue test machine under a constant load amplitude. To determine the effect of crack closure on the FCP rates, the crack closure levels were measured using the compliance method. The δK_th values of the AZ31-F decreased with increasing load ratio. It was found that ΔK_eff,th showed an almost constant value regardless of the load ratio.     

Nd在挤压态AZ31镁合金中的行为及作用

通过金相显微镜、SEM和XRD观察研究挤压态AZ31-xNd镁合金的微观组织和析出相,并测试合金的室温和高温力学性能。结果表明:Nd在合金中以Al2Nd和Mg12Nd化合物形式存在,且随着Nd量的增加,其数量增加;Nd使合金的晶粒细化、室温和高温性能提高。加入0.6%Nd的合金晶粒尺寸由未加Nd时的26μm降至约10μm,加入0.6%Nd合金的室温抗拉强度、屈服强度和伸长率分别为325MPa、247MPa和18.1%。含Nd合金的抗拉强度和屈服强度随温度升高而下降,而伸长率随温度的升高而增加。     

Fatigue crack growth behaviour of pulsed current gas tungsten arc, friction stir and laser beam welded AZ31B magnesium alloy joints

The laser beam welded joints offered better resistance against the growing fatigue cracks compared to friction stir welded and pulsed current gas tungsten arc welded AZ31B magnesium alloy joints. The formation of very fine grains in weld region, higher fusion zone hardness, uniformly distributed fine precipitates and favourable residual stress field of the weld region are the main reasons for superior fatigue performance of laser beam welded joints of AZ31B magnesium alloy.     

Shear bands in magnesium alloy AZ31

During deformation of magnesium at low temperatures, cracks always develop at shear bands. The origin of the shear bands is the { 1011 } twinning in basal-oriented grains and the mobility of this type of twin boundary is rather low. The most frequent deformation mechanisms in magnesium at low temperature are basal slip and { 1012 } twinning, all leading to the basal texture and therefore the formation of shear bands with subsequent fracture. The investigation on the influences of initial textures and grain sizes reveals that a strong prismatic initial texture of (0001) parallels to TD and fine grains of less than 5 8m can restrict the formation and expansion of shear bands effectively and therefore improve the mechanical properties and formability of magnesium.     

Anticorrosive magnesium phosphate coating on AZ31 magnesium alloy

A novel anticorrosive film with a thickness of approximately 50 μm was successfully coated on an AZ31 magnesium alloy by chemical and low-heat treatments (50 °C). The film was a single-phase system of newberyite (MgHPO₄•3H₂O) having an orthorhombic crystal structure. The corrosion current density of the newberyite film coated on the AZ31 magnesium alloy decreased by more than two orders of magnitude as compared to that of the AZ31 magnesium alloy. The static water contact angle of the newberyite film was less than 10°. The average value of the scratch critical load for the newberyite coating was estimated to be approximately 15 mN.     

AZ31镁合金的织构对其力学性能的影响

利用电子背散射衍射(EBSD)取向成像技术,分析AZ31镁合金热挤压棒材和轧制薄板的织构特点;对具有不同初始织构的镁合金棒材和薄板进行力学性能分析,并从织构角度分析棒材的拉压不对称性和薄板的力学各向异性。结果表明:挤压镁合金棒材具有主要以(0001)基面平行于挤压方向的基面纤维织构,存在严重的拉压不对称性,其原因在于压缩时的主要变形方式为{1012}1011孪生;热轧镁合金薄板具有主要以(0001)基面平行于轧面的强板织构,具有显著的力学性能各向异性,其原因在于拉伸时不同方向的基面滑移Schmid因子不同。