SMALL CRACK GROWTH AND FATIGUE LIFE PREDICTIONS FOR HIGH-STRENGTH ALUMINIUM ALLOYS: PART I—EXPERIMENTAL AND FRACTURE MECHANICS ANALYSIS

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests were conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plasticity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions. Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminium alloys was developed and demonstrated using the crack-closure model.     

Initial Magnetization Curve and Hardening Mechanism in the Nanocomposite Nd8Fe85Nb1B6 Ribbon

The melt-spun Nd8Fe85Nb1B6 ribbon was prepared by the single roller method with the tangential speed of 20 m/s. A mixture of Nd2Fe14B and α-Fe phases with the average crystalline grain size of about 20 nm was found to exist in the as-quenched ribbons. The initial magnetization curve of the nanocomposite Nd8Fe85Nb1B6 ribbon can be divided into four sections by the inflection points on it. The magnetically hardening mechanism corresponding to each section was investigated. The initial susceptibility of the Nd8Fe85Nb1B6 ribbon is higher than that of the Nd15Fe85B9 powder, which may be attributed to the reversible magnetization rotation in the central region not influenced by the exchange-coupling effect within the α-Fe grains.The above-mentioned magnetization rotation leads to the formation of equilibrium 180 deg. domain walls at the boundaries of the α-Fe grains. With the increase of applied field, these domain walls are compressed reversibly towards the Nd2Fe14B grains and eventually invade into them.The irreversible movement of the domain walls in the Nd2Fe14B grains accounts for the steepest growth of magnetization with the applied field. Finally, the magnetically inhomogeneous “core regions” are formed in the Nd2Fe14B grains, and the magnetization rotation in these“core regions”indicates the end of the whole initial saturation process.     

高温合金中钨元素的看谱分析技术应用研究

对高温合金中钨元素的看谱分析技术进行了研究,摄制并标定了高温合金中钨元素的彩色图谱,制定了半定量分析测定用强度标。研究结果可用于高温合金中钨元素的定性和半定量分析及材料牌号鉴别。     

Al-6Mg-0.2Sc铝合金高温力学行为研究

研究了Al-Mg-Sc超塑合金在200～500℃的高温力学性能,并对实验残样进行了微观组织分析.结果表明,Al-Mg-Sc合金在250～500℃区间内均具有一定的超塑性;在300～400℃超塑成形时强度较低,成形后制件使用性能较好,工程化条件下可选择使用;合金高温断裂机制由穿晶断裂向沿晶断裂转变的温度点在300℃附近,在此温度以下晶粒强度低于晶界强度,在此温度以上晶粒强度高于晶界强度.     

高效防霉聚硫密封剂的研究

选用了噻唑类、有机金属络合物、咪唑类3种防霉剂应用于聚硫密封剂,考察了聚硫密封剂的防霉效果,并研究了不同防霉剂对密封剂工艺性能和力学性能的影响.结果表明,聚硫密封剂中添加噻唑类防霉剂综合性能最好.     

(Fe0.5Ni0.5)80-xMoxB20非晶态合金的玻璃形成能力与热稳定性

采用XRD,TEM,DSC研究了(Fe0.5Ni0.5)80-xMoxB20 (x＝0, 2, 4, 6, 8)非晶态合金的玻璃形成能力和热稳定性,讨论了非晶态合金玻璃形成能力的表征,探讨了Mo对玻璃形成能力和热稳定性的作用机理.结果表明: Mo的加入可提高(Fe0.5Ni0.5)80B20合金的玻璃形成能力和热稳定性;Tx/Tm可用于表征(Fe0.5Ni0.5)80-xMoxB20非晶态合金的玻璃形成能力;合金组元数目增多、原子尺寸比增大及负混合热是Mo提高玻璃形成能力的主要原因.     

乙炔炭黑填充导电硅橡胶的研究

对乙炔炭黑填充硅橡胶的导电性能进行研究.实验结果表明:在甲基乙烯基硅橡胶中,随着乙炔炭黑用量增加,试样的体积电阻率呈降低趋势;白炭黑对硅橡胶导电性能有较大不良影响;经过热处理后,硅橡胶的导电性能有明显改善.     

Influence of surface integrity on fatigue strength of 40CrNi2Si2MoVA steel

Influence of surface integrity (including surface roughness, residual stresses, and microstructure in surface) on fatigue limit of 40CrNi2Si2MoVA steel specimens is investigated comprehensively in this work according to a systematic consideration. The surface integrity of specimens is changed due to several widely used manufacturing procedures: heat-treatment, grinding, electro-polishing, hard chromium plating and shot peening. In comparison with specimen electro-polished after grinding, the specimen without polishing has 10% lower fatigue limit due to higher surface roughness; while shot peening improves the fatigue limit for about 36% due to inducing of compressive residual stress field in the surface and transferring the fatigue crack source from surface to interior. The fatigue limit of specimen with decarburized layer after grinding is lower about 13%, but the shot peening can eliminate its detrimental effect. Hard chromium plating decreases the fatigue limit dramatically. The shot peening carried before plating can improve the fatigue limit of specimen and cause it to get to a level even higher than that of specimen without plating.     

NiCoCrAlYHf涂层与一种Ni基单晶高温合金循环氧化行为研究

在第三代Ni基单晶高温合金上制备NiCoCrAlYHf(HY5)高温抗氧化涂层,采用扫描电镜(SEM),能谱分析(EDS)和X射线衍射(XRD)等手段,对涂层表面及界面进行组织形貌观察和相结构分析,研究涂层试样的循环氧化动力学和元素互扩散行为。结果表明:采用真空电弧镀(AIP)方法制备的HY5涂层,提高了单晶高温合金的循环抗氧化能力;在1100℃大气环境中扩散600h后,涂覆HY5涂层的单晶高温合金基体中形成了互扩散区IDZ,互扩散主要由Cr,Ni元素从基体向高温抗氧化涂层的外扩散以及Al元素从高温抗氧化涂层向基体的内扩散构成;在涂层下方60μm左右有拓扑密堆相TCP析出,并形成二次反应区SRZ。     

某种钛合金精密成形铸件铸造变形的数值模拟

目的 基于ProCAST，建立钛合金铸件铸造变形的模拟预测方法。方法 以某板状钛合金铸件为例，模拟了充型凝固、型壳内冷却和脱壳后冷却3个过程，并分别对各过程进行了相应假设和参数设置。为验证模拟结果，根据模拟模型设计了浇注验证实验。结果 铸件中间部位向外侧凸起，加强筋部位向内侧凹陷，和实验结果基本一致，变形量预测吻合度在60%~72%之间。结论 通过合理设置模拟流程和材料参数模型，数值模拟可以预测钛合金铸件的变形规律，并为变形量预测提供重要参考。