Flow stress behavior of porous FVS0812 aluminum alloy during hot-compression

The hot deformation behavior of porous FVS0812 aluminum alloy prepared by spray deposition was studied by means of compression tests on a Gleeble 1500 machine. The samples were hot compressed at temperatures ranging from 573 K to 773 K under various true strain rates of 10⁻⁴–10⁰ s⁻¹. The deformation behaviors are characterized by a significant strain hardening during hot-compression due to the progressive compaction of the pores with increasing compressive strain. A revised formula describing the relationships of the flow stress, strain rate and temperature of the porous alloy at elevated temperatures is proposed by compensation of strain. The theoretical predictions are compared with experimental results, which show good agreement.     

Fatigue and damage tolerance behaviour of corroded 2024 T351 aircraft aluminum alloy

The fatigue and damage tolerance behaviour of pre-corroded 2024 T351 aluminum alloy specimens has been investigated and compared to the behaviour of the uncorroded material. The experimental investigation was performed on specimens pre-corroded in exfoliation corrosion environment and included the derivation of S–N and fatigue crack growth curves as well as measurements of fracture toughness. The fatigue crack growth tests were performed for different stress ratios R. To obtain reference material behaviour all mechanical tests were repeated under the same conditions for uncorroded specimens. For the corroded material an appreciable decrease in fatigue resistance and damage tolerance was obtained. The results of the experimental investigation were discussed under the viewpoint of corrosion and corrosion-induced hydrogen embrittlement of the 2024 aluminum alloy. The need to account for the influence of pre-existing corrosion on the material’s properties in fatigue and damage tolerance analyses of components involving corroded areas was demonstrated.     

Complex investigation of deformation twinning in γ-TiAl by TEM and neutron diffraction

A near-γ TiAl based alloy with 2 at% of Nb was investigated by means of collaborative research based on transmission electron microscopy and in-situ neutron diffraction techniques with the aim to study mechanical twinning and its role within the mechanisms governing fatigue response and material properties. In-situ neutron diffraction measurements were performed during low cycle fatigue straining at room temperature. Induced lattice strain related to the formation of deformation twins was detected and used to follow changes in the macroscopic material response caused by the twinning process during cycling. A microscopic insight was realised by using several transmission electron microscopy techniques to reveal in detail an internal deformation microstructure of the material at the beginning as well as at the end of the fatigue life. The study was focused on the first loading cycles where the material shows intense cyclic hardening. The effect of mechanical twinning on the material behaviour at several stages of the fatigue life is discussed for two different total strain amplitudes of 0.2% and 0.4%.     

纳米碳管/铝基复合材料的制备及摩擦磨损性能研究

利用粉末冶金法制备纳米碳管/铝基复合材料,研究不同纳米碳管含量对复合材料硬度和稳态摩擦磨损行为的影响,采用扫描电子显微镜观察复合材料的磨损表面形貌,并对其磨损机制进行探讨.结果表明:随着纳米碳管质量分数的增加,复合材料的硬度呈现先增大而后减小的趋势,含质量分数为2%的纳米碳管复合材料硬度比铝增加约80%;复合材料的摩擦系数逐渐降低,磨损率先减小而后增大;含质量分数为1%的纳米碳管复合材料磨损机制为磨粒磨损和粘着磨损,而含质量分数为2%的纳米碳管复合材料以剥层磨损和疲劳磨损为主.     

经不同表面改性处理的钛合金的微动疲劳和微动磨损行为对比研究

对比研究了钛合金微动疲劳(FF)和微动磨损(FW)失效行为,考察了表面喷丸强化和氮化等表面处理对钛合金微动疲劳和微动磨损性能的影响,探讨了钛合金微动磨损和微动疲劳性能的相关性.结果表明,钛合金微动疲劳和微动磨损损伤表面形貌特征相似;当微动位移幅较大、微动区发生整体滑动时,微动接触区磨损有利于延缓微动疲劳裂纹萌生;而在小位移幅、部分滑移情况下,局部磨损促进微动疲劳裂纹萌生.利用喷丸强化在钛合金表面引入残余压应力,可以在降低摩擦系数的同时,提高钛合金抗微动疲劳和微动磨损失效的能力;氮化处理后钛合金表面硬度提高,有利于改善其微动磨损性能,但表面韧性降低导致抗微动疲劳能力降低.因此,在提高表面硬度的同时,不应忽视表层韧性的降低对钛合金微动疲劳性能的不利影响.     

Characterization of mechanical properties of aluminum cast alloy at elevated temperature

The tensile response, the low cycle fatigue(LCF) resistance, and the creep behavior of an aluminum(Al) cast alloy are studied at ambient and elevated temperatures.A non-contact real-time optical extensometer based on the digital image correlation(DIC)is developed to achieve strain measurements without damage to the specimen. The optical extensometer is validated and used to monitor dynamic strains during the mechanical experiments. Results show that Young's modulus of the cast alloy decreases with the increasing temperature, and the percentage elongation to fracture at 100℃ is the lowest over the temperature range evaluated from 25℃ to 300℃. In the LCF test, the fatigue strength coefficient decreases, whereas the fatigue strength exponent increases with the rising temperature. The fatigue ductility coefficient and exponent reach maximum values at 100℃. As expected, the resistance to creep decreases with the increasing temperature and changes from 200℃ to 300℃.     

Fatigue characteristics of ‘bead-on-plate’ of aluminum and Al---Zn---Mg alloy

Fatigue life of ‘bead-on-plate’ on commercially pure aluminum and Al---Zn---Mg alloy have been investigated. In both cases, the fatigue life of ‘Bead-on-Plate’ specimen was drastically reduced. But when the same weld-bead was flushed off, a marginal improvement in fatigue life was observed. For Al---Zn---Mg alloy, the post weld heat treatment was less effective than flushing of weld-bead for improving the fatigue life of welded specimens. Results are analyzed with the help of S-N diagrams and microphotographs.     

Coupled annealing temperature and layer thickness effect on strengthening mechanisms of Ti/Ni multilayer thin films

A systematic study was performed on mechanical and microstructural properties of Ti/Ni multilayers with layer thickness from 200 nm to 6 nm and annealing temperature from room temperature to 500 °C. Based on the observed hardness evolution, a coupled layer-thickness and annealing-temperature dependent strengthening mechanism map is proposed. For as-deposited films, the deformation behavior follows the traditional trend of dislocation mediated strengthening to grain boundary mediated softening with decreasing layer thickness. For annealed films, grain boundary relaxation is considered to be the initial strengthening mechanism with higher activation temperature required for thicker layers. Under further annealing, solid solution hardening, intermetallic precipitation hardening, and fully intermixed alloy structure continue to strengthen the thin layered films, while recrystallization and grain-growth lead to the eventual softening of thick layered films. For the films with intermediate layer thickness, a strong orientation dependent hardness behavior is exhibited under high temperature annealing due to mechanism switch from grain growth softening to intermetallic precipitation hardening when changing the loading orientation from perpendicular to parallel to the layer interfaces.     

Plastic behavior of a nickel-based alloy under monotonic-tension and low-cycle-fatigue loading

The plastic behavior of an annealed HASTELLOY^® C-22HS™ alloy, a face-centered cubic (FCC), nickel-based superalloy, was examined by in-situ neutron-diffraction measurements at room temperature. Both monotonic-tension and low-cycle-fatigue experiments were conducted. Monotonic-tension straining and cyclic-loading deformation were studied as a function of stress. The plastic behavior during deformation is discussed in light of the relationship between the stress and dislocation-density evolution. The calculated dislocation-density evolution within the alloy reflects the strain hardening and cyclic hardening/softening. Experimentally determined lattice strains are compared to verify the hardening mechanism at selected stress levels for tension and cyclic loadings. Combined with calculations of the dislocation densities, the neutron-diffraction experiments provide direct information about the strain and cyclic hardening of the alloy.     

合金含量对高速车轮材料滚动接触磨损性能的影响

将2种含碳量相同合金含量不同的高速车轮材料分别与钢轨材料匹配,利用滚动接触摩擦磨损试验机测试了各摩擦副的摩擦系数和磨损率,比较研究了组织、硬度和加工硬化等因素对车轮材料滚动接触磨损性能的影响.结果表明：在传统的高速车轮材料中适当地增加Si、Mn的含量,降低Cr的含量可以提高车轮材料的抗磨损性能;硬度高的车轮材料未必耐磨,组织差异对车轮材料的抗磨损性能影响显著;表面裂纹易萌生于高度变形的先共析铁素体组织;加工硬化引起的硬度增加对材料的抗磨损性能影响不大.