电沉积制备纳米镍的拉伸变形行为

为了系统研究纳米材料的超塑性变形特点,用电沉积方法制备了平均晶粒尺寸为70 nm的纳米镍.采用单向拉伸实验研究了其在室温和高温时的力学性能,并用透射电子显微镜TEM、扫描电子显微镜SEM和X射线能谱仪EDS观察分析了纳米镍变形前后的显微组织.实验结果表明:制备的纳米镍在室温时表现出的延伸率很低,但强度可达1000 MPa以上.当温度升高至450℃,应变速率为1.67×10-3 s-1时单向拉伸实验得到380%的延伸率,说明制备的纳米镍具有低温超塑性性能.实验过程中,材料内部的晶粒发生明显的长大与拉长.拉伸过程中形成的氧化物夹杂成为裂纹源,断口表现为沿晶断裂.     

工艺参数对7075铝合金板材时效成形性的影响

为评价7075铝合金板材的可时效成形性并掌握最佳的时效成形工艺参数,基于机械加载时效成形试验工装,开展了时效温度和时间对7075铝合金板材时效成形后构件力学性能和物理性能影响的试验研究.结果表明:7075铝合金板材的时效成形性与工艺参数密切相关,随着时效温度和时间的增大,板材的电导率呈升高趋势,而其拉伸性能则呈降低趋势;且合金的拉伸断裂方式与时效状态有关,过时效初期以沿晶韧窝和穿晶韧窝混合型断裂为主,随着过时效进行主要为韧窝断裂.综合考虑构件时效成形后拉伸性能和电导率等情况,合金最佳时效温度为180℃,且保温时间不宜长于16 h.     

3104铝合金板材不同方向拉伸性能及断裂特征的研究

对3104铝合金冷轧板沿轧制方向0°、45°和90°三个方向进行拉伸性能测试,观察断口形貌和显微组织,并分析粗大第二相分布与拉伸方向之间的关系。结果表明,3104铝合金冷轧板材的断口为切变型韧窝断裂特征,裂纹在粗大相自身断裂分离处或者在粗大相尖端处萌生,并在剪切力作用下,以微孔聚合的方式扩展。0°方向拉伸断口韧窝深度最大,延伸率最大;90°方向拉伸断口韧窝深度最小,延伸率最小;而45°方向拉伸断口韧窝深度居中,延伸率也居中。粗大第二相长轴方向和拉伸方向之间的夹角越小,延伸率越大。     

船板拉伸断口缺陷的成因及预防

针对公司生产的船板的拉伸断口出现分层以及严重的中心偏析等缺陷的现象,通过分析这些拉伸断口的特征从而对它们进行分类,分别阐述了形成异常拉伸断口的原因和采取的预防措施,及拉伸断口缺陷对船板的力学性能和使用寿命产生的影响。     

β-SiC纳米丝拉伸变形的分子动力学研究

采用Tersoff势对具有不同截面尺寸的β-SiC纳米丝的[001]向拉伸力学性能进行了分子动力学模拟,得到了纳米尺度下β-SiC纳米丝的应力-应变演化关系,研究了β-SiC纳米丝的力学性能与特征尺寸的关系.模拟结果表明,β-SiC纳米丝在常温下具有不同于宏观陶瓷材料的室温脆性,在断裂前发生了明显的塑性变形,塑性应变达到11%.截面尺寸对纳米丝的力学性能有显著的影响,截面尺寸越大,初始饧氏模量越大,抗拉强度越高.     

5083/6063异种铝合金焊接头的低温拉伸性能研究

采用拉伸试验机,研究了5083/6063异种铝合金对接焊焊接头低温拉伸力学性能,通过扫描电子显微镜,对拉伸断口的组织形貌进行了观察分析。研究结果表明:实验温度为298 K、113 K、77 K时,抗拉强度依次为129 MPa、175 MPa、220.5 MPa,抗拉强度随温度降低而增大,延伸率随温度降低有稍微提高。对低温断口的微观形貌分析,断口呈现韧窝形貌,断口分析为韧性断裂。     

取向电工钢的低温拉伸变形和断裂行为

测量了普通取向电工钢室温至300℃的拉仲性能并观察了拉伸断口形貌.对比分析和定量计算表明,随着拉伸温度的升高P原子在晶界偏聚的程度逐渐降低,使得断口形貌的沿晶特征减弱、解理特征增强.拉伸温度超过100℃以后沿晶断裂特征消失,基体的屈服强度逐渐低于解理强度,使得断口形貌解理特征减弱、韧窝特征增强,至190℃转变成完全的韧窝断口.在100℃-160℃范围内拉伸变形时位错滑移会拖曳柯垂尔气团随之迁移,造成了随温度升高延伸率下降,且屈服强度的降幅减缓.     

单层石墨烯薄膜拉伸变形的分子动力学模拟

采用Tersoff势对锯齿型(Zigzag)和扶手椅型(Armchair)单层石墨烯薄膜的单向拉伸力学性能进行了分子动力学模拟,得到薄膜应力-应变演化关系,初步研究了其拉伸破坏变形机制。模拟结果表明:驰豫后的二维石墨烯薄膜并非完美的平面结构,表面不完全平整,出现类似波纹状褶皱。模拟计算得到锯齿型和扶手椅型石墨烯薄膜的杨氏模量分别为1031.36GPa和1058.42GPa,与文献报道结果一致。石墨烯薄膜在拉伸载荷作用下,薄膜边缘六角元胞首先转变为五角元胞形成缺陷,随着应变增大缺陷增多,碳-碳键逐渐断裂,最终导致薄膜破坏。     

Strain distribution during tensile deformation of nanostructured aluminum samples

To optimize the mechanical properties, especially formability, post-process deformation by cold rolling in the range 5–50 % reduction was applied to aluminum sheets produced by accumulative roll bonding to an equivalent strain of 4.8. During tensile testing high resolution maps of the strain distribution over the tensile sample gage length were obtained in situ using a commercial ARAMIS system. Significant improvements in total elongation from 6 to 13.3 % and in post-UTS uniform elongation from zero to 4.4 % were observed when introducing a post-process deformation step and the observations were underpinned by the in situ observations of the evolution of strain distribution in the sample during tensile straining. The mechanisms responsible for the enhancement were discussed based on strain rate sensitivity measurements and microstructural observations.     

Internal stress behavior of the short ceramic fiber reinforced aluminum alloy under tensile deformation

The in situ measurement of phase stress under tensile deformation on an A6061 alloy reinforced with SiC whiskers (Al/SiCw MMC: Metal Matrix Composite) was performed using the X-ray diffraction technique. In order to raise a preciseness of measurements, we applied a profile fitting technique to separate the nearby located diffraction peak. Tensile deformation on elastic to plastic range was applied by four points bending device and discussed internal stress behavior in the short ceramic fiber reinforced MMC. Phase stress in Al matrix was increased linearly up to 2800×10⁻⁶ in strain and then saturated immediately. On the other hand phase stress in SiC whiskers shows an unstable stress behavior. It was decreased at first because of the Poisson's effect from Al matrix but reversed over 500×10⁻⁶ applied strain. The measured phase stress behavior in elastic region agreed with the calculations using micromechanics based on Eshelby/Mori–Tanaka model except for this unstable internal stress region. The macro stress behavior in plastic region was extremely small than that of the tensile test results. It supposed that the mechanism of strength is not so much the fiber reinforcing as the dispersion strengthening like the Orowan mechanism. Regarding the fatigue property, ΔK_th of the Al/SiC MMC, this was lower than that of the A6061 alloy. On the Al/SiCw MMC specimen, many micro void formations were observed around the fatigue crack tip even under the ΔK_th of A6061. It was considered that these were caused by the high gradient of residual stress on composite process and the unstable stress behavior in low ΔK region.     

Superplastic deformation of Al-Li-Cu-Mg alloy sheet

Al-2.5 Li-1.2 Cu-0.6 Mg-0.12 Zr (wt%) alloy sheet was cold-rolled, solution heat-treated for 20 min at 510° C, prestrained by 3% and superplastically deformed at 450 to 540° C at strain rates between 1×10^–4 and 2.8×10^–1 sec^–1. The maximum elongation obtained was 300%. Significant cavitation occurred above about 0.5 strain at a rate (void volume/unit strain) of 4% at 540° C and 6% at 500° C. The onset of cavitation coincided with a reduction in the room-temperature tensile properties after reheat-treatment. During annealing at 500 to 540° C, grain coarsening near the sheet surface was associated with magnesium and lithium depletion. Superplastic deformation produced a fine equiaxed microstructure by dynamic recrystallization.     

Tensile deformation behavior of spray-deposited FVS0812 heat-resistant aluminum alloy sheet at elevated temperatures

The tensile deformation behavior of spray deposited FVS0812 heat-resistant aluminum alloy sheet was studied by uniaxial tension tests at temperatures ranging from 250 °C to 450 °C and strain rates from 0.001 to 0.1 s^− 1. The associated fracture surfaces were examined by scanning electron microscopy (SEM). The results show that the degree of work-hardening increases with decreasing temperature, and exhibits a small decrease with increasing strain rate; the strain rate sensitivity exponent increases with increasing temperature. The flow stress increases with increasing strain rate but decreases with increasing temperature. The total elongations to fracture increase not only with increasing temperature, but also with increasing strain rate, which is in marked contrast with the normal inverse dependence of elongation on the strain rate exhibited by conventional aluminum alloy sheets. The SEM fracture analysis indicates that the dependence of elongation on the strain rate may be due to the presence of a transition from plastic instability at lower strain rates to stable deformation at higher strain rates for fine-grained materials produced by spray deposition.     

A study to evaluate and understand the response of aluminum alloy 2026 subjected to tensile deformation

The strain concentration factors were determined for aluminum alloy 2026 in the T3511 temper using multi-hole structural coupon specimens. Samples of the alloy were evaluated for both the 6.25 mm (0.25 in.) thick and 10 mm (0.4 in.) thick specimens and having widths of 50 mm (2 in.) and 100 mm (4 in.), respectively. For the case of the specimens that were 50 mm in width the mechanical tests were conducted for both the open hole and filled hole conditions and the corresponding strain concentration value was determined. Threaded fasteners having collars were used for the case of the filled hole specimens. The fasteners posses a shank diameter that was slightly larger than the nominal hole size in order to provide for some interference. The strain concentration values were evaluated at both the failure strain (ε_f) and the strain at maximum load (ε_max). The average strain concentration value was then used to predict the results for the stack-up tests.     

铝合金汽车板性能及其应用

深入讨论了预处理工艺的影响并说明了该工艺在保证铝合金汽车板使用性能的重要性。介绍了铝合金汽车板在汽车上的典型应用。基于笔者认识和目前的研究进展,提出了汽车用铝合金汽车板的重点研发内容,降低成本和价格,做好应用研究是扩大铝合金汽车板在汽车上应用的重要工作。     

Fracture limit prediction for roller hemming of aluminum alloy sheet

Roller hemming limit is predicted based on ductile fracture criterion in this approach. Plastic deformation in sheets made of aluminum alloy 6061-T6 is studied experimentally. Combined isotropic and kinematic hardening rule is considered in roller hemming numerical analysis. Forming limit stress curve at fracture (FLSCF) is derived from Cockcroft–Latham ductile damage criterion to determine fracture during roller hemming simulation. Serrated strain paths are detected along hemline. The zones where fracture takes place obtained by experiments and FE simulations are compared. It is demonstrated that FLSCF, which is on the basis of ductile damage criteria and basically irrelevant to linearity of strain path could be used to predict fracture of roller hemming correctly.     

Hot deformation behavior of 7150 aluminum alloy during compression at elevated temperature

Hot compression tests of 7150 aluminum alloy were preformed on Gleeble-1500 system in the temperature range from 300 °C to 450 °C and at strain rate range from 0.01 s^? 1 to 10 s^? 1, and the associated structural changes were studied by observations of metallographic and transmission electron microscope. The results show that the true stress–true strain curves exhibit a peak stress at a critical strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic-sine equation with the hot deformation activation energy of 229.75 kJ/mol. In the deformed structures appear the elongated grains with serrations developed in the grain boundaries, decreasing of Z value leads to more adequate proceeding of dynamic recrystallization and coarser recrystallized grains. The subgrains exhibit high-angle sub-boundaries with a certain amount of dislocations and large numbers of dynamic precipitates in subgrain interiors as increasing Z value. The dynamic recovery and recrystallization are the main reasons for the flow softening at low Z value, but the dynamic precipitates and successive dynamic particles coarsening have been assumed to be responsible for the flow softening at high Z value.