The role of microstructure in hydrogen-assisted fracture of 7075 aluminum

Underaged, peak strength (T6), and overaged (T73) microstructures were studied in 7075 plate material. Hydrogen charged and uncharged tensile specimens of longitudinal orientation were tested between −196°C and room temperature. The results confirm a hydrogen embrittlement effect, manifested mainly in the temperature dependence of the reduction of area loss; a classical behavior of hydrogen embrittlement. The maximum embrittlement shifted to lower temperatures with further aging. The effect of hydrogen was largest for the underaged condition and smallest for the overaged, thus following the pattern found for the sensitivity to stress-corrosion cracking in high strength aluminum alloys. The fracture path was predominantly transgranular, with minor amounts of intergranular fracture. J. ALBRECHT, formerly with the Department of Metallurgy and Materials Science, Carnegie-Mellon University     

A study of fracture in high purity 7075 aluminum alloys

Ten different alloys based on the 7075 composition were used to study the effect of purity level, dispersoid type, and heat treatment on fracture toughness. Five purity levels ranging from 0.03 to 0.30 wt pct Fe + Si and two dispersoid types were investigated. Each alloy was given two heat treatments: the standard T651 heat treatment or a special thermomechanical treatment (TMT). Fracture toughness was measured using notched round tensile specimens taken from both the longitudinal and long-transverse directions. The notched round tensile test was modified to give the “plastic energy per unit area”. This fracture toughness parameter gave the same ranking for corresponding alloy/heat treatment combinations as the total energy per unit area measured on precracked Charpy specimens. The fracture toughness ranking for the ten alloys was the same in the longitudinal and long-transverse directions. This suggests the elongated distribution of constituent particles in the rolling direction does not change the failure mechanism. Fractographic evidence showed a bimodal distribution of ductile dimple size in all ten alloys. The number of large ductile dimples decreased with increasing purity level while the number of small ductile dimples increased. This is interpreted to mean that the smaller dispersoid and hardening particles become increasingly important in controlling the fracture toughness as the large intermetallic particles are eliminated by increasing the purity of these aluminum alloys. Since thermomechanical processing controls the amount and type of these smaller particles, it is a useful means for increasing fracture toughness in high purity aluminum alloys.     

Grain refinement in 7075 aluminum by thermomechanical processing

A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.     

Ductile fracture of dispersion-hardened alloys

Conclusions Experimental data have been obtained for the Ag-MgO system demonstrating the existence of a mean critical width L of the necks between adjacent particles. It is noted that this magnitude is related to the critical degree of deformation at fracture, lfr, and to the transverse contraction of alloys with secondary particles, . The screening influence exerted by large-sized particles on the void coalescence process is described. For the alloys investigated, the critical MgO particle size, capable of inducing void nucleation, has been found to lie in the range 30–50 nm.Translated from Poroshkovaya Metallurgiya, No. 10(178), pp. 99–103, October, 1977.     

Properties of friction-stir-welded 7075 T651 aluminum

Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure, a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased, while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength. Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable potential for welding 7075 T651 aluminum.     

The relation between microstructure and toughness in 7075 aluminum alloy

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.     

DC铸造7075铝合金微观偏析的量化分析

采用直接水冷半连续铸造(DC铸造)工艺制备7075铝合金铸锭.利用电子探针(EPMA)观察合金元素在样品中的分布,并通过绘制元素分布图判断合金元素在基体中的分布趋势.由于合金相的形成,Zn,Mg和Cu元素易在晶界偏聚;Cr和Ti作为形核质点,易在晶粒中心偏聚.利用5种排序方法处理收集到的电子探针数据,分别绘制成分曲线....     

Ductile fracture of mechanically alloyed iron-yttria alloys

The influence of Y₂0₃ particle content on the tensile fracture of mechanically alloyed iron has been studied for a series of dispersion-strengthened alloys containing up to 10 vol pct particles. When compared to the behavior of spheroidized steels, the present results indicate that at comparable volume fractions of particles, the Fe-Y₂0₃ alloys exhibit a much decreased tensile ductility. Observations of microscopic damage indicate that this is a consequence of rapid void nucleation at small strains, limited void growth, and enhanced void linking, especially at high particle contents. Analysis of these observations suggests (a) a surprisingly high oxide particle-matrix interfacial bond strength, (b) an influence of rapid strain hardening at small strains in creating high flow stresses, which assist void initiation, (c) enhanced void nucleation at high volume fractions of particles due to neighboring particles and voids, and (d) an accelerated void-linking process at high volume fractions of particles when interparticle spacing approaches particle/ void size. J.B. KOSCO, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Pennsylvania State University.     

7075铝合金板材热冲压成形中的高温摩擦

采用自制的板带高温摩擦试验机模拟实际固溶–冲压–淬火一体化热成形工艺下7075铝合金的高温摩擦过程,分别对上下摩擦头进行冷却和加热以模拟实际热冲压过程对模具和压边圈的冷却和加热,分析了下模加热温度、法向载荷和滑动速度对7075铝合金摩擦行为及磨损机理的影响。结果表明:铝合金摩擦系数随着下模加热温度的升高而增大,磨损机制由300 ℃时的黏着磨损转变为500 ℃时的黏着磨损、氧化磨损和磨粒磨损;施加法向载荷越大,摩擦系数越大,不同载荷下磨损机制均为黏着磨损及轻微的磨粒磨损,且随着载荷增大,黏着磨损程度有所加深;高滑动速度导致了磨损表面局部氧化物的生成,使摩擦系数随着滑动速度增大而减小,滑动速度为30 mm·s?1时,磨损机制主要是氧化磨损、磨粒磨损和黏着磨损。      

采用SECM分析7075铝合金的局部腐蚀行为

使用0.6 mol/LNaCl溶液腐蚀7075铝合金, 采用扫描电化学显微镜进行逼近曲线、面扫描测试, 用X射线衍射仪对合金的形貌进行分析, 用能谱仪分析腐蚀产物成分, 研究7075铝合金局部腐蚀电化学机理.结果显示:逼近曲线呈现正反馈, 探针的电流随着与合金基底距离的减小变大; 合金表面的活性点因为氯离子活化作用不断增加, 形成大范围的点蚀; 氯离子通过吸附在合金表面钝化层并与之反应, 破坏钝化层使得内部裸露, 内部的第2相电极电位更负, 和铝基体构成腐蚀微电池, 第2相的阳极腐蚀溶解降低合金的强度和抗腐蚀能力.      

Fatigue behavior and failure mechanisms of modified 7075 aluminum alloys

The effects of purity level and dispersoid type on the fatigue behavior of 7000 series alloys were investigated. Ten different compositions based on the 7075 alloy were produced with five levels of Fe + Si and either Cr or Zr dispersoids. Notched axial fatigue specimens were tested at room temperature and the fatigue life did not correlate with either purity level or dispersoid type. Specimens failed by three macroscopic modes designated as: slant, vee, or flat fracture. Sectioning analysis showed that the slant, vee, and flat fractures resulted from single, double and multiple initiation, respectively. Both initiation and propagation in all three modes of failures were dominated by slip related fracture on the {111} planes inclined at 35 deg to the tensile axis of the textured material. The same failure mechanisms were observed in smooth fatigue specimens. formerly with the Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH 45433     

7075铝合金的梯度时效及其温度场模拟

设计一种梯度时效方法及装置,采用不同的时效工艺参数,对7075铝合金棒材进行梯度时效处理,测试合金棒材沿轴向的温度以及抗拉强度与伸长率,通过FLUENT软件模拟,获得梯度时效温度场并建立等温云图和抗拉强度云图。结果表明:7075铝合金棒材梯度时效处理时,其温度沿轴向的分布近似于一维稳态分布;当时效温度从66℃升高到121℃时,合金的抗拉强度从517 MPa提高至599 MPa,伸长率从12%降低至8%。FLUENT模拟不同时效工艺参数下7075铝合金的温度场分布,模拟温度与实测温度的误差ε(K)2%,等温云图和抗拉强度云图与实验结果误差较小,可为梯度时效传热参数的选择提供有力的依据。     

液相线半连续铸造7075铝合金的热处理性能

液相线半连续铸造是一种先进的半固态浆制备方法，本文研究了液相线半连续铸造7075Al合金触变形成后的热处理工艺。结果表明，固溶时间对合金力学性能的影响是显著的，时效时间对合金的力学性能也有较大的影响，固溶与时效处理对合金的强化是综合作用的结果，适当延长固溶与时效时间能有效提高合金的抗拉强度，7075合金经475℃，12h固溶淬火，140℃ 22h时效之后，抗拉强度为474.8MPa，达到最佳的强化效果。     

Microstructural effects on hydrogen embrittlement in a high purity 7075 aluminum alloy

A high-purity 7075 alloy was heat treated to produce a variety of precipitate sizes and populations in grain interiors and at grain boundaries. Specimens were then evaluated for slip character and hydrogen embrittlement susceptibility following cathodic precharging or by simultaneous cathodic charging and straining. Embrittlement susceptibility was found to correlate very well with the size and type of matrix precipitates, consistent with the rationale that slip character controls the extent of dislocation transport of hydrogen and thereby may control and dominate the distribution of hydrogen in the material and the degree of hydrogen embrittlement. The results also indicate that a smaller role is played by grain boundary precipitates in controlling hydrogen embrittlement susceptibility.     

7075铝合金在不同pH值NaCl溶液中的腐蚀行为

用Tafel曲线和电化学阻抗谱(EIS)研究7075铝合金在不同pH值(pH=3、5、7、9、11) 的0.6 mol/L NaCl溶液中的腐蚀行为.结果表明,溶液pH值在3~7时,腐蚀电位正移,pH值在7~11时,腐蚀电位负移,pH值为11时腐蚀电位最负. pH值在3~11时,腐蚀速度呈现先降低后增大的过程,pH值为11时腐蚀速度最大,达到2.122 2 mm/a.在强酸溶液中,电化学阻抗谱中出现明显的感抗弧,表明存在不均匀点蚀现象. pH值为7和9时,电化学阻抗谱中只出现1个容抗弧,表明铝合金腐蚀属于金属基体溶解过程. pH值为11时,阻抗谱中出现2个容抗弧,表明铝合金腐蚀伴随铝合金的自溶解行为.      

A TEM study of microstructural changes during retrogression and reaging in 7075 aluminum

Microstructural changes occurring during retrogression, and during retrogression plus reaging in 7075-T6 aluminum alloy have been investigated by means of transmission electron microscopy, and related to mechanical properties. TEM results indicate that the drop in strength during the initial stage of retrogression was due to the partial dissolution of G.P. zones while the growth of the semi-coherentη ′ was responsible for the rapid recovery of strength. It is suggested that the retrogression and reaging treatment resulted in the increase in volume fraction of G.P. zones and especially η′ precipitates over both the T6 and retrogressed conditions, therefore significantly improving the strength of the alloy.     

Work of fracture in aluminum metal-matrix composites

The effect of isothermal exposure and thermal cycling on the toughness of B/Al (1100), B/Al (6061), and A1₂O₃/A1 composites has been investigated. In B/Al (1100), isothermal exposure at 773 K for 45 × 10⁴ s (125 hours) reduced toughness, measured by the work of fracture, from 76 kJm^-2 to 10 kJm^-2, and a similar reduction occurred after equivalent thermal cycling. The corresponding reduction in toughness after isothermal exposure in B/Al (6061) was from 44.5 kJm^-2 to 8 kJm^-2; however, the effect of thermal cycling was less detrimental. In the FP-A1₂O³/A1 composite, the work of fracture was insensitive to both forms of thermal treatment. Changes in the toughness of the B/Al composites have been correlated with and analyzed in terms of modifications to matrix, fiber, and interface properties, in particular, matrix softening, interface reaction products, and fiber notch sensitivity. The latter currently on The latter currently on