Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Fracture toughness and fatigue crack propagation behavior of 8090 Al-Li alloy

We present results of fatigue tests of high-strength 8090 Al-Li alloy and data on its fatigue crack growth resistance. High strength combined with fairly high crack growth resistance and endurance limit results in much better service characteristics compared to other high-strength aluminum alloys. We discuss results of tensile and impact tests of Charpy specimens and the critical values of theJ-integral andK _1c for 10-mm-thick specimens in the T-L and L-T orientations subjected to complete and partial aging. The experimental results are compared with published data for 8090 and other high-strength aluminum alloys. We suggest a numerical method for the evaluation of fatigue strength according toda/dN-K diagrams.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 45–58, January – February, 1995.     

热应变对2090铝锂合金焊接热影响区显微组织和力学性能的影响

采用焊接热模拟实验方法研究了在焊接热循环和应力－应变循环同时作用下，２０９０铝锂合金妆热影响显微组织和力学性能的变化特性，结果表明，在焊接热应变作用下，应变使合金中强化数量增多，晶车化，从而降低了２０９０铝锂合金焊接热影响我的软化倾向。     

Effect of specimen thickness on the crack propagation and crack branching in delayed failure

The crack propagation and crack branching behaviors in delayed failure have been investigated on the specimens with various thickness ($\text{B = 1.5–10}\text{mm}$).The crack propagation velocity reveals a maximum value at a medium specimen thickness ($\text{B = 5}\text{mm}$). This fact can be understood by assuming the compound effect of two factors that the triaxiality of stress at crack tip as a driving force for hydrogen diffusion increases with increase of specimen thickness $\text{B}$, and that the invasion of hydrogen atoms from specimen surface increases with decrease of $\text{B}$.The stress intensity factor at crack branching, $\text{K}{}_{\mathrm{IB}}$, increases with decrease of specimen thickness $\text{B}$, and when B is 1.5 mm, the specimen fractures without showing the crack branching. The latter fact can be explained by connecting the necessary and sufficient conditions for crack branching with the decrease in height of plastic region at the crack tip in thin specimens.     

Effect of microstructure on the fatigue crack growth behaviour of a near-α Ti alloy

Fatigue crack growth behaviour of Ti–6Al–2Zr–1.5Mo–1.5V (VT-20 a near-α Ti alloy) was studied in lamellar, bimodal and acicular microstructural conditions. Fatigue crack growth tests at both increasing and decreasing stress intensity factor range values were performed at ambient temperature and a loading ratio of 0.3 using compact tension samples. Lamellar and acicular microstructures showed lower fatigue crack growth rates as compared to the bimodal microstructure due to the tortuous nature of cracks in the former and the cleavage of primary α in the latter. The threshold stress intensity factor range was highest for acicular microstructure.     

A finite element analysis on the post-deflection behaviour of a T-shaped crack formed by crack deflection in layered composites

Post-deflection behaviour of a T-shaped deflected crack formed by the interaction between a primary crack and a defect in a bi-material interface has been investigated. It is predicted that behaviour of the deflected crack is significantly influenced by the globally applied loading and specimen geometry from the early stage of its growth and that it may kink out of the interface before any significant growth along the interface depending on the interfacial fracture energy. This implies that simply achieving crack deflection in early stage of loading is not sufficient and prevention of kinking of the deflected crack out of the interface during its growth is also practically important to obtain improved toughness. Hence it is suggested that post-deflection behaviour of the deflected crack must be considered in addition to the crack deflection criteria in determining the critical conditions for toughening of layered composite. Toughening criteria based on these results has been derived and compared with the existing experimental results.     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

The shear strength () of overlap shear test pieces made by solid state diffusion bonding or by machining thin (2.5 or 4 mm thick) Al-Li 8090 alloy sheet has been determined for various overlap lengths (/). When / < 3 mm, was independent of / and equal to 188 to 202 MPa for the bonded joint and 199 to 209 MPa for the base metal sheet. The lower mean shear strength of the bonded joint was caused by the lower resistance of intergranular fracture in the planar grain boundary at the bond interface. The bond strengths were, however, greater than those previously reported for joints in 8090 alloy made by solid-state or liquid-phase diffusion bonding and about a factor of 7 greater than those for adhesive bonded joints.     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

Al-Li 8090 alloy overlap shear test pieces machined from 3 mm thick diffusion-bonded sheets showed two fracture zones at the bond interface. Zone 1 at the ends of the overlap showed predominantly intergranular fracture and zone 2 at the centre of the overlap showed peel-type fracture. The load appeared to be carried entirely by zone 1. Only zone 1 fracture was obtained in the base metal test piece. The fracture zones were caused by the non-planar stress distribution and by the bending moments associated with this type of test piece. The planar bond interface may accentuate the tendency in these alloys towards low ductility and toughness in the short transverse direction.     

Effects of microstructure on the R-curve behaviour of sintered silicon nitride

The R-curve behaviour of sintered silicon nitride was investigated by using short bar specimens. The samples were fabricated by sintering at 1700° C for 2 or 8 h in a nitrogen atmosphere, varying the initial contents of the silicon nitride powder. The R-curve was evaluated with a loading and unloading technique to calculateK _R during stable fracture. Steep R-curves were observed in the specimens made from the high initial content powder. Thus, the grainbridging effect behind the crack seems to contribute to the R-curve, because a steep R-curve corresponds to fibrous texture development. ApparentK _c values with an assumption of linear elastic fracture mechanics for the specimens undergoing 8h sintering, are greater than those undergoing 2 h sintering. These results can be attributed to microcracking.     

Effect of ceramic reinforcement on the ageing behaviour of an aluminium alloy

The effect of a 20vol % alumina microsphere particulate on the age-hardening characteristics of a 6061 Al matrix composite was investigated — based on microhardness, electrical resistivity and X-ray mapping — and the composite is compared to the unreinforced 6061 Al alloy. It is shown that this ceramic reinforcement can affect the age-hardening behaviour of the matrix alloy by significantly accelerating the kinetics of precipitation. This acceleration is related to a decrease in nucleation time and to an increase in the precipitate-growth rate. The relative amounts of age-hardened precipitates are also observed to be affected by reinforcement addition.     

电场固溶处理对1420铝锂合金组织和性能的影响

研究了电场固溶处理对１４２０铝锂合金组织和性能的影响。试验结果表明,电场固溶处理可以提高１４２０铝锂合金时效后的强度,但塑性略有下降;随固溶处理温度的提高,合金的强度提高而塑性降低;固溶处理上同时,随电场强度的提高,合金的强度提高而塑性下降,经电场固溶处理后的试样时效析出的δ＇相更细小弥散;电场的存在有利于抑制亚晶粒的长大。     

The effect of microstructure on the fatigue crack growth behaviour of an Al-Zn-Mg-(Zr) alloy

The effect of microstructure on the fatigue crack growth behavior of an Al-Zn-Mg-(Zr) alloy was investigated. The major effects were observed during testing at low stress intensities. An overaged microstructure, containing large, partially coherent and incoherent particles, exhibited the slowest crack growth rates when compared to a maximum strength and an underaged condition. This improved fatigue resistance was attributed to the occurrence of dislocation looping and cross slip as the primary deformation mode, causing a larger degree of homogeneous deformation. As the stress intensity increased, the differences in the crack growth rates among the three aging conditions became less as evidence of dislocation banding began to disappear in the underaged and maximum strength aged conditions. At intermediate stress intensity levels no significant crack growth rate differences were observed.The observed crack growth rates were compared to the values predicted by various crack growth rate equations which use low cycle fatigue and microstructural parameters. Several equations were tested, but only two types appear to be acceptable. Formulae which predict straight line dependence of the crack growth rate on the stress intensity level give an excellent fit over a segment of the da/dN vs. K plot. However, these equations have numerical constants in them which necessitate the actual fatigue propagation study to be conducted. The second type of equation, which gives reasonably accurate growth rates for given K values, uses only measurable monotonic and cyclic material parameters and a microstructural parameter. Although these equations do not fit measured crack growth rates with the accuracy of the first type of equation, they do permit the crack growth rates to be estimated without actually conducting the fatigue crack propagation tests.

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Résumé L'effet de la microstructure sur la croissance d'une fissure de fatigue dans un alliage Al–Zn–Mg–(Zr) a été étudié. Les principaux effects ont été constatés au cours d'essais à faible intensité de contrainte. Une microstructure vieillie, comportant des particules n' importantes et partiellement cohérentes ainsi que des particules n incohérentes, a montré la plus faible vitesse de propagation de fissure lorsqu'on établit la comparaison à une contrainte maximum déterminée et à des conditions non vieillies. Cet accroissement de la résistance à la fatigue est attribuée à l'apparition de boucles de dislocation et de bande de glissement représentant un mode de déformation primaire et causant un degré important de déformation homogène. Lorsque l'intensité de contrainte augmente, les différences dans la vitesse de propagation de la fissure entre 3 conditions de vieillissement diminuent car la formation en bandes des dislocations commence à disparaitre dans des conditions sous-vieillies et vieillies à contrainte maximum. A des niveaux intermédiaires de l'intensité de contrainte, on n'observe plus de différences significatives dans la vitesse de propagation de la fissure.Les vitesses de propagation de fissure observées on été comparées aux valeurs prédites par diverses équations de vitesse de propagation qui utilisent les paramètres de la fatigue olygocyclique et des paramètres microstructuraux. Diverses équations ont été testées mais il n'apparait que deux types d'équations acceptables. Les formules qui prédisent une dépendance en ligne droite de la vitesse de propagation d'une fissure en fonction du niveau d'intensité de contrainte fournissent une représentation excellente sur un segment d'un graphe da/dN par rapport à K. Cependant, ces équations comportent des constantes numériques qui rendent nécessaire l'étude de la propagation en fatigue sur des éprouvettes. Le deuxième type d'équation, qui fournit des vitesses de propagation raisonnablement exactes pour des valeurs K déterminées, recourt seulement à des paramètres mesurables, monotoniques et cycliques du matériau ainsi qu'à un paramètre microstructural. Bien que ces équations ne recontrent pas les vitesses de propagation de fissure mesurées avec la précision fournie par le premier type d'équation, elles permettent d'estimer les vitesses de propagation de fissure sans devoir conduire des essais de fatigue.

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Glossary a total crack length - a _i crack length at i ^th measurement - A Paris coefficient - b fatigue strength exponent - B breadth of sample - C Coffin-Manson exponent - COD crack opening displacement at maximum load - da change in crack length - dN change in number of cycles - E modulus of elasticity - H half height of specimen - k cyclic strain hardening coefficient - K stress intensity level - K _I stress intensity level for mode I loading - m Paris exponen This research was supported by the U.S. Air Force Office of Scientific Research (ASFC) under Grant No. AFOSR-74-2615.     

AZ31镁合金板材轧制过程边裂形成原因分析

针对AZ31镁合金板材轧制过程中出现边部裂纹的问题,采用数值模拟方法研究了AZ31镁合金板材轧制过程中轧制温度对板材边裂的影响,利用实验室热轧试验方式研究了AZ31镁合金板坯宽厚比、轧制道次数以及工作辊直径等工艺参数对镁板边部裂纹的影响.研究表明,边裂的产生多数情况是由于几种因素共同作用的结果,其主要影响因素有轧制温度、道次加工率、轧辊直径以及板坯宽度和厚度等.在其他条件不变的情况下,减少板材宽厚比,可降低边部所受拉应力,有利于减少横向裂纹产生;当b(R·△h)~(1/2)时,随着板材宽度增加,轧制力逐渐升高,边部产生横向裂纹的几率增加;对于相同规格板坯,随着辊径增大,轧制过程中板坯的宽展量和所受摩擦力逐渐增加,有利于发挥板材塑性从而减小边部裂纹产生的趋势.     

Gust severity effects on fatigue crack propagation in aluminium alloy sheet materials

Flight simulation fatigue tests were carried out on specimens of two aluminium alloys to investigate the effect of differing gust load experiences on fatigue crack propagation in 7075-T6 and the effect of gust load alleviation on 2024-T3. Two gust spectra were used: the Fokker F-27 spectrum for 7075-T6; and the reference spectrum TWIST for 2024-T3. There was a large systematic effect of gust severity on crack propagation rates in 7075-T6, and this effect correlated with the stress intensity factor for the root mean square of the gust amplitudes. Gust alleviation increased the crack initiation and total lives for 2024-T3, but decreased the crack propagation lives owing to a lessening of crack growth retardation following severe flights.