Evaluation of the creep properties of an Al–17Si–1Mg–0.7Cu alloy (original) (raw)
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Analysis of the creep response of an Al–17Si–4Cu–0.55Mg alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
The creep response of a hypereutectic Al-Si alloy containing 4% Cu and 0.55% Mg was investigated between 553 and 653 K. Minimum creep rate as a function of applied stress by using the power-law equation suggested the existence of two different regimes: a low-stress regime characterised by a stress exponent close to 4-5, and a high-stress regime with a higher stress exponent. Although the magnitude of the stress exponent in the low-stress regime was equivalent to that observed in pure Al, the apparent activation energy for creep was higher (Q = 210 kJ/mol) than the activation energy for self-diffusion in Al (Q d = 143 kJ/mol). The microstructural analysis suggested a similarity between this alloy and Al-matrix discontinuously reinforced composites. This approach, based on threshold-stress concept, permitted to rationalise both the magnitude of the stress exponent and the apparent activation energy for creep. In addition, the substantial similarity between hypereutectic Al-Si alloys and Al-based composites with similar composition was confirmed.
Effect of Cu on the Creep Behavior of Cast Al-15Si-0.5Mg Alloy
JOM, 2019
In the present article, the effect of Cu on the microstructure and creep properties of Al-15Si-0.5Mg cast alloy was studied by using optical microscopy, scanning electron microscopy, energy dispersion spectrometry and the impression creep test. The microstructure is changed considerably in the presence of copper. The number of the Cu-rich phases increased noticeably with rising Cu content. Formation of a dendritic structure of a(Al) and modification of the eutectic Si were the other effects of increasing the Cu amount. The results showed that creep properties of the alloy increase considerably with the Cu addition. Calculating the stress exponent (n) and creep activation energy (Q) revealed that pipe diffusion climb-controlled creep is the dominant creep mechanism of the main alloy. Although Cu had no effect on the dominant creep mechanism at the lower stress, it was changed to lattice diffusion climb-controlled dislocation creep with increasing stress.
High temperature creep behavior of metal matrix AluminumSiC composites
Acta Metallurgica et Materialia, 1993
The tensile creep behavior of A1-SiC metal matrix composites has been investigated and analyzed over the temperature range from 230 to 525°C. It is shown that plastic flow in these materials is lattice-diffusion controlled dislocation creep in the aluminum matrix. All data on A1 SiC have been assessed by a creep relation developed for creep of metals at constant structure with the added contribution of a threshold stress. The threshold stress for creep in A1-SiC composites is not a thermally-activated process and is shown to have a linear dependence with temperature becoming zero at 470°C. The threshold stress is higher for the whisker composites than for the particulate composites. The origin of the threshold stress is not well understood and cannot be explained by contemporary dislocation models involving dislocation bowing or unpinning around particles sites. The observed interparticl~interwhisker spacing is shown to influence the creep rate in the same way as observed for mechanical alloyed (MA) AI base materials.
Metallurgical Transactions A, 1993
The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 ~ to 250 ~ At 150 ~ reinforcement led to an improvement in creep resistance, while at 250 ~ both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 ~ and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 ~ can be attributed to diffusional relaxation processes at the higher temperature.
Materials Science and Engineering: A, 2009
The effect of Al content and Si addition on the microstructural and creep properties of Mg-Al-RE alloys was investigated in this study. The steady state creep rates were specified and it was found that the creep behavior of the alloy, which is dependent on the stability of the near grain boundary microstructure, was improved by the RE and Si addition. For the AZ91 alloy, the results indicate a mixed mode of creep behavior, with some grain boundary effects contributing to the overall behavior. However for the RE and Si added samples, sliding of grain boundaries was greatly suppressed and the dislocation climb controlled creep was the dominant deformation mechanism. Analysis of creep rates also showed that the Si addition resulted in formation of Mg 2 Si particles (in Chinese Script form) which have a high thermal stability. After Si addition the steady state creep rates were decreased and the creep resistance was improved. This was due to formation of Mg 2 Si particles which change the deformation mechanism at elevated temperatures. Addition of cerium rich misch metal to AZ91 alloy resulted in formation of needle shape particles, which also had a very high thermal stability, providing increased creep resistance and superior mechanical properties compared to AZ91 magnesium alloy. As a result, the grain boundaries were less susceptible for grain boundary sliding at high temperatures. By decreasing the Al content of the alloy having 2 wt.% RE from 9 to 4 wt.%, the steady state creep rate was also decreased compared to AZ91 + 2% RE alloy. The fracture mechanism was also investigated and it was observed that although the Si addition improves the creep resistance, it can make the alloy brittle at ambient temperature.
Creep behavior of an aluminum 2024 alloy produced by powder metallurgy
Acta Materialia, 1997
Creep tests were conducted over a range of temperatures from 523 to 603 K on an unreinforced 2024 Al alloy fabricated by powder metallurgy processing. The creep curves under all testing conditions exhibit a brief quasi-steady-state condition and then a very extended tertiary stage leading to failure. A logarithmic plot of the minimum creep rate against the applied stress leads to a high and variable stress exponent and a high apparent activation energy. Prior to creep testing. the specimens contained large particles identified as AbCu and AbCuMg.
A unified description of solid solution creep strengthening in Al–Mg alloys
Materials Science and Engineering: A, 2012
It is proposed that the creep strengthening of aluminium due to the addition of Mg atoms in solid solution and the variation of the stress exponent, n, with the stress (from n≈5 to n≈3) is due to a unique microstructural feature, that is, the stress variation of the total to mobile dislocation density ratio. To support this idea, creep data recorded from the literature of pure Al-Mg alloys and of pure aluminium have been analyzed in the frame of the Strength Difference Method, SDM. A strengthening proportional to the applied stress is found. On this basis, a model which considers a change of the dislocation density/velocity due to the presence of the Mg atoms in solid solution and the solute drag and climb forces for dislocation motion was assumed. The new model, which also takes into account published data of the dislocation density measured at different applied stress, describes naturally the curvature of experimental Al-Mg creep data, associated traditionally to the change in deformation mechanism from dislocation glide controlled (n=3) to dislocation climb controlled (n=3) mechanism. The model does not undermine the relevance of aluminum self diffusion for dislocation climb process (vacancy diffusion) as the creep controlling mechanism in this solid solution alloy.
Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite
Metallurgical and Materials Transactions A, 1996
The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an AI-10 vol pct SiCp composite has been studied. The A1-4 wt pct Mg-10 vol pct SiC, composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol-L These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol-~, at a constant stress, cr = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the AI-Mg-A1203r composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the A1-Mg-SiC e composite are compared with those of the A1-Mg-AIzO3/and 6061 AI-SiCe.w composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the AI-SiC, composite.
Microstructure and dislocation analysis after creep deformation of die-cast Mg–Al–Sr (AJ) alloy
Materials Science and Engineering: A, 2009
The microstructure and creep behavior of Mg/Al composite crankcases cast with three alloy formulations of the Mg-Al-Sr alloy AJ62 have been investigated. Overall 12 components were used within this study. Multi-level creep tests were conducted to evaluate the creep properties at stresses up to 90 MPa and temperatures up to 473 K. Microstructure observations including phase characterization and in-depth dislocation analyses were performed in the as cast condition and after creep testing. The tensile creep testing revealed a distinct primary creep and a high stress exponent up to a value of 10. The threshold stress concept was applied, which yields to an effective stress exponent of 5 indicating a strengthening effect due to particle-dislocation interaction. Transmission election microscopy (TEM) of the microstructure revealed the continuous precipitation of -Mg 17 Al 12 in the ␣-Mg matrix near the interdendritic regions during creep. In addition, a fine-dispersed nano-scaled Al-Mn phase, probably Al 8 Mn 5 , was observed in the ␣-Mg matrix in all samples under investigation. According to an in-depth TEM analysis of the dislocation structure, slip of non-basal a dislocations and c + a dislocations is activated in addition to basal slip even at 423 K and very low stress (15 MPa). Furthermore, the TEM images reveal a strong interaction between dislocations and the Mg-Al and Al-Mn matrix precipitates. Hence, matrix strengthening by well-distributed precipitates could be one factor for the excellent creep resistance of AJ-alloys. Despite of the matrix precipitates, the substitution of the eutectic phase Al 4 Sr by Mg 9 Al 3 Sr in one of the alloys seems to be the major difference in the investigated alloys and should therefore account for the differences in creep rate and creep strain.
Metallurgical and Materials Transactions A, 1996
Tensile creep tests were conducted on two Al-Si alloys produced by rapid solidification: an Al-Si-Ni-Cr alloy and an Al-Si-Cu-Fe alloy, designated alloys A and B, respectively. The creep curves of these two alloys in the temperature range from 493 to 573 K were markedly different, with alloy A exhibiting a normal creep curve with a very short tertiary region and alloy B exhibiting an extended tertiary stage associated with strain localization. The minimum creep rates varied, with the applied stress raised to exponents of ϳ9.0 and ϳ8.5 for the two alloys, respectively. The hardness of alloy B decreased with time during the creep testing, but there was little or no change in the hardness of alloy A. These differences in the creep and hardness characteristics are attributed to the evolution of precipitates within the two alloys during creep testing. A detailed analysis shows that, over the temperature range examined experimentally, alloy A exhibits a creep strength that is superior to conventional Al-based alloys and comparable to, or even higher than, some SiC-reinforced Al composites.