Microstructures developed during steady-state creep of Al-Mg alloys (original) (raw)
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Creep curves of AlMg alloys at very low stresses
Materials Science and Engineering, 1985
Creep tests were conducted on A1-Mg alloys at very low stresses, where newtonian behavior of the Harper-Dorn type was previously reported, under the condition of large creep strains. The creep curves obtained show the presence of large transient strains before steady state is reached. This finding indicates that the earlier creep data on these alloys obtained from short-term tests (strains less than 1%) are not representative of steady state creep.
Creep behavior of pure magnesium and Mg–Al alloys in active environments
Materials Science and Engineering: A, 2005
Environment-enhanced creep, which we have called "corrosion creep" (CC), was investigated in pure Mg and die-cast AZ91D, AM50 and AS21 alloys in a borate buffer solution and 3.5% NaCl at room temperature. In contrast to the data in the air demonstrating only the first stage of creep process, in corrosive solutions, secondary and tertiary creep due to the plasticization effect of the solution was observed. The lifetime of pure Mg increases by one order of magnitude in the buffer solution in comparison with that in 3.5% NaCl. Elongation-to-fracture in the former was approximately twice as high as that in NaCl, probably, due to hydrogen embrittlement in the latter. In corrosive solutions, the creep life and elongation-to-fracture of Mg alloys decreases with increasing aluminum content from 2.3 to 8.4% in AS21, AM50 and AZ91D alloys. Cracking and final creep-rupture of pure Mg originate in a transcrystalline manner in comparison with intercrystalline fracture of the alloys. Cracks are observed in pure Mg at the primary creep stage, but their depth and amount are relatively small. The effect of environment on the creep behavior of magnesium is connected, mainly, with plasticization of metal assisted by chemical reactions. Also, anodic dissolution of Mg is enhanced by creep stress.
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.
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.
Materials Science and Engineering: A, 2009
Creep specimens prepared of magnesium alloy AE42 were investigated under constant load in compressive and tensile creep respectively. Material was cast via the squeeze casting process in order to obtain a dense microstructure without pores. Creep tests were performed at constant temperatures between 150°C and 240°C and constant applied stresses between 40 MPa and 120 MPa until minimum creep rate ε & s was reached. It could be seen that the minimum creep rates of compressive creep tests were smaller compared to tensile creep tests, and the difference increased with increasing applied stress. Stress exponents, n, were determined according to the Norton-equation and it was found that a threshold stress σ had to be introduced into the analysis. The threshold stress is based on strengthening by Al-RE (aluminum-rare earths) precipitates. Calculating the true stress exponent, n , deformation mechanisms during creep could be clarified.
The measurement of internal stresses during creep of al and Al-Mg alloys
Acta Metallurgica, 1971
Current experimental techniques are described for measuring average int~erdialocation internal stresses. Measurements of internal stresses during steady stat,e creep of Al and Al-Mg are reported. The predicted reoovery ram based on the measured internal stress dependence of the steady state creep rate agrees with Fried&s network growth model when one accounts for the position dependence of the int,ernal st,ress. The variation of internal stress during normal and inverted primary creep is reported. From transient, creep measurements it, is possible to evaluate the stress dependence of the average dislocation velocity and mobile dislocation density. Results of measurement's on Al 5% Mg indicate that t'he average dislocation velocity depends linearly on t,he effective stress while the mobile dislocation den&y varies reciprocally with the internal st,ress. ~TI~SURES DES (~O~TRAI~TES INTER?;ES AU CGURS DU FLUAGE DES ALLIAGES Al ET A-Mg Lea autours decrivent les techniques experimentales utilistes couramment pour mesurer les contraintes internes moyennes entre dislocations, et donnent les resultats obtenus pour les mesures des contraintes imernes de fluage pour Al et Al-Mg. La vitesse de revenu prevue it partir de la variation de la vitesse de Auage stationnaire avec la contrainto in&me mesuree est, en accord avec la modele de Friedel pour la croixsance du reseau, si on co&d&e que la contrainte interne depend de la position. Lex auteurs domlent la variation de la contraint'e intern0 au tours du fluage primaire normal et inverse. A partir des mesures du fluage transitoire il est possible d'evaluer la variation aver la contrain& de la vitasse moyenne des dislocations et de la densite des dislocations mobiles. Les r&&tats des mesures sur AI-5 % Mg montrent que la vitesse moyenne des dislocations depend lineaircment de la contrainte effective alorx que la densite des dislocations mobiles varie inversement aver la cont,raint,e intnrne. MESSUNG INNERER SPA?iNlJ?\'GEN WdHREND DES KRIECHENS VGN Al UpiD Al-Mg-LEGIER~T~G~~ Experimentelle Methoden zur Messung van mittleren inneren Spannungen zwischen Versetzungen werden beschrieben. Es wird iiber Messungen von inneren Spannungen wahrond des stationiircn Kriechens von Al und van Al-Mg berichtet. Die aus der gemessenen Abhangigkeit der station&m Kriechgeschwindigkeit van inneren Spannungen vorhergesagte Erholungsrate stimmt mit Friedels Model1 iiber das Wachstum von Versetzungsnetzwarken iiberein, wenn man die Ortsabhiingigkeit~ der inneren Spannungen berii?ksichtigt. Uber die Anderung der inneren Spannung bei normalem und invertiertem prim&rem Kriechen wird berichtet.. Aus Messungen des ~~bergan~skriechens ka.nn man die Spannungsabh~ngigkeit dar mittleren Versetzungsgeseh~~indigkeit und der Dichte beweglicher ~~ersetzungen bestimmen. Ergebnisse an Al-5% Mg deuten darauf hin, da& die mittlere Versetz~lngsgeschwindigkeit linear van der effektiven Spannung abhiingt, wiihrrnd die Dichte beweglicher Versetzungen reziprok mit der inneren Spannung variiert.
Physica B: Condensed Matter, 2001
Al-10 wt% Ag and Al-22 wt% Ag alloys were used to trace the effect of Ag content on the behavior of both transient and steady state creep. After solid solution treatment, specimens of both alloys were aged at 563, 613 and 683 K for different periods of time (5-120 min) and creep tests were performed at different testing temperatures (353-413 K) by applying a stress of 115 MPa. A higher Ag content in the Al-Ag system is assumed to accelerate the rate of dissolution of GP zones and the rate of formation and growth of g 0-and g-precipitates which generally reduced the creep rate, a matter that is confirmed by TEM investigations. Values of the activation energy of the mechanisms operating in both transient and steady state stages were found in the range of 10-23 kJ/mol and 40-55 kJ/mol, respectively.
Creep of Al-3wt.%Mg as measured with the incremental loading method
Materials Science and Engineering: A, 1991
The deformation behaviour of A1-3wt.%Mg polycrystals was studied in the temperature interval from 295 to 674 K on a creep apparatus, applying the incremental loading method. The temperature dependences of the stresses which first cause plastic strain as well as the stress sensitivities of the strain rate indicate that different deformation mechanisms are operating. At temperatures below 0.5 T m, the intersection of forest dislocations is the most likely rate-controlling mechanism, superimposed at the lowest applied temperatures by effects of dynamic strain ageing. Above 0.5 T m there is diffusion-controlled high-temperature dislocation creep. The differences in results due to the use of the method and its limitations are discussed. 0921-5093/91/$3.50