Comparison of S-phase coarsening in alloy 2618A during ageing, creep, and fatigueWednesday (08.11.2017) 15:00 - 15:20 Part of:
The aluminium alloy 2618A is an Al-Cu-Mg alloy designed for long-term operation at elevated temperature in transportation and aerospace industries. The desired properties, e.g. creep behavior and hardness, are controlled by fine S-phase precipitates formed from GPB zones within the matrix. Due to the overageing of the rod-shaped S-phase precipitates (Al2CuMg) and the conversion of the GPB zones into the S-phase the strength of the material declines during exposure to elevated temperatures1. In this work the S-phase rod radii were investigated to improve the understanding of the overageing process of alloy 2618A at application relevant temperatures and mechanical loads under different ageing conditions with statistical accuracy.
The nm-sized Al2CuMg precipitates form as rods along the 001 direction of the Al matrix and were selectively imaged by dark-field TEM. Radii distributions were obtained from multiple images using a standard procedure involving several steps. The radii distributions show that the initially narrow distribution (T61) broadens during isothermal heat treatments of 2500 h and 5000 h at 190 °C and the distribution maxima move to higher radii. A lognormal distribution fits the radii distributions well and allows the determination of average radii which increase with increasing time. The crept samples in comparison exhibit an accelerated increase in average radii with time (1820 h, 4172 h, about 15 %). Measurements of fatigued samples show a significant increase of the average radii for short test durations (4.3 h, 17.5 h).
A fast increase of the average precipitate radii is observed during the initial stage of ageing for both the heat treated samples without external load and the crept samples. The authors suggest that the accelerated coarsening during the initial phase of ageing elapses fast due to the combined coarsening of the S-phase precipitates and the transformation of GPB zones into S-phase precipitates. After about 2500 h only S-phase precipitates remain and the coarsening process slows down. In case of the fatigued samples the significant increase of the average radii for short fatigue durations may suggest that the expected large dislocation density induced by the LCF experiments play a role in accelerating the coarsening process.
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|Short Paper||This paper describes the degradation process of alloy 2618A due to coarsening of the S-phase. To quantitatively investigate the coarsening process dark-field transmission electron microscopy (DFTEM) was performed to investigate samples in different ageing states. The resulting particle size distributions allow a statistically relevant evaluation of the average particle radii. A fast increase of the average precipitate radii was found during the initial state of ageing for both the samples heat treated without external load and the crept samples. The authors suggest that the fast coarsening during the initial phase of ageing elapses fast due to the combined coarsening of the S-phase precipitates and the transformation of GPB zones into S-phase precipitates. After about 2500 h only S-phase precipitates remain and the coarsening process slows down.||471 KB||Download|