Thermal stability in bulk cryomilled ultrafine-grained 5083 Al alloy

Abstract

Thermal stability in bulk ultrafine-grained (UFG) 5083 Al that was processed by gas atomization followed by cryomilling, consolidation, and extrusion, and that exhibited an average grain size of 305 nm, was investigated in the temperature range of 473 to 673 K (0.55 to 0.79 T m , where T m is the melting temperature of the material) for different annealing times. Appreciable grain growth was observed at temperatures Ͼ573 K, whereas there was limited grain growth at temperatures Ͻ573 K even after long annealing times. The values of the grain growth exponent, n, deduced from the grain growth data were higher than the value of 2 predicted from elementary grain growth theories. The discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination of the microstructure of the alloy suggests that the origin of the pinning forces is most likely related to the presence of dispersion particles, which are mostly introduced during cryomilling. Two-grain growth regimes were identified: the low-temperature region (Ͻ573 K) and the high-temperature region (Ͼ573 K). For temperatures lower than 573 K, the activation energy of 25 Ϯ 5 kJ/mol was determined. It is suggested that this low activation energy represents the energy for the reordering of grain boundaries in the UFG material. For temperatures higher than 573 K, an activation energy of 124 Ϯ 5 kJ/mol was measured. This value of activation energy, 124 Ϯ 5 kJ/mol, lies between that for grain boundary diffusion and lattice diffusion in analogous aluminum polycrystalline systems. The results show that the strength and ductility of bulk UFG 5083 Al, as obtained from tensile tests, correlate well with substructural changes introduced in the alloy by the annealing treatment.

Thermal Stability of Cryomilled Al-Mg-Er Powders

“…The activation energy of 5.6 kJ⋅mol −1 was determined for milled 5083 powder where there is no strain relaxation. Similar to the findings of Roy et al [34], the increase in activation energy of the Al-Mg-Er powders compared to the milled 5083 powder could be attributed to some strain relaxations taking place with increased temperature. Annealing at 150 ∘ C resulted in limited grain growth in both powders (Figures 9 and 15) and reduced microstrain from the as-milled state (i.e., 0.041% in 0.1 Er and 0.066% in 0.5 Er).…”

“…Another study on a bulk UFG 5083 alloy reported = 25 ± 5 kJ⋅mol −1 for the low temperature regime, while = 124 ± 5 kJ⋅mol −1 was found for higher temperatures ( > 573 K) [5,34]. Roy et al [34] suggest that the low activation energy at low temperatures is indicative of highly unstable and nonequilibrium grain boundaries. Unstable grain boundaries could require smaller driving force for rearrangement of the grains and grain boundaries, while nonequilibrium boundaries may possess greater atomic mobility due to excess dislocations [34].…”

“…In Tellkamp et al 's study of a milled 5083 alloy powder, ≈ 5.6 kJ⋅mol −1 was reported for the low temperature regime ( < 654 K), while ≈ 142 kJ⋅mol −1 was reported for the higher temperature regime ( > 654 K) [4]. Another study on a bulk UFG 5083 alloy reported = 25 ± 5 kJ⋅mol −1 for the low temperature regime, while = 124 ± 5 kJ⋅mol −1 was found for higher temperatures ( > 573 K) [5,34]. Roy et al [34] suggest that the low activation energy at low temperatures is indicative of highly unstable and nonequilibrium grain boundaries.…”

“…Roy et al [34] suggest that the low activation energy at low temperatures is indicative of highly unstable and nonequilibrium grain boundaries. Unstable grain boundaries could require smaller driving force for rearrangement of the grains and grain boundaries, while nonequilibrium boundaries may possess greater atomic mobility due to excess dislocations [34].…”

Microstructure and Mechanical Properties of Al-5Mg-0.8Mn Alloys with Various Contents of Fe and Si Cast under Near-Rapid Cooling

“…Extensive research to improve the AA5083 alloy had been conducted with emphasis on the microstructures and mechanical properties [8][9][10][11], superplasticity [12][13][14][15], ultrafine grain [16][17][18][19][20] and adding element [21][22][23][24], etc. Recently, research was carried out on the formation of the Fe-and Mn-rich intermetallics in the AA5083 type Al-Mg-Mn alloy [2].…”