Browsing by Subject "Annealing"

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Development of nano/submicron grain sizes in an AISI 301 austenitic stainless steel
(2004-08-16) Johannsen, Dana Lynn; Ferreira, Paulo J. (Paulo Jorge)
Nano/Submicron austenitic stainless steels increasingly attracted attention over last few years with their potential to revolutionize traditional materials design in many applications via fine structural control to tailor the engineering properties. At these nano/submicron grain scales, the fraction of atoms at the grain boundaries is of the same order as those inside the grains, and thus grain boundary strengthening can be significant (1000 MPa for 0.5 μm grain size). To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90% cold rolled and subsequently annealed at temperatures varying from 600 to 900ºC for a dwelling time of 30 minutes. The effect of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation is also determined herein. The analysis of the as-cold rolled microstructure reveals that a 90% cold reduction produces a combination of the lath type and dislocation cell type martensitic structure. For the annealed samples, the average austenite grain size increases from 285 nm at 600ºC to 5.85 μm at 900ºC. On the other hand, the amount of reverted austenite exhibits a maximum at 750ºC, where austenite grains with an average grain size of 1.7 μm compose 95% of the microstructure. Annealing temperatures above 750ºC show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)₂₃C₆ nanoscale carbides form within the grains and at the grain boundaries.
The effects of processing conditions on static abnormal grain growth in Al-Mg alloy AA5182
(2011-05) Carpenter, Alexander James; Taleff, Eric M.; Bourell, David L.
Static abnormal grain growth (SAGG) was studied in Al-Mg alloy AA5182 sheet by varying four processing parameters: deformation temperature, strain rate, annealing temperature, and annealing time. SAGG is a secondary recrystallization process related to geometric dynamic recrystallization (GDRX) and requires both deformation at elevated temperature and subsequent static annealing. A minimum temperature is required for both SAGG and GDRX. Recrystallized grains only develop at strains larger than the critical strain for SAGG, [epsilon]SAGG. The size of the recrystallized grains is inversely related to and controlled by the density of SAGG nuclei, which increases as local strain increases. The results of this study suggest that SAGG is controlled by two thermally-activated mechanisms, dynamic recovery and recrystallization. During deformation, dynamic recovery increases as deformation temperature increases or strain rate decreases, increasing the critical strain for SAGG. SAGG is subject to an incubation time that decreases as annealing temperature increases. SAGG can produce grains large enough to reduce yield strength by 20 to 50 percent. The results of this study suggest strategies for avoiding SAGG during hot-metal forming operations by varying processing conditions to increase [epsilon]SAGG.
The effects of radiation damage and annealing on the diffusion of helium from zircon
(2018-12) Goldsmith, Adam S.; Stockli, Daniel F.; Ketcham, Richard Alan, 1965-; Lin, Jung-Fu; Garver, John I; Gautheron, Cécile
The relationship between radiogenic ⁴He in zircon, and its associated radiation damage to the crystal lattice, have been of scientific interest since the earliest attempts at geochronology, and remain an active topic of research today in the pursuit of understanding the timing of low-temperature geological processes. Recently, the complex interrelationships between ⁴He diffusion kinetics, time, temperature, and radiation damage were quantified into a complete model for the zircon (U-Th)/He (ZHe) thermochronometric system: the Zircon Radiation Damage Accumulation and Annealing Model (ZRDAAM). Many researchers have taken issue with the predictions of this model, however, particularly at moderate to high radiation doses (~3 × 10¹⁷ – 2 × 10¹⁹ α/g). In this submission, the relationship between radiation damage and ⁴He diffusion kinetics are examined in greater detail to identify sources of discrepancy between observations and model predictions. Using zircons from three geologically distinct localities, this work represents the largest study on the diffusion kinetics of ⁴He from zircon to date, including 26 cycled step-heating analyses. A new behavior is observed in diffusion kinetics behavior, termed ‘rollover’: a thermally activated decrease in diffusivities, interpreted as low-temeprature annealing. Comparing the results of modeled and observed ZHe dating and step-heating analyses, we find significant discrepancies with model predictions in agreement with previous research. The source of these discrepancies is identified as low-temperature annealing, a process distinct from epitaxial recrystallization, resulting in significant effects on diffusional behavior, and not accounted for in the ZRDAAM. Results reveal that the damage threshold beyond which model diffusivities increase is too high, likely as a result of extensive prior geological annealing of the high-damage Sri Lankan zircons used to calibrate the ZRDAAM. Furthermore, the use of a single alpha dose appears inappropriate for capturing the full complexity of the ZHe system. Lastly, these results strongly suggest that the diffusion of ⁴He from zircon proceeds in a manner similar to a homogeneous gas, in spite of the fact that natural ⁴He concentrations are often strongly heterogeneous. This work is a significant step forward in the improvement of ZHe thermochronology, and will yield to the recovery of more accurate and robust thermal histories.