Confirmation of Dynamically Recrystallized Grains in Hexagonal Zirconium through Local Internal Friction Measurements
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TRODUCTION
HEXAGONAL Zirconium (a-Zr) is used in the structural components in thermal nuclear reactors.[1,2] The actual components are typically fabricated through a series of hot and warm working steps followed by cold forming.[3,4] More specifically, the as-cast a-Zr is subjected to forging or extrusion to break up the cast structure. This initial stage is followed by subsequent hot and warm extrusion to provide tubes for subsequent draw bench or pilgering and annealing. It is important to note that the microstructure and crystallographic texture obtained through warm-hot working is altered, though not very significantly, during the final cold working plus annealing stages.[5] The warm/hot working of Zr may occur in singlephase bcc-Zr (b-Zr), but also as a + b-Zr and as aZr.[6,7] Lower working temperatures have the specific advantage in improved dimensional tolerance.[8]
K.S. THOOL, A. PATRA, and I. SAMAJDAR are with the Metallurgical Engineering and Material Science Department, Indian Institute of Technology-Bombay, Powai, Mumbai 400076, India. Contact e-mail: [email protected] K.V. MANI KRISHNA is with the Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. D. SRIVASTAVA is with the Nuclear Fuel Complex, Hyderabad 500062, India. R.D. DOHERTY is with the Metallurgical Engineering and Material Science Department, Indian Institute of Technology-Bombay and also with the Department of Materials Science and Engineering, Drexel University, Philadelphia PA 19104. Manuscript submitted April 2, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
However, it typically involves higher loads. Working at higher temperatures, to take advantage of the lower loads, leads to either a saturation of the flow stress or often an actual fall. Classical text books[9,10] and review articles,[11,12] mainly based on studies of cubic alloys, attributed the former to dynamic recovery; while flow softening was related to dynamic recrystallization (DRx).[13] During dynamic recovery, the original grain structure undergoes a change in shape as determined by the strain imposed. The DRx, on the other hand, involves formation of initially strain-free and usually equiaxed grains. The high-temperature deformation involves complex interplay between dynamic recovery and DRx. For example: extended recovery, often observed in fcc metals with high stacking fault energy, inhibits DRx and leads to flow stress saturation. The fcc metals with lower staking fault energy, on the other hand, undergo DRx and show flow softening after an early peak flow stress. Most reviews of hot deformation[10,14,15] related DRx with flow softening and corresponding appearance of an inflection in the stress dependence of strain hardening, that is d2h/dr2 = 0,[15,16] where r is true stress, e is true strain, and h represents dr/de. d2h/dr2 = 0 is observed prior to the peak stress that corresponds to the formation of a significant fraction of the recrystallized grains. The origin of ‘new’ grains in hot deformed microstructures was related to various proce
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