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Polygonization in zirconium
M E T A L L O G R A P H Y 4, 457-461 (1971)
457
SHORT C O M M U N I C A T I O N S
Polygonization in Zirconium P. MUKHOPADHYAY, S. BANERJEE AND R. KRISHNAN Metallurgy Division, Bhabha Atomic Research Center, Trombay, Bombay, India
Douglass, t in his excellent review on the physical metallurgy of zirconium, has stated that "the study of the recovery behaviour of zirconium and its alloys has been very limited due to the rapid recrystallization kinetics and low temperatures for recrystallization." However, Bostrom and Kulin, z from electrical resistivity measurements of crystal bar zirconium cold-swaged to 97% reduction, reported substantial changes in this property even at temperatures as low as 100°C. But no metallographic evidences of structural changes could be observed even at annealing temperatures up to 400°C. McGeary and Lustman 3 believe that the increase in x-ray intensity for the rolling texture due to low-temperature annealing is caused by the reorientation of crystals into a more perfect rolling texture and suggest that this is possible only if polygonization takes place. It appears that until now no evidence of direct observation of polygonization in zirconium has been reported. This note shows clearly evidences for polygonization from transmission electron microscopic observations. Iodide-purity crystal bar zirconium slices were rolled at room temperature. Some slices were given 25% reduction in thickness, and others were cold-rolled to about 72%. The samples were vacuum-annealed at 480°C for times of 5 minutes, 25 minutes, and 125 minutes. Thin foils suitable for transmission electron microscopy were prepared by a chemical polishing in a solution of HF, HNO3, and water followed by an electrolytic polishing in a perchloric-acetic acid bath. Numerous thin foils have been examined, and, generally speaking, it can be stated that the processes of recovery and recrystallization were strikingly inhomogeneous in the scale of electron microscopic observations. Though the recrystallization process was found to occur rather rapidly, one could clearly see evidences of polygonized structures, some of which are reproduced here. Figure 1 shows the structure obtained after a 25-minute anneal of a 25% coldworked sample. An extensive array of dislocation network can be seen, suggesting that the material is in a polygonized state. No evidence of recrystallization could Copyright © 1971 by American Elsevier Publishing Company, Inc.
458
P. Mukhopadhyay, S. Banerjee and R. Krishnan
FIc. 1 . - - T h e recovery structure of zirconium showing dislocation network. Magnification 54,000 × .
FIG. 2.--Micrograph showing ordered arrays of dislocations and dislocation networks which are typical of a polygonized structure. Magnification 25,000 × .
Polygonization in Zirconium
459
be detected. Figure 2, which is a structure obtained on annealing the same material for 125 minutes, shows a polygonized structure as evidenced by dislocation networks (marked by a circle) and ordered arrays of dislocations at many places (marked by arrows). One can see that the dislocation nodes are not extended, indicating that the stacking-fault energy of the material is high. At places marked A, one can see nuclei for recrystallization emerging, and it appears that certain cells grow at the expense of others, as can be seen from the vanishing cell boundaries. This observation is consistent with that of Bailey, 4 who observed subgrain formation preferentially in the microband regions by a coalescence process during which some sub-boundaries disappear without migration. Figure 3 is another example of ordered array obtained under identical treatment.
FIc. 3. Arrangement of parallel dislocations to form a tilt boundary. Magnification 46,000 x. Figures 4 and 5 correspond to structures obtained with 72% reduced material after an anneal of 5 minutes at 480°C. One can see clearly recrystallized grains growing at the expense of polygonized structure. Cell structure and vanishing sub-boundaries along with well-developed high-angle boundaries can be seen at many places. One can also notice that tangled dislocation walls have become thinner at several places. In addition to these structural observations, selected area diffraction patterns were taken from just-recrystallized material (corresponding to a treatment of 25 minutes of a 72% reduced material), and the texture developed in this
460
P. Mukhopadhyay, S. Banerjee attd R. Krishnan
FIG. 4. Coalescence of subgrains due to vanishing subboundaries. Magnification 38,000 ×
FIG. 5. A polygonized region surrounded by few expanding recrystallized grains. Sharp cell walls between the polygonized grains appear prominently. Magnification 25,000 ×.
Polygonization in Zirconium
461
material was determined. This was found to be different from the recrystallization texture of zirconium as reported from x-ray measurements, suggesting the possibility that recrystallization in zirconium proceeds more by an oriented growth than by an oriented nuclei mechanism. We should like to thank Drs. M. K. Asundi and V. K. Moorthy for their keen interest and encouragement during the course of these investigations.
References 1. D. L. Douglass, At. Energy Rev., 1 (1963) 71. 2. W. A. Bostrom and S. A. Kulin, Zirconium and Zirconium Alloys, American Society for Metals, Cleveland, Ohio, 1953, p. 186. 3. R. K. McGeary and B. Lustman, Trans. AIME, 197 (1953) 284. 4. J. E. Bailey, Electron Microscopy and Strength of Crystals, John Wiley, 1963, p. 535.
Accepted February 5, 1971
457
SHORT C O M M U N I C A T I O N S
Polygonization in Zirconium P. MUKHOPADHYAY, S. BANERJEE AND R. KRISHNAN Metallurgy Division, Bhabha Atomic Research Center, Trombay, Bombay, India
Douglass, t in his excellent review on the physical metallurgy of zirconium, has stated that "the study of the recovery behaviour of zirconium and its alloys has been very limited due to the rapid recrystallization kinetics and low temperatures for recrystallization." However, Bostrom and Kulin, z from electrical resistivity measurements of crystal bar zirconium cold-swaged to 97% reduction, reported substantial changes in this property even at temperatures as low as 100°C. But no metallographic evidences of structural changes could be observed even at annealing temperatures up to 400°C. McGeary and Lustman 3 believe that the increase in x-ray intensity for the rolling texture due to low-temperature annealing is caused by the reorientation of crystals into a more perfect rolling texture and suggest that this is possible only if polygonization takes place. It appears that until now no evidence of direct observation of polygonization in zirconium has been reported. This note shows clearly evidences for polygonization from transmission electron microscopic observations. Iodide-purity crystal bar zirconium slices were rolled at room temperature. Some slices were given 25% reduction in thickness, and others were cold-rolled to about 72%. The samples were vacuum-annealed at 480°C for times of 5 minutes, 25 minutes, and 125 minutes. Thin foils suitable for transmission electron microscopy were prepared by a chemical polishing in a solution of HF, HNO3, and water followed by an electrolytic polishing in a perchloric-acetic acid bath. Numerous thin foils have been examined, and, generally speaking, it can be stated that the processes of recovery and recrystallization were strikingly inhomogeneous in the scale of electron microscopic observations. Though the recrystallization process was found to occur rather rapidly, one could clearly see evidences of polygonized structures, some of which are reproduced here. Figure 1 shows the structure obtained after a 25-minute anneal of a 25% coldworked sample. An extensive array of dislocation network can be seen, suggesting that the material is in a polygonized state. No evidence of recrystallization could Copyright © 1971 by American Elsevier Publishing Company, Inc.
458
P. Mukhopadhyay, S. Banerjee and R. Krishnan
FIc. 1 . - - T h e recovery structure of zirconium showing dislocation network. Magnification 54,000 × .
FIG. 2.--Micrograph showing ordered arrays of dislocations and dislocation networks which are typical of a polygonized structure. Magnification 25,000 × .
Polygonization in Zirconium
459
be detected. Figure 2, which is a structure obtained on annealing the same material for 125 minutes, shows a polygonized structure as evidenced by dislocation networks (marked by a circle) and ordered arrays of dislocations at many places (marked by arrows). One can see that the dislocation nodes are not extended, indicating that the stacking-fault energy of the material is high. At places marked A, one can see nuclei for recrystallization emerging, and it appears that certain cells grow at the expense of others, as can be seen from the vanishing cell boundaries. This observation is consistent with that of Bailey, 4 who observed subgrain formation preferentially in the microband regions by a coalescence process during which some sub-boundaries disappear without migration. Figure 3 is another example of ordered array obtained under identical treatment.
FIc. 3. Arrangement of parallel dislocations to form a tilt boundary. Magnification 46,000 x. Figures 4 and 5 correspond to structures obtained with 72% reduced material after an anneal of 5 minutes at 480°C. One can see clearly recrystallized grains growing at the expense of polygonized structure. Cell structure and vanishing sub-boundaries along with well-developed high-angle boundaries can be seen at many places. One can also notice that tangled dislocation walls have become thinner at several places. In addition to these structural observations, selected area diffraction patterns were taken from just-recrystallized material (corresponding to a treatment of 25 minutes of a 72% reduced material), and the texture developed in this
460
P. Mukhopadhyay, S. Banerjee attd R. Krishnan
FIG. 4. Coalescence of subgrains due to vanishing subboundaries. Magnification 38,000 ×
FIG. 5. A polygonized region surrounded by few expanding recrystallized grains. Sharp cell walls between the polygonized grains appear prominently. Magnification 25,000 ×.
Polygonization in Zirconium
461
material was determined. This was found to be different from the recrystallization texture of zirconium as reported from x-ray measurements, suggesting the possibility that recrystallization in zirconium proceeds more by an oriented growth than by an oriented nuclei mechanism. We should like to thank Drs. M. K. Asundi and V. K. Moorthy for their keen interest and encouragement during the course of these investigations.
References 1. D. L. Douglass, At. Energy Rev., 1 (1963) 71. 2. W. A. Bostrom and S. A. Kulin, Zirconium and Zirconium Alloys, American Society for Metals, Cleveland, Ohio, 1953, p. 186. 3. R. K. McGeary and B. Lustman, Trans. AIME, 197 (1953) 284. 4. J. E. Bailey, Electron Microscopy and Strength of Crystals, John Wiley, 1963, p. 535.
Accepted February 5, 1971