Anomalous pore morphologies in liquid-phase-sintered Al-Zn alloys
- PDF / 1,114,694 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 22 Downloads / 244 Views
addition to this, there is also a shadowing effect; i.e., there is a narrow shadow just below a column, where no droplets impinge. The porous column formation can be prevented by avoiding a mold or deposit surface, which lies nearly parallel to the spray axis, or by directing the spray axis perpendicular to the mold or deposit surface.
The authors thank Ford Motor Co. (Grant No. FMC22868) for funding this investigation, and ZHL appreciates the support of KAIST for his sabbatical visit at the University of California–Irvine. REFERENCES 1. E.J. Lavernia and Y. Wu: Spray Atomization and Deposition, John Wiley & Sons, Inc., New York, NY, 1996. 2. P.S. Grant: Progr. Mater. Sci., 1995, vol. 39, pp. 497-545. 3. K.M. McHugh: Solidification 1998, TMS, Warrendale, PA, pp. 427-38. 4. R.L. Kozarek, M.G. Chu, and S.J. Pien: Solidification 1998, TMS, Warrendale, PA, pp. 461-71. 5. P. Mathur, S. Annavarapu, D. Apelian, and A. Lawley: Mater. Sci. Eng. A, 1991, vol. 142, pp. 261-76. 6. B.C. Moon and Z.H. Lee: J. Kor. Inst. Metall. Mater., 1994, vol. 32, pp. 923-30.
of tungsten heavy alloys under space-based microgravity conditions can produce a number of unusual microstructural features.[1,2,3] One such observation was spherical pores trapped inside solid grains. While grains do commonly contain pores during solid-state sintering under normal gravitational conditions, it is an unusual occurrence during liquid phase sintering, where pores are either mobile and are removed by bouyancy forces or they become attached to solid liquid interfaces.[4] The spheroidicity of the internal pores is not unexpected since that would provide the minimum energy. The location, not the morphology, is the puzzling phenomenon, although similar pore locations have been observed in Mo-Ni, which had been cycled between 1300 8C and 1460 8C.[5] The mechanism in that case was supposed to be Ostwald ripening. A second unusual observation of microgravity sintered liquid phase microstructures is the coexistence of liquid and vapor phases inside a solid grain. Here, pores are contained in liquid pools contained within solid grains. This was explained on the basis of microgravity and its effect on buoyancy, mass transport, and capillarity. Notwithstanding a definitive model, these remain unusual microstructures for liquid-phase-sintered systems. Here, we show that similar microstructural features can develop by a different mechanism during terrestrial experiments in Al-Zn. Specimens with a composition of Al-8Zn-2.5Mg-1Cu (all compositions in wt pct) were compacted from premixed, elemental powders in a floating, cylindrical die at 125 MPa with 0.5 pct stearic acid admixed as a die lubricant. The powder characteristics are presented in Table I. Samples were dewaxed at 300 8C for 20 minutes, sintered at 620 8C under dry nitrogen (dew point ,240 8C) for times up to 2 hours, and water quenched. The heating rate was 10 8C/min. In order to examine the microstructure during sintering, the cycle was interrupted at various stages by quenching samples into water. Samples tha
Data Loading...
