The Influence of Growth Rate on Porosity in Al-Pd-Mn Icosahedral Quasicrystals
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The Influence of Growth Rate on Porosity in Al-Pd-Mn Icosahedral Quasicrystals. Amy R. Ross†, Ian R. Fisher*, Paul C. Canfield†‡, and Thomas A. Lograsso†§ † Ames Laboratory and, ‡ Department of Physics and Astronomy, and § Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011-3020, USA, * Stanford University, Stanford, Ca 94305, USA
ABSTRACT Growth experiments have been carried out to characterize the occurrence and development of porosity in Bridgman and flux grown Al-Pd-Mn icosahedral quasicrystals. The porosity level has been observed to fluctuate between values of 0.0 and 3.75 percent along the length of Bridgman single crystals implying that the development of porosity is affected by the local growth conditions. Experiments were conducted to evaluate the influence of the rate of solidification on the occurrence of porosity. Alloys were solidified with different growth rates, 1mm/hr and >10 mm/hr, using the Bridgman configuration and at different cooling rates, ranging from 0.29°C/hr to 10°C/hr, using the flux growth method. Porosity levels were analyzed via optical image analysis. These experiments indicate that porosity percentages are greatly influenced by cooling rates and crystal size. INTRODUCTION Many experiments have been conducted to determine the source of porosity in Al-Pd-Mn quasicrystals since first reported by Beeli in 1992 [1]. Beeli, Gödecke and Lück proposed that quasicrystal pores formed via thermal vacancy migration and condensation [3]. Additional work by Beeli et al. confirmed that pore size and spacing corresponded to diffusion models [4]. Two other hypotheses regarding the origin of porosity were based on quasicrystalline structural arguments [5,6]. The self-similar model proposes that porosity develops from specifically located vacancies that are inherent in the quasicrystal structure [5]. A modification of the selfsimilar model, the random covering theory, maintains the inherent and specifically located vacancies but increases the ability for deviation from the perfect arrangement by reducing the strict scaling laws [6]. Bridgman experiments studying how growth conditions affected porosity characteristics determined that some aspect of solidification influenced porosity content [7]. These experiments which documented the porosity occurrence, distribution, and characteristics, revealed a striking fluctuation in porosity level within any given sample. In contrast, the flux-grown quasicrystals were found to be virtually pore free [10]. The significant difference in the two crystal’s porosity characteristics invite comparison, however, crystals grown with these two processes cannot be directly compared because their thermal histories are not the same. Bridgman crystals are grown with cooling rates greater than those of the flux method and undergo some annealing as the crystal is continuously cooled to room temperature. Alternatively, crystals grown with the flux method are quenched after cooling slowly through a predetermined temperature range. Two sets of e
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