Crystal Nucleation in Pd-Si Alloys
- PDF / 284,720 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 27 Downloads / 201 Views
81
CRYSTAL NUCLEATION IN Pd-Si ALLOYS A.J. DREHMAN AND D. TURNBULL Division of Applied Sciences, Harvard University, Cambridge, MA 02138, USA ABSTRACT We report here a study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd84.5Si15.5 eutectic composition. Molten droplets are released at the top of a drop tube and solidify (to either a This provides crystalline or glassy state) during descent. a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. We found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with our previously published study of nucleation in 0.06mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube. INTRODUCTION A glass is formed when a liquid is cooled to a temperature at which it "freezes" kinetically while bypassing crystallization. In order to avoid crystallization of the undercooled liquid, cooling must be sufficiently rapid that significant nucleation and subsequent growth of the crystalline phase does not occur. As yet there is little information on whether crystal nucleation in Heteroglass forming melts is effected homogeneously or heterogeneously [1]. geneous nucleation can, in principle, be avoided by the removal of all heterogeneities (container walls, motes, surface oxides, etc.) while only sufficiently high cooling rates can prevent significant homogenous nucleation and growth. Previously [2] we reported the solidification behavior of undercooled Pd83Si and Pd 8 2 Si 1 8 liquid droplets falling through a He-H 2 atmosphere in a 1.2 meter drop tube. Solidified droplets were found to be either entirely crystalline or entirely glassy, based on the calorimetric measurement of the heat of crystallization. The fraction of glassy droplets, of diameter d, was found to This behavior is consistent with the asfit the relation: x = exp[-(d/d 0 ) 2]. sumption that crystallization is initiated entirely by a time independent number 2
of pre-existing surface nuclei, N5 , where Ns = 1/rd . This interpretation is supported by the observation that the value of N5 required to fit the data was decreased by a factor of 3.4 when the gas inside the drop tube was dried and the graphite atomizer (which formed the droplets) was baked out. No evidence was found for the occurrence of homogeneous nucleation, even in 0.33 mm Pd82Si18 droplets cooled at 5 x 103 K/sec. This implies that the maximum homogeneous nucleation frequency in Pd82Si1 is less than 5 x 105 nuclei/cm 35 3sec, which is
82
He Gas In Pd 8 2 Si18 Scan rate: 10 K/min
IIIU1I
%A 01
i
l
y
t
Pyrex Tube Molten Droplet
H
Baseline•
One-Way Valved;n
630 To Vacuum Pump Brass Bottom Fig.1. Schematic of drop tube used for containe
Data Loading...
