Crystallization Pathway in the Bulk Metallic Glass Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5
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Mat. Res. Soc. Symp. Proc. Vol. 455 ©1997 Materials Research Society
time- and temperature evolution of the decomposition process. In particular, we show that above a critical temperature crystallization behavior changes significantly. Experimental Amorphous samples were prepared from a mixture of the pure elements by induction melting on a water-cooled silver boat under Ti gettered Ar atmosphere. The ingots were remelted in a silica tube with an inner diameter of 10mm and then quenched with a cooling rate of about 10K/s. For the SANS investigations disks with a thickness of 2.7mm were cut from the these rods and isothermally annealed at temperatures between 608K and 673K for different times. The SANS measurements were carried out at the Intense Pulsed Neutron Source at Argonne National Laboratory. Results and Discussion Figure 1 shows SANS data of an as prepared Zr,41 2Ti13 Cu125NiBe22.5 alloy and samples isothermally annealed for different times at 613K. The data for the as- prepared sample only depict background scattering, for the aged ones broad maxima appear. For early stages a maximum of scattering intensity is observed at q=0.05A,1, indicating spatially correlated chemical inhomogeneities in the amorphous state.
1.2 1.0
E 0.8 0
"- 0.6 T0.4 0.)
0.2 0.0 4! 0.00
0.05
0.10
0.15
0.20
0.25
q(A-1) Fig. 1: SANS data of the Zr 41 2Ti 138Cu 12.5 Ni1oBe
225 alloy
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aged at 613K for different times.
The wavelength corresponding to the maximum intensities is about 125A. The position of qmax shifts slightly towards smaller values with increasing annealing duration suggesting a coarsening of the chemical inhomogeneities during isothermal heat treatment. We assume a decomposition process being responsible for the scattering contrast [7]. From estimations of the increase of scattering intensity and 62Ni isotope experiments we conclude that the initial increase is mainly associated with a decomposition process of Ti and Be atoms [8].Two different time laws are involved in the process (Fig. 2). First, the scattering cross section for a constant q value grows exponentially during annealing, and the interference peak shifts slightly to lower q. In the very early stages of decomposition the slowly rising exponential time law mimics an incubation time of about 150min at 613K. During that time no changes in structure or composition are observed by SANS or thermal analysis [7]. Then the scattered intensity increases very rapidly. Later a much slower process overtakes control on the increase of scattering cross section. It is not clear yet whether the time dependence for the later stages is also exponential or a power law. Cahn's theory of spinodal decomposition gives such an exponential growth law [9] but does not predict a shift of the interference peak to lower q with increasing time. Such a behavior is proposed in [10]. During phase separation, composition and short-range atomic order [11] in at least one of the decomposed regions approach those of a competing crystalline phase and the nucleation probabil
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