Decomposition and primary crystallization in undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 melts

Abstract

bulk metallic glasses were prepared by cooling the melt with a rate of about 10 K/s and investigated with respect to their chemical and structural homogeneity by atom probe field ion microscopy and transmission electron microscopy. The measurements on these slowly cooled samples reveal that the alloy exhibits phase separation in the undercooled liquid state. Significant composition fluctuations are found in the Be and Zr concentration but not in the Ti, Cu, and Ni concentration. The decomposed microstructure is compared with the microstructure obtained upon primary crystallization, suggesting that the nucleation during primary crystallization of this bulk glass former is triggered by the preceding diffusion controlled decomposition in the undercooled liquid state. © 1995 American Institute of Physics.In the last decade, phase separation in the amorphous or undercooled liquid state has been studied in a large variety of metallic glasses. Decomposition was, for example, reported in the Ti-Zr-Be system, 1,2 in Zr-Cu, 3,4 , Zr-Co, 5,6 and lately in the La-Al-Ni 7 and the Zr-Y-Ni-Al 8 bulk metallic glass ͑BMG͒ forming systems. In BMG forming alloys, phase separation becomes an important factor in understanding the crystallization process and the glass forming ability.The novel multicomponent BMG formers that, for example, additionally include Zr-Al-Ni, 9 Zr-Al-Cu-Ni, 10and Zr-Ti-Cu-Ni-Be, 11 exhibit an extremely high glass forming ability. For the Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 alloy, a cooling rate of only 1 K/s is required to suppress crystallization and to form a metallic glass.12 If one assumes classical nucleation from a homogeneous undercooled liquid state below the melting point or a supercooled liquid state above the glass transition, this implies very small nucleation rates for crystallization at relatively high diffusion constants 13 and should result in a rather coarse microstructure upon crystallization. However, in contradiction to this, there are indications in several BMG forming alloy systems that primary crystallization results in a microstructure consisting of nanocrystals with a very high number density embedded in an amorphous matrix. 14 -16 In this letter we will show that the Zr 4.12 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 alloy decomposes during undercooling in the liquid state. This decomposition appears to be a necessary precursor for the nucleation of crystals and further results in a uniform distribution of crystals on a very fine length scale during primary crystallization. Results of atom probe field ion microscopy ͑AP/FIM͒ and transmission electron microscopy ͑TEM͒ will be shown to support this description.Amorphous alloy ingots with a nominal composition of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 were prepared from a mixture of the elements of purity ranging from 99.5% to 99.9% by induction melting on a water-cooled silver boat under a Ti gettered argon atmosphere. Several pieces were remelted together in a 1 2 inch inner diameter silica tube and then water quenched, resulting in a 4 ...

Pronounced asymmetry in the crystallization behavior during constant heating and cooling of a bulk metallic glass-forming liquid

“…15,16,4 These concentration modulations partially shift the composition towards the composition of the primarily solidified phase, which causes an increase of the nucleation probability. The resulting high nucleation rate would also lead to a faster crystallization process upon reheating.…”

“…In a complex system like Vit 1 an adequate description of the nucleation process will certainly have to go beyond the concept of steady-state nucleation. 5 Further influences such as, for example, decomposition processes, have to be considered [15][16][17] to explain the fine microstructures of Vit 1. 5 At this point, however, to discuss the different growth kinetics on crystallization the simplified approach of steady-state nucleation serves as a sufficient model.…”

Experimental determination of a time–temperature-transformation diagram of the undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy using the containerless electrostatic levitation processing technique

“…Based on DSC experiments, AP/FIM results, 10 and transmission electron microscopy and small angle neutron scattering studies, 11 we believe that for these undercoolings a phase separation ͑event I͒ into Be-poor and Be-rich regions precedes the nucleation of crystals. According to Schneider et al, 11 the primary crystallization ͑event II͒ can be attributed to the formation of a nanocrystalline Be-poor and Tirich f.c.c.…”

“…9 This phase separation was confirmed by atom probe field ion microscopy ͑AP/FIM͒ measurements on samples cooled with a rate of approximately 10 K/s. 10 The schedule of the construction of the TTT diagram is added in Fig. 1, indicating the isothermal experiments to determine the time required to reach the onset of crystallization at dif- ferent isothermal temperatures.…”

Small-angle x-ray-scattering study of phase separation and crystallization in the bulk amorphousMg62Cu25Liu1, Johnson2, Schneider3 et al. 1999Phys. Rev. BSelf Cite

“…In addition, crystallization mechanisms can be delineated in great detail in these systems. In a number of multicomponent metallic-glass-forming systems such as La-Al-Ni, 2 Zr-Y-Ni-Al, 3 and Zr-Ti-Cu-Ni-Be, [4][5][6] phase separation in the amorphous or supercooled liquid states has been observed and identified to be closely related to the primary crystallization process. Studies of phase separation therefore become important in understanding the thermal stability and the crystallization processes in metallic glasses.…”