Crystallization kinetics of amorphous magnesium-rich magnesium-copper and magnesium-nickel alloys
- PDF / 371,061 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 23 Downloads / 172 Views
TATE phase transformations are important means for the adjustment of the microstructure and, thus, the tuning of the properties of materials. In order to exploit this tool to its fullest extent, much effort is spent on the modeling of phase transformations. A general phase transformation model is presented in Reference 1. This model was shown to describe the crystallization of a quaternary metalmetalloid glass very well.[2] As another, possibly less complicated, model system, crystallization of binary metal-metal glasses has been studied in the present project. The considered model systems are MgyX1⫺y alloys (X ⫽ Ni or Cu and 0.82 ⬍ y ⬍ 0.89), which can be made amorphous around the Mg-rich eutectic composition.[3,4] The availability of thermodynamic[3,5–7] and crystallographic[8,9,10] data is helpful for the modeling of the crystallization of the alloys. The thermodynamic stable phases in the studied systems are Mg and Mg2X, which are crystallographically similar.[8] However, during the crystallization of Mg-Ni, an unknown metastable phase is formed, which is not the case in the Mg-Cu system.[3,4]
polymorphous crystallization, the compositions of the crystalline product phase and the amorphous parent phase are equal; no volume diffusion takes place. Upon primary crystallization, one crystalline product phase of composition different from the overall alloy composition precipitates from the parent amorphous matrix. Long-range material transport is needed during primary crystallization. Further, at least one subsequent process (for crystallization of the remaining amorphous matrix) is needed to arrive at the thermodynamic stable state. Upon eutectic crystallization, crystalline product phases of different composition form simultaneously. Depending on the temperature gradient in the material and on the growth velocity of the new phases, the microstructure becomes lamellar or fibrous (absence of long-range diffusion), or globular (occurrence of long-range diffusion).[12] Apart from the inherent occurrence of two steps during a primary crystallization process,[13,14] another reason for two steps in a crystallization process is the precipitation of a metastable phase, followed by transformation of the metastable phase to a phase of higher stability.[11] B. Phase Transformation Kinetics
II. THEORETICAL BACKGROUND A. Crystallization The crystallization of an amorphous alloy takes place in a polymorphous, primary, or eutectic fashion.[11] Upon
In general, the progress of a phase transformation, defined as the fraction transformed, f depends on a path variable, . This variable,  is given by the integral of the temperature (T ) dependent, reaction-rate constant, k, over time (t):[1,15]
⫽ A.T.W. KEMPEN, Postdoctoral Student, Max Planck Institute for Metal Research and Institute for Physical Metallurgy, H. NITSCHE, Doctoral Student, Max Planck Institute for Metal Research and Institute for Physical Metallurgy, F. SOMMER, Professor and Research Associate, Max Planck Institute for Metal Research, and E.J. MITTEMEIJER, Directo
Data Loading...
