applied pressure on metallic glasses

In general, applied pressure on metallic glasses might have three effects ^[1]:

• The first effect is densification, which reduces the free volume in the glass by pressure. This effect could favor the crystallization process because crystallization of metallic glasses involves densification.
• The second effect is suppression of atomic mobility at high pressures, which reduces the atomic diffusion in metallic glasses. At least for eutectic or primary crystallization processes this effect retards crystallization since atomic diffusion is required for these crystallization processes.
• The third effect is due to changes in relative Gibbs free energies of the glassy phase and crystalline phases by pressure, and in the energy barrier for crystallization. This effect could alter the relative amounts of the crystalline phases in the crystallized samples, and could be of either sign with regard to dTx /dp.

The effects can alter the sequence or the relative volume fraction of crystalline phases in the hot-pressed samples. For a certain amorphous phase, its crystallization behavior under pressure may depend on which is the dominating factor among the three effects.

1. J.Z. Jiang, J.S. Olsen, L. Gerward, S. Abdali, J. Eckert, N. Schlorke-de Boer, L. Schultz, J. Truckenbrodt and P.X. Shi, J. Appl. Phys. 87 (2000), pp. 2664–2666.

Sintering mechanisms for Metals

Sintering mechanisms for Metals (right for metallic glasses):

• Evaporation -> vapor diffusion --> Condensation
• Surface diffusion from surface to neck
• Bulk diffusion from surface to neck
• Grain boundary diffusion from grain boundary to neck
• Bulk diffusion from grain boundary to neck
• Plastic flow by dislocation motion

Alumina densification rates (0.8 and 1.3 mm mean, log-normal) illustrate sintering stages:

Stage I: Initial sintering

• Surface smoothing
• Grain boundary formation, neck growth
• Rounding of interconnected open pores
• Small, slow reduction of porosity, increase in density

Stage II: Intermediate sintering

• Shrinkage of open pores intersecting grain boundaries
• Lots of fast porosity reduction
• Small amount of grain growth

Stage III: Final sintering

• Pores close at 92% dense
• Closed pores shrink to a limited size, or vanish
• More grain growth, leading to internal pores within grains
• Metals often dissolve all gases, leading to 100% density

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