| An investigation into the effect of A₃-transformation on sintering has been carried out for the compacts prepared from powder s of pure Fe and Fe+Cu mixture, and for the Cu-infiltrated Fe powder compacts. The approach has been to employ the techniques of density and porosity measurement, and metallographic examination for the specimens sintered at various stages under isothermal (at 895℃: α-range and 925℃: γ-range) and non-isothermal (cyclic between 895℃ and 925℃) conditions. Results obtained can be summarized as follows; (1) For pure Fe powder compacts, the repetition of A₃-transformation during sintering caused a negative effect on densification while isothermal heating at the temperature of α-range showed a relatively high densification. Isothermal sintering at γ-range, however, was found to be similar to that of cyclic sintering. (2) In the cases for mixed and infiltrated specimens, densification was considerably improved by the cyclic heating as compared to either of isothermal sintering at α-or γ-range. This enhancement in densification was high especially for the case of infiltrated specimen. The above mentioned negative effect on densification is believed to be resulted from the irreversible destruction of contact points due to the volume changes at the moments of allotropic transformation α⇔γ, and the lack of a suitable constraint to maintain or to improve sintering condition during the cyclic treatment. And the improved sinterability upon the cyclic heating for mixed and infiltrated specimens can be explained as follows: (1) Bonding between Fe and Cu at early stage could provide the proper constraint to the particles in a compact, (2) under this constraint, particles in the sinter mass may experience the repeating local stress at the contact points, and (3) these stresses could be responsible for the transformational superplasticity and also for the enhanced diffusion mechanisms, such as grain-boundary-short-circuiting, viscous creep, and so on. |
|
