Superplastic deformation of boron doped Fe–18at.%Si

Superplastic deformation behavior of boron doped Fe–18at.%Si alloy with D03 structure was investigated in tension at strain rates from 2.34×10−5 to 2.34×10−3 s−1 and at temperatures from 973 to 1223 K. The samples with a large initial grain size of 72 μm were H2 gas-quenched immediately after high temperature deformation for microstructural observation and discussion of the mechanism of superplastic deformation. The stress–strain curves are classified into the following two types: continuous work softening and fracture following a sharp stress peak, and steady-state flow following rapid work softening just after yielding. In the continuous work softening type, fine grains less than 10 μm are produced by dynamic recrystallization (DRX) taking place only near prior grain boundaries, followed by fracture occurring along prior grain boundaries. In the steady-state flow type, relatively coarse DRX grains ranging from 28 to 155 μm are developed in entire gage area. A large elongation over 200% is obtained when the DRX grain size is 38 μm. It is concluded from the microstructural observations that superplastic deformation of boron doped Fe–18at.%Si is not associated with grain boundary sliding of DRX grains, but with repeated formation of relatively coarse DRX grains containing subgrain structures, and so retardation of cavity formation and growth.