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Synopsis:Tensile deformation behaviour of two-phase iron alloys with ductile second phase is studied and tried to be understood by the difference in strain between the harder phase and the softer phase. The alloys used are classified into the three groups; Fe-Cr-Ni alloys, Fe-Ni-C alloys and Fe-C alloys composed of austenite and ferrite, austenite and martensite, and ferrite and martensite respectively with various volume fractions. The mean effective grain size of each alloy is adjusted to be nearly same by heat treatment .The main results obtained are as follows:(1) The tensile deformation behaviour of a two-phase alloy is dependent on the C-value (=0.2% proof stress of the harder phase/0.2% proof stress of the softer phase), volume fraction and shape of the harder phase. This is because the degree of strain-inequality m (m =e2/e1:e1 is the mean strain of the softer phase and e2 is that of the harder phase) is affected by these factors.(2) The flow stress of a two-phase alloy with small C-value (smaller than about 3) is estimated by the strain-equal model (m=1).(3) The m-value of a two-phase alloy with nearly spherical second phase decreases with an increase in C-value and its tensile deformation behaviour approaches to the stress-equal model (m=0).In this case, the m-value is decreased with decreasing the volume fraction of the harder phase. When the volume fraction of the harder phase becomes close to unity, the deformation of the softer phase is strongly restricted by the around harder phase, so that the tensile deformation behaviour of a two-phase alloy approaches to the strain-equal model, independent on C-value.(4) The m-value is slightly affected by the overall strain of a two-phase alloy. It is small at small strain and increases with an increase in strain.(5) The tensile deformation behaviour of a two-phase alloy with long fiberous or disc second phase whose largest axis is parallel to the tensile direction is nearly shown by the strain-equal model even if Cvalue is large.

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