The Mechanism of Deformation of Fe-Ni Alloys in a Low Temperature Range

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  • Mukai Shizuo
    Department of Metallurgy, Kyushu Institute of Technology
  • Fujiwara Makoto
    Department of Metallurgy, Kyushu Institute of Technology. Present address: Takada Industries, Ltd., Kitakushu

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  • Fe-Ni 合金の低温域における変形機構の考察
  • Fe-Ni ゴウキン ノ テイオンイキ ニ オケル ヘンケイ キコウ ノ コウサツ

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In order to make clear the deformation mechanism of Fe-Ni alloys contained Ni to 9% and commercial 9%Ni steel in a low temperature range, the effects of compositions, metal structures and strain-levels on the activation parameters in deformation have been investigated using tensile strain rate controlled tests, and some considerations of the ductile-brittle transition have been attempted. The results obtained are as follows:<BR>(1) The rate-process dislocation mechanism of Fe-Ni alloys in a low temperature range is considered to be the Peierls-Nabarro stress.<BR>(2) The deformation characteristics of alloys which have mixed structures of ferrite and martensite, containing few percent of austenite like a 9%Ni steel, are the same as those of all ferrite structure.<BR>(3) The stress increment Δτ with increasing strain rate tends to increase gradually with an in crease in austenite content in the ferrite matrix, but no significant variation in activation volume with austenite content is observed.<BR>(4) From the viewpoint of physical meanings, the value of TV*=0 obtained by extrapolating V* to zero in the V* vs T1⁄2 diagrams appears to be related to the micro-crack transition temperature Tm. There is a good correspondence between TV*=0 and Tm in Fe and Fe-3.3%Ni. For Fe-6.9%Ni, Fe-9.4%Ni and 9%Ni steel, their correspondence is not made clear because the brittle fracture cannot be observed even at 77°K and accordingly TV*=0 is estimated below 77°K.

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