Yielding Phenomenon in a 3 wt%Si-Fe Alloy at High Temperatures

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  • Sakai Taku
    Department of Mechanical Engineering, The University of Electro-communications, Chofu-shi
  • K\={o}zai Masafumi
    Graduate School, The University of Electro-communications
  • \={O}hashi Masayuki
    Department of Mechanical Engineering, The University of Electro-communications, Chofu-shi
  • Sakui Seita
    Department of Mechanical Engineering, The Science University of Tokyo

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Other Title
  • 3%Si-Fe合金における高温降伏現象
  • 3パーセント Si Fe ゴウキン ニ オケル コウオン コウフク ゲンショウ

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In a 3%Si-Fe alloy with mean grain sizes from 13.6 to 80.9 μ, a new yielding accompanied by a prominent yield drop was observed when it was pulled only under a certain test condition at high temperatures. The characteristics of this high-temperature yielding phenomenon were studied by tensile tests at various temperatures from room temperature to 800°C with strain rates varying from 1 to 10−5 l/sec. The results were as follows: (1) The relation between the yield stress and grain size was approximately represented by the Hall-Petch equation from room temperature to 800°C. (2) In the region where the high-tmperature yielding was observed, the grain size effect Ky. D−1⁄2, was nearly independent of temperature and strain rate, but the friction stress, σ0, was strongly sensitive to them. The yielding phenomenon was observed in the region of lower temperature and higher strain rate in this thermal deformation range or only in the region of friction stress between an upper bound and a lower bound, independent of the actual temperature or strain rate. At the lower temperatures and higher strain rates furthermore, friction stress was nearly independent of temperature and strain rate, indicating the athermal deformation behaviour. (3) Friction stress, σ0, in the thermal deformation range can be correlated with temperature, T, and strain rate, \dotε, approximately by the follwing deformation equation.<BR>(This article is not displayable. Please see full text pdf.) <BR>\oindentwhere τ0=1⁄2·σ0, ΔH0′ is 93.5 kcal/mol and the values of exponent m decrease from 5.1 to 4.0 with increasing temperature. ΔH decreases from 93.5 kcal/mol to 79.5 kcal/mol with increasing τ0 from 1 kg/mm2 to 9 kg/mm2 and the activation area calculated using the above equation decreases from about 140 b2 to 35 b2 with increasing τ0 from 2 kg/mm2 to 9 kg/mm2. (4) The heterogeneous deformation related to this yielding phenomenon was not observed and the density of etch-pit which was considered to be related to each dislocation increased rapidly at the yielding.<BR>From these experimental results it is considered that the values and strain rate dependence of friction stress in the region where the high-temperature yielding occurs are sufficiently large and mobile dislocations multiply rapidly at the yielding and therefore this phenomenon may be interpreted as arising from the mechanism proposed originally by Johnston. The yield deformation in this region is likely to be controlled by the process based on a short range interaction between gliding dislocations and localized obstacles.

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