チタン合金の変形応力:ひずみ曲線におけるひずみ速度依存性

Abstract

Quantitative expression of stress-strain curve including strain rate dependence for Ti and its alloys was studied at temperatures between 77 and 355 K. In order to evaluate an independent component of the strain rate, in the first place, it was attempted to obtain the stress-strain curve as the curve at zero strain rate through relaxation tests under constant crosshead displacement. The relaxation-saturated stress-strain points made a single curve. This single curve was named the 'base curve'. The base curve was good fitted to the Swift's equation in the following form: sigma(sub Base)(epsilon) = A(epsilon + b)(exp n), where sigma(sub Base)(epsilon) is the stress on the base curve, epsilon the plastic strain, n the exponent, and A and b are coefficients. The stress-strain curves at a strain rate between 2.8 x 10(exp -5) and 3.0 x 10(exp -2)/sec were parallel to the base curve. Namely, the strain-rate-dependent component, sigma(sup *), was independent of strain at a constant strain rate. The relation between sigma(sup *) and strain rate, gamma, is expressed in the following form: sigma(sup *) = B x gamma(exp m), where B is a coefficient and m is the exponent. Finally, the equation, sigma = A(epsilon + b)(exp n) + B x gamma(exp m), is derived for the expression of flow stress-plastic strain relation under the deformation at a constant strain rate.

チタンおよびチタン合金に関し、ひずみ速度依存性を含めた応力-ひずみ曲線の定量的表式を77?355Kの温度範囲で研究した。ひずみ速度の非依存成分を評価するために、まず、定クロスヘッド変位条件下の緩和試験を通してひずみ速度零における曲線としての応力-ひずみ曲線を求めることを試みた。緩和-飽和応力-ひずみ点は1重曲線を作る。この1重曲線を「Base曲線」と名付けた。Base曲線は以下の形のSwiftの方程式、σ(sub Base)(ε)=A(ε+b)(exp n)とよく合致した。ここで、σ(sub Base)(ε)はBase曲線上の応力、εは塑性ひずみ、nは指数、Aとbは係数である。ひずみ速度が2.8×10(exp -5)?3.0×10(exp -2)/秒の範囲の応力-ひずみ曲線はBase曲線に平行であった。すなわち、ひずみ速度依存成分σ(sup *)は定ひずみ速度のひずみには依存しない。σ(sup *)とひずみ速度γの関係を以下の式、σ(sup *)=Bγ(exp m)で表した。ここで、Bは係数、mは指数である。最後に、定ひずみ速度での変形条件下における流れ応力-塑性ひずみ関係を表す方程式、σ=A(ε+b)(exp n)+Bγ(exp m)を導出した。