The Flow-Stress Asymmetry of Ultra-Pure Molybdenum Single Crystals

• Published in: Materials Transactions, JIM Vol. 41; no. 1; pp. 141 - 151
• Main Authors: Seeger, A., Hollang, L.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Sendai The Japan Institute of Metals 2000; Japan Institute of Metals
• Subjects: anomalous slip; Applied sciences; Cross-disciplinary physics: materials science; rheology; cyclic deformation; Deformation, plasticity, and creep; Exact sciences and technology; flow-stress asymmetry; kink-pair formation; Materials science; Metals. Metallurgy; molybdenum; Peierls potential; Physics; straining asymmetry; Treatment of materials and its effects on microstructure and properties; twinning–anti-twinning asymmetry; Monocrystals; Shear stress; Mechanical properties; Cyclic deformation; Twinning; Experimental study; Molybdenum; Critical resolved shear stress; Flow stress
• ISSN: 0916-1821; 2432-471X
• DOI: 10.2320/matertrans1989.41.141

Cyclic deformation experiments by the Mughrabi–Ackermann technique (predeformation with plastic shear-strain amplitude εpl0⁄2=1×10−3 at 530 K till saturation) have been performed on a molybdenum single crystal of ultra-high purity (residual resistivity ratio RRR0≈4×105; angle χ=29° between the plane of maximum resolved shear stress and the 〈111〉{110} slip system with largest Schmid factor), covering the temperature range 123K≤T≤460K and seven logarithmically spaced strain rates (1.0×10−6s−1≤|\dotεpl|≤1.0×10−3s−1). The analysis of the effective flow stress σ* in terms of the kink pair-formation theory gave the same results as the earlier experiments on Mo crystals of the same high purity but with smaller χ [Hollang, L., M. Hommel, and Seeger, A., phys. stat. sol. (a) 160 (1996) 329] both for the energy of two isolated kinks in 〈111〉a0⁄2 screw dislocations, 2Hk=1.27 eV, and for the kink height a, which agreed with the distance a{112} between neighbouring Peierls valleys on the {112} planes. The transition between the elastic-interaction approximation and the line-tension approximation of the kink pair-formation theory, which is responsible for the strain rate-dependent “upper bend” in the σ*-T relationship, occurred at \hatσ*=110 MPa. In agreement with theoretical predictions, at effective stresses less than \hatσ* the flow stress was the same in tension and compression, i.e., there was no flow-stress asymmetry. Below the upper-bend temperature, \hatT, the flow-stress asymmetry Δσ (=algebraic sum of the positive flow stress in tension and the negative flow stress in compression) increased with decreasing temperature, was positive in agreement with the “twinning–anti-twinning rule”, and reached a plateau at the lowest temperatures investigated. Since for reasons of symmetry, slip on {110} cannot give rise to a flow-stress asymmetry, together with a=a{112} this result confirms that over the entire temperature range investigated the elementary slip steps take place on {112} planes. In the other two ultrahigh-purity crystals investigated (χ=0°, 21°), the flow-stress asymmetry has the opposite sign (in violation of the “twinning–anti-twinning rule”) and, at low temperatures, much smaller absolute values than at χ≈30°. Furthermore, Δσ<0 extends to temperatures well above \hatT. Hence there must exist, in addition to the “twinning–anti-twinning asymmetry”, a further asymmetry-producing mechanism (dubbed “straining asymmetry”), for which the following model is proposed. In the bcc metals the screw-dislocation cores of lowest energy have {110} slip planes; the configuration with {112} slip planes is populated by thermal activation. Tensile strain normal to the {112} slip plane reduces the difference in the line energy of the two configurations and, as a consequence, the Peierls potential on the {112} slip plane. Compression strain has the opposite effect.