Temperature and strain rate sensitivity of the SD effect in saturation cycling

The strength differential effect during push-pull ~2 pct strain cycling was studied in AISI 310, α-brass, Cu, Ni, Al, Nb and Fe (Armco and Ferrovac E) at ~175 K. There is a small SD effect in Nb and Cu, none in the remaining fcc alloys, and a sizeable one in the interstitial Fe alloys. The latter were further investigated in the temperature range from 294 to 83 K; in addition values ofda/d In ε were determined in the Armco iron by stress relaxation. A direct manifestation of the SD effect is that the values determined in compression of the thermally activated stress term σ* and ofdσ/d In ε are at all temperatures in the ratio 1.14:1 to the homologous values obtained in tension. It is concluded that the SD effect is caused by a linear variation of σ* with hydrostatic stress; σ* appears to be the only plastic deformation parameter directly affected by it. Formerly a Post Doctoral Fellow on a Fellowship provided by the Netherlands National Science Foundation FOM, Laboratorium voor Fysische Metaalkunde     

Temperature and strain rate dependence of the portevin-le chatelier effect in a rapidly solidified Al alloy

The Portevin-Le Chatelier (PL) effect was studied for a rapidly solidified aluminum alloy in a temperature range of 193 to 423 K and a strain rate range of 10~5 to 10~’ s’1. Emphasis was put on the dependence of the critical strain (ɛ _c) for the PL effect on temperature (7) and strain rate (ɛ). It is found that the overall ɛ_c -T-ɛ relation is grouped into two categories: ɛ_c increases with increasing ɛ but with decreasingT in the low T/high ɛ regime and ɛ_c increases with increasingT but with decreasing ɛ in the high T/1ow ɛ regime. The underlying dynamic strain aging (DSA) mechanism is discussed to account for the two different yet unified categories by calculating activation energies and by introducing the transition conditions. Formerly with the Department of Materials Science, Delft University of Technology.     

The effect of solidification rate on structure and high-temperature strength of the eutectic NiAl-Cr

The eutectic NiAl-Cr, consisting of chromium rods in a NiAl matrix, was directionally solidified at rates varying from 1/4 to 30 in. per hr. The inter-rod spacing and elevated temperature tensile properties were measured and the structure determined for each solidification rate. The spacing, λ, obeyed the relation λ²V= Constant, whereV is the solidification rate. AtV greater than 1 in. per hr, a cell or colony structure formed and the cell size decreased with increasing solidification rate. At 600°C, the tensile strength increased substantially with decreasing inter-rod spacing, reaching 100,000 psi at the highestV. At 800° and 1000°C, the strength first increased then decreased at the highest solidification rates reaching maximum strengths of 63,000 psi and 47,000 psi, respectively. At 1200°C, the strength decreased slightly with increasing solidification rate. Fracture occurred by shear along grain boundaries and cell walls. The decreased strength at the highest solidification rates may be related to the increased cell boundary area.     

Temperature dependence of the strength-differential effect in hardened steels

The strength differential (SD) has been measured as a function of temperature in a fully hardened 0.2C, 6 Ni steel, quenched and then tempered at 250°C. It is found appropriate to express the results in terms of the intercept flow stresses as extrapolated back to zero plastic strain. The corresponding SD increases significantly with decreasing temperature below the ambient, and the data are well fitted by separating the SD into an athermal component (~3 pct) and a thermally activated component (up to ~15 pct at the liquid-nitrogen temperature). The latter type of contribution is thought to arise from a dilatation which occurs during the activation process for dislocation motion and which causes an increase in the activation energy under compressive loading and a decrease under tensile loading. The parameters obtained via this treatment are consistent with a double-kink Peierlsbarrier mechanism of plastic flow at low temperatures. This paper is based on a doctoral thesis presented in August 1972 by F. B. Fletcher to the Department of Metallurgy and Materials Science at the Massachusetts Institute of Technology.     

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5 · 10⁻⁵ s⁻¹≤ε≤1 · 10⁻² s⁻¹. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε≲10⁺⁴ s⁻¹), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials-Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5·10⁻⁵ s⁻¹≤ε≤1·10⁻²s⁻¹. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε<~10 ⁺⁴s⁻¹), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed. This article is based on a presentation given in the symposium entitled ‘Dynamic Behavior of Materials-Part II,” held during the 1998 Fall TMS/ASM ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Temperature and strain rate dependence of stress-strain behavior in a nickel-base superalloy

The temperature and strain rate dependence of yield and work hardening behavior in elevated temperature stress-strain testing was investigated in the nickel-base superalloy UDIMET 115. This alloy was heat treated to produce a bimodal distribution of hyperfine and coarse γ¹ precipitates. Yield behavior is shown to be controlled primarily by dislocation pair cutting of the hyperfine precipitates. Stage II work hardening appears to be governed mainly by the dislocation networks generated on the coarse precipitates as these are bypassed under the Orowan mechanism. Analysis of Stage III work hardening using ϕvs σ plots shows that a steady state exists for the stress-strain deformation of this system. Results are also reported on extremely strong serrated flow at moderately elevated temperatures and a high temperature ductility minimum. RUSSELL R. JENSEN, formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University, New York, NY An erratum to this article is available at .