Tensile strain-rate sensitivity of

The tensile strain-rate sensitivity of continuous-tungsten-fiber reinforced niobium composites (W/Nb), fabricated by an arc-spray process, was studied in the 1300 to 1600 K temperature range. The tensile properties of the fiber and matrix components, as well as of the composites, were measured and compared to rule of mixtures (ROM) predictions. The deviation from the ROM was found to depend upon the chemistry of the tungsten alloy fibers, with positive deviations for thoria-dispersed W wire (ST300) reinforced Nb composite(i.e., stronger composite strength than the ROM) and negative or zero deviations for lamp-grade W wire (218) reinforced Nb composite. In addition, it was found that the composites tested at higher crosshead speeds exhibited a strain-rate sensitivity greater than that of the free fibers tested at the same crosshead speeds, even though the composite tensile strength is determined mainly by the fiber component.     

Solid solution strengthening in Ti-Al alloys

Aluminum was found to strengthen textured polycrystalline α-Ti linearly with concentra-tion over the 78 to 810 K temperature range. Between 300 and 530 K about 90 pct of the strengthening effect is athermal and the athermal component, Δτ_a, follows Δτ_a = ϕμε′c over the 0 to 15 at. pct Al range where ϕ is a dimensionless constant near unity, μ. is the modulus, ε′ is the misfit parameter, andc is concentration. It is proposed that edge kinks in screw dislocations interact with solute atoms and give rise to the observed linear athermal strengthening. Above about 530 K, atmosphere and strain aging effects are associated with aluminum.     

Solid solution creep behavior of Sn-xBi alloys

This study details the steady-state creep properties of Sn-1 wt pct Bi, Sn-2 wt pct Bi, and Sn-5 wt pct Bi as a function of stress and temperature. All data, including previous work on pure Sn, are described by the following empirical equation:

Fatigue crack propagation in iron and Fe-Mo solid solution alloys (77 to 296 K)

Fatigue crack propagation data were obtained for iron and Fe-Mo solid solution alloys through the ductile-brittle transition temperature (77 to 296 K). The fatigue crack growth rates were found to decrease with decreasing temperature for each alloy and to decrease with increasing solute content at 296 K but to increase with solute content at 77 K. These results are discussed in terms of the wide range of mechanical properties observed for the materials. It is shown that the fatigue crack growth rates are inversely proportional to a plastic work term and the square of a characteristic stress approximated by the yield or ultimate stress. Formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University, Evanston, Ill. This paper is based on a thesis submitted by L. H. Burck in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

Operant strain-rate sensitivity during tensile necking

Precisely machined tensile specimens of aluminum-killed steel sheet were used to measure the continuous strain-rate sensitivity,m _c, in a series of isothermal tests at different crosshead speeds.m _cwas found to be independent of strain and strain rate, in contrast with the “jump” test instantaneous strain sensitivity, m_i, which was found to vary strongly with strain rate. A series of matched tensile specimens was also photogridded and deformed at three rates and terminated at four elongations. The strain distributions obtained from these tests were compared with Finite Element Modeling (FEM)—calculated ones based on several strain-rate sensitivity formulations. Comparison of calculations with experiments revealed that the opérant rate sensitivity during tensile localization,m _t, was intermediate betweenm _candm _iat each rate and elongation. Once the effective rate sensitivity was established, detailed predictions of strain distributions and failure elongations agreed very well with experiment. A qualitative model of strain-based stress transients was proposed for both strain-rate and strain-state path changes. Formerly Staff Research Scientist, General Motors Research Laboratories. Formerly Senior Staff Scientist, General Motors Research Laboratories.     

Thermodynamics and solubility of carbon in ferrite and ferritic Fe-Mo alloys

A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of α-Fe, Fe-0.48 wt pct Mo and Fe-1.16 wt pct Mo specimens which were equilibrated with a series of methane-hydrogen gas mixtures. The equilibria investigated involved the ferrite phases of the alloys between 682 and 848‡C. The experimental results permitted direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of other thermodynamic functions, without the necessity for any correction factors. The results have been compared with the experimental measurements of a number of other investigators. In ferrite, the partial molar enthalpy and entropy of solution of carbon are found to be 26,800 cal/mole, (112, 130 J/mole), and 30.59 cal/K-mole, (127.99 J/K-mole) respectively at temperatures below about 727‡C. Above this temperature, the values are 25,200 cal/mole and 29.13 cal/K-mole, respectively. The addition of molybdenum results in an increase in these properties below 727‡C and a decrease in the values above 753‡C, and the changes are found to be proportional to the molybdenum content. The solubility of carbon in α-Fe is found to be 0.0176 wt pct at the eutectoid temperature. Molybdenum increases the solubility relative to the Fe-C system at temperatures above the eutectoid and decreases it below the eutectoid.     

The origin of strain-rate sensitivity in OFHC copper

This investigation was conducted to determine the nature of the strain-rate sensitivity of OFHC copper at room temperature. In particular, the relative magnitudes of the dynamic contribution (particle inertia, suppression of thermal assistance, and so forth) and the nondynamic contribution (namely, the accelerated rate of strain hardening observed at high strain rates) to the strain-rate sensitivity were determined. Specimens were dynamically compressed using the Hopkinson pressure bar technique, and then were reloaded quasistatically to determine their respective yield strengths. The dynamic contribution to strainrate sensitivity was taken as the difference between the peak dynamic flow stress and the flow stress of the same specimen when reloaded quasistatically. The nondynamic contribution to strain-rate sensitivity of the flow stress was taken as the difference between the quasistatic flow stress in reloading of a specimen prestrained ε₀ dynamically and the flow stress at ε₀ for a sample deformed in uniaxial quasistatic compression. The room temperature dynamic flow-stress of OFHC copper, deformed at 500s⁻¹, was found to be 25 pct higher than the conventional quasistatic flow stress for this metal over a strain range of 0.08 to 0.20. The nondynamic contribution to strain-rate sensitivity was found to be about 60 pct of the total flow stress increase and has been attributed to a difference in strainhardening at different strain rates. Thus, it appears that in OFHC copper at room temperature and at strain rates of about 500s⁻¹, the nondynamic contribution to strain-rate sensitivity is more significant than the dynamic contribution.     

Anneal hardening and flow softening in beta zirconiumniobium alloys

The flow properties of β-Zr-Nb (Cb) alloys were investigated by means of compression testing in the strain rate range 10^-1 to 10^-5 s^-1 and from 725 to 1025°C. The flow curves obtained on Zr-Nb alloys containing 10, 15 and 20 pct Nb exhibited flow softening, and the magnitude of this effect decreased as the temperature was increased. All three alloys also exhibited anneal hardening, i.e. an increase in flow stress at 825°C with annealing time at 1000°C. Neither the flow softening, nor the anneal hardening could be associated with environmental effects, as in Zr-Mo alloys, nor could they be attributed to texture changes or to the occurrence of dynamic recrystallization. On the basis of X-ray and microprobe investigations, as well as grain size measurements, it is concluded that the anneal hardening is due to the combined effect of grain growth and the formation of solute clusters during annealing. The occurrence of flow softening is attributed to the destruction of the solute clusters by straining. Stress-strain curves were also determined for Zr-2.5 pct Nb. Unlike the high Nb alloys, these materials exhibited neither flow softening nor anneal hardening. The flow stresses were found to be highly strain rate dependent, with stress sensitivities of about 5.5 for yielding and 4.5 for steady state flow.     

Solid solution decomposition mechanisms in cast and microcrystalline Al-Sc alloys: I. Experimental studies

The structure and the electrical and mechanical properties of Al-Mg-0.22 wt % Sc-0.15 wt % Zr alloys with various magnesium contents (0, 1.5, 4.5 wt %) are experimentally studied during the decomposition of the solid solution of scandium and zirconium in the states after solidification from a melt (cast ingots) and after subsequent multicycle equal-channel angular pressing (microcrystalline structure). The dependences of electrical resistivity ??, microhardness HV, macroelasticity limit ??₀, and yield strength ??_y on the annealing temperature and time are analyzed.     

Solid solution decomposition mechanisms in as-cast and microcrystalline Al-Sc alloys: IV. Effect of the decomposition of a solid solution on the mechanical properties of the alloys

The effect of the decomposition of the solid solution in Al-X wt % Mg-0.22 wt % Sc-0.15 wt % Zr (X = 0, 1.5, 4.5 wt %) alloys on their mechanical properties in the as-cast and microcrystalline (MC) states is studied. The contribution of the particles that precipitate during the decomposition to the mechanical properties of the as-cast and MC alloys is determined. For this purpose, the dependences of the volume fraction and the size of the particles precipitating during the decomposition on the annealing time and temperature and the magnesium content in the as-cast and MC alloys are calculated. An additional contribution to the hardening of the MC alloys is shown to be related to the internal stress fields created by the defects that accumulate at the disperse particles distributed in grain boundaries during grain-boundary migration. The calculated macroelasticity limits in the as-cast and MC alloys are compared with the experimental data.     

Experimental study of the origin of strain-rate sensitivity in a common Pb-Sn alloy

The origin of strain-rate sensitivity existing in the room-temperature plastic distortion of a 60 pet Pb-40 pet Sn alloy is examined experimentally using simple tension loadings. It is found that the response of this material to both constant stress and constant strain-rate loadings is accurately described by a single uniaxial constitutive relation which is independent of the instantaneous level of straining rate and dependent explicitly only on stress, strain and lapse-time variables. The rate sensitivity exhibited by this alloy during room-temperature quasistatic tensile deformations is thus concluded to be explainable directly in terms of a combination of the competing effects of strain hardening and time-dependent thermal softening without the necessity of introducing explicit strain-rate mechanisms of any kind.     

Solid solution decomposition mechanisms in cast and microcrystalline Al-Sc alloys: III. Analysis of experimental data

The results of experimental studies of the decomposition of the solid solution in cast and microcrystalline (MC) Al-X wt % Mg-0.22 wt % Sc-0.15 wt % Zr (X = 0, 1.5, 4.5) aluminum alloys that were described in part I of this work are analyzed. The data on the electrical resistivity of the alloys are used to determine the volume fraction of Al₃Sc(Zr) particles precipitating in the temperature range 240–400°C. The model developed in part II of this work is used to reveal the mechanism of particle precipitation in the cast and MC alloys. It is shown that the particles in the cast and MC alloys precipitate mainly on dislocations. The differences in the decomposition kinetics of the solid solution in the cast and MC alloys are explained by different mechanisms of dislocation structure recovery mechanisms occurring in them.     

The problem of the negative strain-rate sensitivity of metals under the portevin-lechatelier deformation conditions

Experimental data are used to show that the ‘negative’ strain rate sensitivity of metals, under conditions producing ‘jerky flow’, is only an apparent value. Taking into account the mode of propagation of slip-bands, a simple model of the phenomenon is postulated. Also discussed are the most important features of non-uniform deformation in Cu-base and Al-base alloys.     

Analysis of the softening of heterophase aluminum alloys with a eutectic component

The effect of interparticle distance, volume fraction, and the average particle size of phases of eutectic origin in cold-rolled sheets of alloys of the Al-Ni, Al-Ni-Ce, Al-Mg-Si, and Al-Cu-Ce systems on strengthening and structural changes during heating at temperatures of 0.6?C0.8 T _m is investigated. The dependence of the intensity of softening on the particle parameters in the investigated alloys is established, and their ability to accelerate or retard recrystallization is analyzed using these results.     

Analysis of softening alloys of the Al-Ni system containing particles of variable dispersity

Regularities of the deformation strengthening and softening of aluminum alloys containing second-phase Al₃Ni particles 0.3 to 2.2 μm in size with a volume fraction from 0.03 to 0.1 are investigated during cold deformation and subsequent annealing at 0.6t _m. It is shown that the largest hardness increment is observed for alloys with a maximal fraction of fine particles (d = 0.3 μm) after rolling deformation larger than 0.4. Fine particles prevent the development of crystallization upon true deformation up to 2.3, thereby effectively inhibiting softening. An increase in the particle size to 1.2–2.2 μm stimulates nucleation during recrystallization, substantially accelerating this process. For example, in order to ensure recrystallization uniformly over the entire sheet volume at d = 2.2 μm, cold deformation with ? = 0.4 is sufficient.     

The effect of hydrogen on the solid solution strengthening and softening of nickel

The effects of hydrogen on the plastic deformation of nickel and nickel-carbon alloys were studied using plastic deformation techniques over a wide range of strain rates at about 300 K. The emphasis of the study was on the behavior at very low strains and low strain rates. Hydrogen was introduced as a solute element by quenching from a gaseous H₂ atmosphere or by testing in a gaseous H₂ atmosphere. The behavior of a number of different purities of nickel with hydrogen additions was examined. The most significant impurity element seemed to be C and this element was varied over a wide composition range by annealing in different atmospheres. Both solution softening and solution strengthening was observed depending on the amount of H in solution relative to the amount of C in solution. The nature of this solution softening effect is discussed.