Correlation of the hot-hardness with the tensile strength of 304 stainless steel to temperatures of 1200°C

It is shown that the ultimate tensile strength, σ, of 304 stainless steel can be correlated with hot-hardness measurements at temperatures up to 1200°C using the expression σ = (H/3.0)(n/0.2n) ⁿ whereH is the diamond pyramid hardness number andn is the strain hardening coefficient. The strain hardening coefficient was obtained from a Meyer’s hardness coefficient at the room temperature test condition and for test temperatures up to about 0.5T _m , from the empirical relationshipn =k/λ. Herek is a constant equal to approximately 0.2 microns and λ is the subgrain dimension of the deformed specimen as obtained by transmission electron microscopy. Formerly with General Electric Company, Cincinnati, Ohio     

Effect of internal boundaries on the yield strengths of spheroidized steels

The combined effects of cementite particles, grain boundaries, and subgrain boundaries on the room temperature yielding behavior of spheroidized, plain carbon steels were investigated. Spheroidization by austenitizing and quenching, followed by annealing at temperatures just under theA ₁ temperature, produced a subgrain-connected cementite particle distribution. The subgrain size, λ_l,p, stabilized by the particles governs the yield stress via the relationσ _y = 9.5 + 1.33 λ_l,p ^-1/2, kgf/mm² In contrast, austenitizing and quenching, followed by thermal cycling about theA ₁ temperature, produced microstructures with a large fraction of intraboundary, subgrain-free cementite particles. The lower yield stress of these steels could not be accounted for by either the Orowan or the Ansell-Lenel theory. The yield stress is predominantly controlled by the ferrite grain size, λ_g, via the relationσ _y = 12.4 + 1.87 λ_g ^-1/2, kgf/mm² The intraboundary particles contribute only a small strain-hardening term which increases the value of the friction stress (12.4 kgf/mm²) over that associated with grain boundary strengthening alone (8.8 ± 0.8 kgf/mm²). Formerly affiliated.     

The application of a dislocation model to the strain and temperature dependence of the strain hardening exponentn in the Ludwik-Hollomon relation between stress and strain in mild steels

With the aid of a dislocation model for the stress-strain relationship of α-Fe, analytical expressions for the strain and temperature dependence of the exponentn in the relation, σ= K · ε ⁿ, are derived. These account quite accurately for experimental results obtained with several low alloy steels. It is shown thatn varies continuously with strain but that the theoretical and experimental log σ-log ε curve in most cases can be approximated by two straight lines in accordance with the well-known “double-n” behavior. The strain, ε₁ at which the two lines intersect is equal to the strain at which the theoretical n(ε) curve has an inflection point. With the model presented it is also possible to account for the temperature dependence ofn(ε) and of ε₁ within the temperature range −78° to 500°C.     

On the stress exponent and the rate-controlling mechanism of high-temperature creep in some solid solutions

The internal stress, σ_i, and the effective-stress exponent of the dislocation velocity,m*, have been determined during creep of Fe-3.5 at. pct Mo alloy at 1123 K under 10.8 to 39.2 MN/m² and of Ni-10.3 at. pct W alloy at 1173 K under 19.6 to 88.2 MN/m². Both alloys have been classified among class I alloys under a certain condition including the present one, because the applied-stress exponent of the steady-state creep rates,n, is almost 3. Values of σ_i obtained by stress-transient dip test were small and almost independent of the applied stress, σ_c, in Fe-3.5 Mo alloy. On the other hand, in Ni-10.3 W alloy σ_i increased with increasing σ_c as in the case of many pure metals. The value ofm* obtained by analyzing stress-relaxation curves immediately after creep deformation was unity in Fe-3.5 Mo alloy, whereas in Ni-10.3 W alloy it was about 2.5. These results indicate that the rate-controlling mechanisms in creep are different from each other in these two alloys and that the classification according ton-value does not always coincide with the classification according to the rate-controlling mechanisms. It is concluded that the fact thatn ≃ 3 is not a sufficient evidence supporting that creep is controlled by one of microcreep mechanisms.     

Ordering and the strength of an aligned Ag3Mg-AgMg eutectic

A fine lamellar structure with interlamellar spacings from 1 to 7μ has been produced by directional solidification of an Ag₃Mg-AgMg eutectic alloy. The tensile properties were measured as a function of test temperature, interlamellar spacing,λ, and degree of order in the Ag₃Mg phase. The dependence of flow stress onλ ^-1/2 increased sharply with ordering of Ag₃Mg and this strengthening persisted at elevated temperatures. Work hardening rate and ductility of the eutectic at low temperatures also were affected, leading to the conclusion that ordering changes the compatibility of slip across interphase boundaries.     

Combined hardening of alloy Ti-6Al sheet workpieces during reversible hydrogen alloying

The effect of the initial hydrogen concentration, warm rolling, and vacuum annealing conditions on the formation of the phase composition, structure, and mechanical properties of rolled sheet workpieces made of a Ti-6Al α alloy is studied. When the initial hydrogen concentration increases to C _Hⁱⁿⁱ = 0.3–0.9%, the grain size decreases and the phase composition of the alloy is complicated. In the grain size range 27–5 μm, the yield strength of the alloy obeys the Hall-Petch relation with the lattice friction stress σ _i = 662 MPa. When the initial hydrogen concentration increases, the grain-boundary hardening intensity and the yield strength increase. At an average α grain size of 5 μm, the yield strength increases from 770 MPa in the alloy with C _Hⁱⁿⁱ = 0.004% to 970 MPa in the alloy with C _Hⁱⁿⁱ = 0.7%. The maximum yield strength (σ_y = 1064 MPa) is obtained for the alloy with C _Hⁱⁿⁱ= 0.5% after vacuum annealing at 650°C. The conditions and contributions of solid-solution hardening, grain-boundary hardening, precipitation hardening induced by the formation of the α₂ phase, and strain hardening to the total hardening of the alloy are considered.     

The Normal spectral emittance of yttrium,lanthanum, cerium,praseodymium and neodymium above 1000 K

The normal spectral emittance (X = 0.645 μm) of yttrium, lanthanum, cerium, praseodymium and neodymium was determined at temperatures above 1000 K in an argon atmosphere at a pressure slightly above atmospheric. The measured normal spectral emittances are: yttrium, (solid), ε _nλ = 0.368 ± 0.005; (liquid), ε _nλ = 0.368 ± 0.005; lanthanum, (solid), ε _nλ = 0.409 ± 0.049; (liquid), ε _nλ = 0.257 ± 0.015; cerium, (liquid), ε _nλ = 0.309 ± 0.020; praseodymium, (solid), ε _nλ = 1.609 - 1.0964 x 10⁻³ T (K); (liquid), ε _nλ = 0.284 ± 0.005; neodymium, (solid), ε _nλ = 1.090 - 5.6 x 10⁻⁴ T (K); (liquid), ε _nλ = 0.394 ± 0.007. Formerly NSF Presidential Intern and PostDoctoral Fellow Formerly Undergraduate Research Helper, Prepared for the Energy Research and Development Administration under Contract No. W-7405-eng-82.     

On the contribution of concurrent grain growth to strain

Flow behavior and microstructural evolution in an Al-Cu eutectic alloy of equiaxed grains were investigated over ε ≃ 2× 10⁻⁶ to 2 × 10⁻² s⁻¹ andT = 400° to 540 °C. Depending on the test conditions, there appeared either strain hardening or strain softening predominantly in the early part of the σ-ε curves. The microstructural observations showed evidence for grain growth, development of zig-zag boundaries, dislocation interactions, and cavitation. The grain growth adequately accounts for the observed strain hardening at higher temperatures and lower strain rates. However, at lower temperatures the strain hardening can be only partly accounted for by the observed grain growth; under this condition, some dislocation interactions also contribute to the strain hardening. The presence of cavitation causes strain softening predominantly at higher strain rates. Therefore, to develop a proper understanding of the superplastic behavior of the Al-Cu eutectic alloy, it is necessary to take into account the influence of dislocation interactions and cavitation along with that of grain growth.     

The stress-dependence of high temperature creep of tungsten and a tungsten-2 Wt pct ThO2alloy

Constant-load creep tests were conducted with pure tungsten and a W-2 wt pct ThO₂ alloy at temperatures between 1600° and 2200°C and at strain rates of about 1 × 10^-8 to 4 × 10^-5 sec^-1. The results were evaluated by the empirical correlations of Robinson and Sherby and also Mukherjeeet al. which describe the stress dependence of the creep of metals and alloys. The agreement of the present experimental data with these correlations was found to be poor. However, when the following empirical relationship was used: •ε _c =A’(σ _c /σ _f ) ⁿ the present creep data for tungsten and the tungsten alloy at various temperatures were much better correlated. Here, •ε _c is the experimental creep rate, σ_c is the applied stress for creep, σf is the flow stress of the material at the same temperature in a constant strain rate tensile test, andA’ is function of temperature, structure, and strain rate.     

Evolution of dislocation structures and deformation behavior of iron at different temperatures: Part I. strain hardening curves and cellular structure

The deformation behavior of iron has been investigated at different temperatures by means of tension tests. There exist two temperature ranges for deformation. In the low-temperature range(T < 293 K), the flow stress σ, the work-hardening rate θ at ε = 0.06, and the yield stress σ_y decrease with increasing temperature, but in the higher temperature range(T ≥: 293 K), σ and θ at the same strain increase while σ_y decreases more slowly. The change of dislocation density, with temperature, atε = 0.06 exhibits the same tendency as that of the flow stress. The strainhardening rates decrease almost linearly with increasing stress up to necking in the low-temperature range, except the initial strain range. At the higher temperature range, the hardening rates decrease linearly with stress only at the early stage of deformation, but above certain strains, the decreases become more gradual; that is, the G-cr curves deviate from the linear region. The evolution of dislocation structure has also been observed by transmission electron microscopy (TEM). The results show that a substructural transition takes place in the nonlinear range of G-cr curves. In the linear decreasing region of strain-hardening curves, the deformation is controlled by the uniformly distributed dislocations or cell multiplication prevails. However, in the nonlinear region of G-cr curves, cell multiplication seems to be balanced by cell annihilation.