Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 x 10^-4 oxygen equivalent) and commercial purity (6 x 10^-3 oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the (c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. A. M. GARDE, formerly a Post Doctoral Fellow at the University of Florida. E. AIGELTINGER, formerly a Post Doctoral Fellow at the University of Florida. B. N. WOODRUFF, formerly a student at the University of Florida.     

Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 × 10⁻⁴ oxygen equivalent) and commercial purity (6 × 10⁻³ oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the(c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a student at the University of Florida.     

Dynamic strain aging in carbide-strengthened molybdenum alloys

A study was made of the dynamic strain aging processes in carbide strengthened molybdenum alloys. Strengthening due to dynamic strain aging was observed in the temperature range 1500° to 2400°F. The strengthening is believed to arise from the pinning of mobile dislocations by metal-carbon pairs or clusters and/or carbide precipitates. This immobilization of the dislocations led to a high work hardening rate and the observed strengthening.     

Dynamic strain aging and hydrogen-induced softening in alpha titanium

Compression tests were carried out on samples of commercial-purity titanium charged with up to 4.7 at. pct hydrogen. Strain rates of 10⁻³ to 1 s⁻¹ were employed and testing was limited to the α phase field at temperatures of 773 to 973 K. The dependences of the flow stress on strain, strain rate, and temperature were determined. A plateau or bulge appeared in the temperature and strain-rate dependences of the flow stress, and the work-hardening rate also showed peaks. Serrations were observed on some of the stress-strain curves. All these features indicated that dynamic strain aging (DSA) was occurring. Analysis of the results (together with data from other authors) indicates that there are three ranges of DSA behavior in this material within the experimentally investigated temperature range; these appear to be associated with the diffusion of iron, carbon, and oxygen, respectively. Alloying with hydrogen decreases the magnitude of the DSA attributable to these elements and displaces the phenomenon to higher temperatures and/or to lower strain rates. The dependence on strain rate and temperature of the relative softening attributable to hydrogen addition was determined. The results indicate that hydrogen-induced softening is related to the occurrence of DSA in this temperature range. Possible explanations for this relationship are discussed. O.N. SENKOV, on leave from the Institute of Solid State Physics, Russian Academy of Sciences, Moscow Region 142432, Russia     

Dynamic strain aging effects in niobium microalloyed steel

Abstract

The dynamic strain aging behaviour of a niobium microalloyed steel has been examined. Hot tensile testing was carried out on heat treated and as received specimens. Heat treated specimens were austenitised at 1000°C for 1 h, and then cooled in air or in a stainless steel cylinder to obtain various amounts of free or uncombined interstitial solutes in solid solution, to examine the effect on the dynamic strain aging behaviour of the steel. It was found that dynamic strain aging takes place in niobium microalloyed steel during tensile testing at temperatures ranging from ambient temperature to 450°C at a crosshead displacement rate of 2 mm min^-1. As a result, the ultimate tensile strength and initial work hardening rate exhibit maximum values at temperatures between 200 and 350°C. Also, load-extension graphs for tested specimens show serrated behaviour and yield points at 200, 250, and 300°C. It is believed that dynamic strain aging in niobium steel is caused by interaction between dislocations and interstitial solutes (nitrogen and carbon) or solute pairs consisting of one interstitial and one substitutional solute atom (for example Mn-C and Mn-N).     

Dynamic strain aging during creep of α-Zr

In the temperature range 723 to 823 K (450° to 550°C) annealed, crystal bar α-Zr exhibits anomalous behavior with respect to both single stress and incrementally stressed creep tests. The nature and extent of the anomalous behavior depends on temperature, stress, and impurity content. Specimens with low oxygen content exhibit: 1) normal, three-stage creep behavior during single stress tests, and 2) normal transients during incremental stress and temperature tests. Specimens with higher oxygen contents exhibit: 1) multi-stage creep curves whose shapes depend on temperature and stress, 2) inverse transients following stress and temperature increments, and 3) peaks in activation energy-tempera-ture curves. The nature of the anomalous behavior is consistent with a model for strain aging in which the possibility of localized depletion of the strain aging species exists. In the material being studied oxygen is probably responsible for the observed effects. R. D. WARDA, formerly of Department of Metallurgy, University of British Columbia     

Dynamic and static strain aging effects in Fe-modified Ll2 Al3 Ti

Serrations have been observed in the stress-strain curves of Fe-modified Ll₂ Al₃ Ti-based alloys at intermediate temperatures and related to dynamic interactions between mobile dislocations and solute atoms, i.e. dynamic strain aging effects. Serrations appear only at intermediate, not at high or low temperatures, because of the following reasons: at high temperatures the diffusivity of the solute atoms is high enough that the solute atoms and dislocations move together, thereby requiring no break-away of the dislocations. At low temperatures the diffusivity of the solute atoms is so low that no dynamic interaction can occur, but a static interaction is observed, instead. Thus the entire range of yielding phenomena can be consistently explained in terms of interactions between solute atoms and mobile dislocations and requires no assumptions about changes in the nature of dislocation cores. The solutes giving rise to this effect are likely of substitutional type but have not been identified.     

Dynamic strain aging during wire drawing and its effect on electrochemical behaviour

Second stage dynamic strain aging is observed during cold drawing of low carbon steel wire rods with higher free nitrogen content. The effect is prominent in thinner wires which have experienced higher strain and are thereby heavily stressed. Consequently, these severely deformed wires with higher nitrogen content are prone to early environmental degradation.     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C. Formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C.     

Influence of strain rate on ductility of metallic materials

The flow behaviour of iron and steel X 2 CrNiMo 18 10 is studied in impact tension tests with strain rates between 1000 and 7000 s^?1 on smooth specimens and on notched bars. The neck formation is discussed on the basis of the theory of imperfections. The temperature increase during the adiabatic process promotes deformation localisation and instability. However, the stabilising effect of the strain rate sensitivity is dominating and leads to an increase in ductility. On the other hand, increased strain rates reduce the local fracture strain leading to an increased dynamic notch sensitivity. Through combination of experiment and simulation, the location of void initiation could be estimated and the influence of strain rate on the local failure strain could be determined.     

Dislocation dynamics in strain aging alloys

The problem of dynamic strain aging is treated using a new approach to the aging kinetics. The moving dislocations are considered as an ensemble of double loops which overcome the forest dislocations by thermal activation. This implies an exponential distribution of waiting times in front of the obstacles during which additional pinning agents arrive at the dislocation. The temporal development of the mean number of pinning points per loop can be calculated from an appropriate master equation which is based on the dynamic transition rates for pinning and depinning of the dislocation segments. It is shown that the two pinning mechanisms, lineal pinning and forest strengthening discussed in the literature, can be treated in a unified way. This leads to the definition of a single parameter, the mobility factor, which completely describes the effect of dynamic aging on dislocation velocity and plastic strain rate. The role of local instabilities and the problem of strain rate sensitivity are discussed.     

Relationship between contractile strain ratioR and texture in zirconium alloy tubing

The crystallographic textures of zirconium alloy tubing used as cladding in nuclear reactor fuel are commonly characterized by the quantitative texture numbersF (Källström) and f_r (Kearns) which are derived from the direct and inverse pole figures. The texture numbers of zircaloy 2 and 4 tubes have been correlated experimentally with the value of the contractile strain ratioR which is a measure of the plastic anisotropy of the tube. The correlations were based on the results of 20 different tubing lots. Thef _r-R correlation shows much less data scatter than theF-R correlation. By assuming a simple plastic deformation model for zirconium alloys the following relations between texture and anisotropy are obtained:F=R- 1/R+1 and f_r = R/R+1 The theoretically derived relations are in good agreement with the experimental data. The procedure of correlating texture with plastic anisotropy is not limited to zirconium alloy tubing, but should be equally applicable to textured sheet and plate materials and other alloys with a limited number of slip systems.     

Thermal gradients,strain rate,and ductility in sheet steel tensile specimens

The effect on ductility of strain rate and thermal gradients arising from deformation is examined in tensile specimens of 1008 AK steel. The total elongatione _tot is taken as the measure of ductility, since it reflects changes in the strain hardeningn and strain-rate sensitivitym. Tensile specimens are pulled to failure in 23 °C air, at initial strain rates from 10⁻³ to 10⁻¹ s⁻¹, with thermocouples recording temperature along the 50.8 mm gauge section. The maximum temperature is ∼110 °C just prior to failure at the highest rate. Thee _tot, however, remains fairly constant with rate at ∼40 pct. When thermal gradients are prevented by immersing the specimens in circulating water at 23 °C,e _tot, increases with rate to a maximum of ∼54 pct at 10⁻¹ s⁻¹. Direct measurements of isothermal values ofm at 23, 60, and 90 °C show thatm increases with rate.e _tot, therefore, would be expected to increase with rate. Since under nonisothermal conditionse _tot does not change, it appears thatm and thermal gradients are competing influences on ductility at higher rates. Enhanced ductility in stampings should be possible by suppressing gradients, either by controlling die temperature or by heat transfer properties of a lubricant.     

Effect of titanium,niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys

An attempt has been made to develop a cobalt base casting superalloy (30Cr-10Ni-7W-Co) having high creep rupture strength and ductility for first stage nozzles of gas turbines. In cobalt base superalloys, there was found to exist a close correlation between the creep rupture strength and MC type carbide forming elements such as Ti, Nb and Zr. In cobalt base alloys with 0.25 wt pct C, precipitation and coarsening of carbides can be reduced by addition of Ti, Nb and Zr. Therefore, by adding the optimum amount of Ti, Nb and Zr, precipitation of carbides in the alloy reaches such an amount as to give the highest creep rupture strength. Excess addition of Ti, Nb and Zr does not improve the creep rupture strength. By adding C, creep rupture strength of the cobalt alloy with Ti, Nb and Zr can be improved and becomes the highest at 0.40 wt pct. C. According to the experimental results, the creep rupture strength becomes the highest at a value of (Ti + Nb + Zr)/C (atomic ratio) of about 0.3. Contrary to the expectation, it was found in this experiment that the ductility in creep rupture tests increases with increasing carbon content up to 0.6 wt pct.     

Postirradiation aging affects stress and strain in polyethylene components

Ultrahigh molecular weight polyethylene components oxidatively degrade because of gamma radiation sterilization and subsequent shelf aging in air. The effects of shelf aging on the stresses and strains associated with surface damage in tibial and acetabular components were examined. A material model was developed to predict the stress and strain relationship of oxidatively degraded polyethylene as a function of density using samples of polyethylene that were gamma radiation sterilized and evaluated immediately after irradiation and after 42 months of shelf aging. The finite element method was used to determine the stresses and strains before and after shelf aging for two tibial components with different conformities between the articulating surfaces and for an acetabular component. The stresses increased by 10% to 14% in the conforming tibial model after 42 months of aging, whereas the stresses in the nonconforming tibial model and in the acetabular model increased by only 4% to 8%. Aging decreased the principal strains by 5% to 10% in both tibial models and by 15% to 17% in the acetabular model. Postirradiation aging during shelf storage of polyethylene joint components is likely to worsen long term wear, based on the increased stresses and decreased strains predicted to occur as a result of aging.     

Yield points and strain aging in hexagonal-based particulate composites

Initial and strain-aging yield points have been observed and studied for three hexagonal-based particulate composites: Cd-30 vol pct B, Mg-25 vol pct B, and Zn-15 vol pct A1₂O₃. The strain-aging yield points begin to appear at 0.5 to 0.6T_m and persist up to the melting temperature. These effects are associated with the large volume fraction of hard particles, which are considered to be a primary source of dislocations and vacancies; strain aging yield points are attributed to the resulting decrease in mobile dislocation density, by annihilation or interaction with imperfections other than impurity atoms, which occurs during aging after straining.