Nitrogen strengthening of a stable austenitic stainless steel

The low-temperature flow stress of mechanically stable austenitic stainless steels increases with increasing concentration of nitrogen in solution and with decreasing temperature. This phenomenon has been studied in a series of FeNiCrMo alloys with nitrogen contents between 0.04 and 0.36 wt% by measuring the flow stress and the thermal activation parameters for plastic flow as a function of stress, plastic strain and nitrogen concentration in stress relaxation and strain-rate change experiments. Care is taken, when analyzing the data, to distinguish between athermal and thermal effects. The significant increase of the athermal flow stress with increasing nitrogen concentration is attributed to short-range ordering of chromium and nitrogen atoms. The thermally activated component of the flow stress is also dependent on the nitrogen concentration and is thought to be due to localized, predominantly modulus interactions between lattice disturbances in the immediate vicinity of nitrogen atoms and slip dislocations. The thermally activated component is suitably described by Friedel's model of solid solution strengthening.     

Stress relaxation and solid solution hardening of cubic ZrO2 single crystals

Solid solution hardening in cubic ZrO₂ single crystals of varying Y₂O₃ contents (12.7, 15.2, 17.7, and 20.5 mol %) oriented for easy 100 〈011〉 slip has been studied at 1400°C. Strain rate cycling and stress relaxation experiments have been performed to characterize the thermally-activated deformation processes. The strain rate sensitivity is very low at small strains but increases with increasing strain; the values measured by stress relaxation are greater than those derived from the strain rate cycling experiments, and the relaxation curves show “inverse” curvature at small strains. The athermal component of the flow stress originating from long-range dislocation interactions was estimated from dislocation densities obtained from etch pit micrographs. The dislocation density increases with increasing Y₂O₃ concentration, but the densities are too small to cause the appreciable athermal component of the flow stress; we believe that significant recovery must have occurred during cooling. The stress relaxation data can be interpreted by assuming that the deformation itself is mainly athermal, but that thermally-activated recovery takes place during the deformation; the Y₂O₃ solute may cause hardening by decreasing the diffusion kinetics. Alternatively, it is possible that the flow stress is controlled by the intrinsic lattice resistance of secondary slip systems.     

Athermal Solid Solution Hardening in Tantalum

The influence of solute atoms on the athermal component of the flow stress, determined by means of dip-tests (incremental unloading), has been investigated at room temperature and slightly above in binary Ta-Re, Ta-Mo, Ta-W, Ta-Hf, Ta-Zr, and Ta-Nb alloys and in ternary Ta-W-Re, Ta-W-Mo, Ta-W-Hf, and Ta-W-Nb alloys. Binary athermal substitutional solid solution hardening in tantalum is linear up to high concentrations of solute and is dominated by the atomic size misfit parameter, in agreement with the authors’ recent model for binary athermal solid solution hardening in bcc metals at temperatures where the Peierls stress is still important. In this model, solid solution hardening is caused by interactions of solute atoms having a size misfit with polarity reversing kinks and constrictions in 〈111〉 screw dislocations. The observed solid solution hardening in the ternary alloys is well described by the authors’ phenomenological model for multicomponent solid solution hardening. L.A. GYPEN, formerly with the Departement Metaalkunde, Katholieke Universiteit Leuven, Belgium     

The effect of zirconium on the strength of thorium

The flow stress of thorium-zirconium alloys containing up to 12 at. pct zirconium was studied over a range of temperatures and strain rates. It was found that the flow stress could be analyzed as the sum of a thermally activated and an athermal component. The thermally activated component was nearly the same in the base metal and the zirconium alloys. The major effect of zirconium was a considerable increase in the athermal component. The magnitude of this increase was directly proportional to the zirconium content. Zirconium also retarded the recovery and recrystallization processes. D. C. ZABEL is a former Graduate Assistant, Ames Laboratory, USAEC, Iowa State University, Ames, Iowa. 50010.     

The effect of grain boundaries on the athermal stress of tantalum and tantalum-tungsten alloys

The temperature dependence of the yield stress of polycrystalline Ta, Ta-2.47 wt pct W (Ta-2.5W), and Ta-9.80 wt pct W (Ta-10W) was measured to study the effect of grain boundaries and tungsten concentration on athermal strength components. Compression tests were performed over a temperature range from 77 to 1223 K at strain rates of 10⁻⁴ and 10⁻¹ s⁻¹. The test results show that the yield stress of Ta becomes independent of temperature above about 400 K, indicating an “athermal” regime. In contrast, the temperature dependence of yield stress was still significant for Ta-10W up to the maximum test temperature. An analysis of the test data using single-crystal data in conjunction with Taylor factors was performed to assess the effect of grain boundaries on the athermal component of flow stress at 600 K. The results indicated that the long-range athermal stress at the yield point due to grain boundaries is approximately 13 to 41 MPa for the study materials and decreases with an increase in tungsten concentration. These results are discussed with regard to constitutive modeling of flow stress.     

Effect of nitrogen on the strength of thorium

The effect of nitrogen in solid solution on the plastic deformation of polycrystalline thorium was studied by measuring the flow stress from 4.2 K to 773 K at several strain rates. Nitrogen was found to behave similarly to carbon in thorium and increased the thermally activated component of the flow stress. The activation energy was 1.3 eV for overcoming nitrogen atoms in solution. Aging the higher nitrogen alloys produced an age hardening contribution to the flow stress which was a thermally activated component at about 600 K but became an athermal component at lower temperatures.     

Effect of nitrogen on the strength of thorium

The effect of nitrogen in solid solution on the plastic deformation of polycrystalline thorium was studied by measuring the flow stress from 4.2 K to 773 K at several strain rates. Nitrogen was found to behave similarly to carbon in thorium and increased the thermally activated component of the flow stress. The activation energy was 1.3 eV for overcoming nitrogen atoms in solution. Aging the higher nitrogen alloys produced an age hardening contribution to the flow stress which was a thermally activated component at about 600 K but became an athermal component at lower temperatures. D. R. McLACHLAN, formarly a Graduate Assistant, Ames Laboratory, ERDA, Iowa State University.     

Comparison between high and low strain-rate deformation of tantalum

To understand the constitutive behavior of tantalum, compression tests are performed over the range of strain rates from 0.0001/s to 3000/s, and at temperatures from 296 to 1000 K. The flow stress is seen to be representable as the sum of a thermal, an athermal, and a viscous drag component. At high strain rates (3000/s), the thermal component is observed to be expressible in terms of the temperature and the strain rate, whereas the athermal component is independent of these variables. At lower strain rates, however, such a separation of the effects of the strain, strain rate, and temperature on the flow stress is not easily achieved. At high enough temperatures, i.e., temperatures above which the thermal component is essentially zero, viscous drag appears to have a significant effect on the flow stress. 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.     

Comparison between high and low strain-rate deformation of tantalum

To understand the constitutive behavior of tantalum, compression tests are performed over the range of strain rates from 0.0001/s to 3000/s, and at temperatures from 296 to 1000 K. The flow stress is seen to be representable as the sum of a thermal, an athermal, and a viscous drag component. At high strain rates (3000/s), the thermal component is observed to be expressible in terms of the temperature and the strain rate, whereas the athermal component is independent of these variables. At lower strain rates, however, such a separation of the effects of the strain, strain rate, and temperature on the flow stress is not easily achieved. At high enough temperatures, i.e., temperatures above which the thermal component is essentially zero, viscous drag appears to have a significant effect on the flow stress. was formerly with the Center of Excellence for Advanced Materials, University of California, San Diego. 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.     

The temperature independent plateau stress of solid solution crystals

A calculation of the plateau stress in solid solution crystals is presented assuming an arbitrarily oriented dislocation loop of lengthL, that moves under an applied stress. At high concentrations of solute atoms the dislocation segment does not interact with an individual solute atom but instead with all the solute atoms along the dislocation segment within a certain radius. The macroscopic flow stress is assumed to be determined by the maximum force that is encountered when a dislocation is moved over a distance equal to the distance between the position at zero stress and the critical position of an activated Frank-Read source. If the dislocation segment is assumed to be large compared to atomic distances, the interaction with groups of atoms will lead to an athermal process and therefore can explain the origin of the temperature independent flow stress in solid solution crystals. From this model the flow stress can be calculated with the help of statistical methods similar to those used in calculations of the movement of Bloch walls in magnetic materials. Besides the proper temperature dependence of the plateau stress the above model yields a dependence of the plateau stress upon the square root of the solute concentration, a result that is in good agreement with the measurements on silver, gold, and copperbased alloys. A linear relation between the solid solution hardening parameter dT/d√c and the strength of the solute atoms is obtained which is confirmed by the experimental results on copper-based alloys.     

The deformation of polycrystalline zirconium

The deformation of polycrystalline zirconium has been examined in terms of the power-law and the thermally activated rate equation approach to deformation. It is shown that the thermal component of the flow stress is independent of strain and that the athermal component is the main contributor to work hardening. The deformation behavior is in general agreement with Fleischer’s force-distance diagram for tetragonal defect-dislocation interaction. Electron microscopy however, showed that the majority of slip activity occurred on the prism planes and interstitials in octahedral positions do not interact strongly with dislocations on the prism planes. It is therefore suggested that deformation is controlled by solute atoms interacting with the dislocation core and that this interaction has a force-distance relationship equivalent to that suggested by Fleischer.     

The effect of solutes on the strength and strain hardening behavior of alloys

The effect of solutes on the yield strength and strain hardening behavior of a wide variety of alloys is considered. A discussion of the athermal and thermal mechanisms that appear to explain the observed peaks and plateaus in the temperature dependence of the flow stress is presented. Several explanations based on thermally activated mechanisms are found to provide superior flexibility in modelling the strength versus temperature behavior shown by the available data. These explanations are based on: (1) precipitation during deformation, (2) solute enhanced dislocation multiplication, (3) reduced rate of recovery due to solutes, or (4) enhanced effectiveness of barriers to dislocation motion by solutes. Models that simulate the temperature dependence of the flow stress (in the context of the MATMOD constitutive equations) are presented for each of the explanations cited above.     

Fracture toughness and deformation of titanium alloys at low temperatures

Experimental data concerning the influence of compositional, microstructural and textural variations on the fracture toughness and deformation kinetics of α and α-β titanium alloys are presented. In particular, the influence of these parameters on the thermal and athermal components of the flow stress is compared to their influence on fracture toughness. The similarities in deformation behavior among alloys are noted and contrasted to the dissimilarities in fracture toughness behavior.     

Lüders bands formation in a rapidly solidified Ni3al alloy ribbon

The phenomenology of Lüders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Lüders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Lüders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures (<470 K), a flat plastic region with few instabilities was seen; but at higher temperatures (>470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σ_eff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Lüders front motion and responsible for serration appearance.     

Analysis of Strain Rate Sensitivity of Ultrafine-Grained AA1050 by Stress Relaxation Test

Commercially pure aluminum sheets, AA 1050, are processed by accumulative roll bonding (ARB) up to eight cycles to achieve ultrafine-grained (UFG) aluminum as primary material for mechanical testing. Optical microscopy and electron backscattering diffraction analysis are used for microstructural analysis of the processed sheets. Strain rate sensitivity (m-value) of the specimens is measured over a wide range of strain rates by stress relaxation test under plane strain compression. It is shown that the flow stress activation volume is reduced by decrease of the grain size. This reduction which follows a linear relation for UFG specimens, is thought to enhance the required effective (or thermal) component of flow stress. This results in increase of the m-value with the number of ARB cycles. Strain rate sensitivity is also obtained as a monotonic function of strain rate. The results show that this parameter increases monotonically by decrease of the strain rate, in particular for specimens processed by more ARB cycles. This increase is mainly linked to enhanced grain boundary sliding as a competing mechanism of deformation acting besides the common dislocation glide at low strain rate deformation of UFGed aluminum. Recovery of the internal (athermal) component of flow stress during the relaxation of these specimens seems also to cause further increase of the m-value by decrease of the strain rate.     

Dislocation mechanics-based constitutive equations

A review of constitutive models based on the mechanics of dislocation motion is presented, with focus on the models of Zerilli and Armstrong and the critical influence of Armstrong on their development. The models were intended to be as simple as possible while still reproducing the behavior of real metals. The key feature of these models is their basis in the thermal activation theory propounded by Eyring in the 1930’s. The motion of dislocations is governed by thermal activation over potential barriers produced by obstacles, which may be the crystal lattice itself or other dislocations or defects. Typically, in bcc metals, the dislocation-lattice interaction is predominant, while in fcc metals, the dislocation-dislocation interaction is the most significant. When the dislocation-lattice interaction is predominant, the yield stress is temperature and strain rate sensitive, with essentially athermal strain hardening. When the dislocation-dislocation interaction is predominant, the yield stress is athermal, with a large temperature and rate sensitive strain hardening. In both cases, a significant part of the athermal stress is accounted for by grain size effects, and, in some materials, by the effects of deformation twinning. In addition, some simple strain hardening models are described, starting from a differential equation describing creation and annihilation of mobile dislocations. Finally, an application of thermal activation theory to polymeric materials is described. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

Deformation behaviour of zircaloy-4 between 77 and 900 K

Tensile tests were carried out on zircaloy-4 over a range of temperatures 77–900 K, flow stress values at various strain rates were used to determine the activation parameters. Two thermally activated zones were observed between 77 and 600 K and the deformation continues to be thermally activated above 700 K. Within the temperature range 600–700 K, an athermal region occurs with a hump, the position of which is a function of strain rate. The various mechanisms which can be responsible for the thermal and athermal zones are discussed in the light of experimental results. The mechanical behaviour of zircaloy-4 and zirconium-oxygen alloys were observed to be generally similar.     

The shape memory effect and pseudoelasticity in polycrystalline Cu-Zn alloys

The shape memory effect associated with the reverse transformation of deformed martensite, pseudoelastic behavior involved in stress-induced martensite formation and the reversion of strained martensite after an applied stress is relaxed aboveA _f have been studied. Grain size and specimen geometry effects have been related to the above phenomena. Although recoverable strains as high as 10.85 pct were observed in coarse-grained (“bamboo” type) specimens, the shape memory effect is restricted in fine-grained specimens because of permanent grain boundary deformation and intergranular fracture which occurs at relatively low strains. A fine grain size also acts to suppress pseudoelastic behavior because permanent, localized deformation is generated concurrent with the formation of stress-induced martensite which inhibits reversion of the latter upon release of stress. The apparent plastic deformation of martensite belowM _f can be restored by transforming back to the original parent phase by heating toA _f (shape memory) or alternatively, can be recovered belowM _f by applying a small stress of opposite sign. Martensite deformed belowM _f with the same stress maintained while heating persists aboveA _f, but reverts to the parent phase in a pseudoelastic manner when the stress is relieved. The athermal thermoelastic martensite, which forms in groups composed of four martensite plate variants, undergoes several morphology changes under deformation. One of the variants within a plate group cluster may grow with respect to the others, and eventually form a single crystalline martensitic region. At a later stage pink colored deformation bands form in the same area and join up with increasing stress, resulting in thermally irreversible kinks. The clusters of plate groups may expand like grain growth or contract as a whole during deformation, or act as immobile “subgrains” which lead to permanent deformation at their boundaries. Stress-induced martensite usually forms as one variant of parallel plates which join up with increasing stress to form single crystalline regions. Further stress leads to pink colored deformation bands, similar to those in the deformed athermal martensite. Other similarities and differences between the stress-induced and athermal martensite have been investigated and are discussed.     

Formation and reversion of stress induced martensite in Ti-10V-2Fe-3Al

The formation and reversion behavior of stress induced orthorhombic martensite (α″) in Ti-10V-2Fe-3Al has been studied, and several interesting features found. Reversion appears to take place in four stages. First, there is an athermal reshearing of α″ back to the b.c.c. parent phase, which is accompanied by a significant one-way shape memory effect between 165° and 240°C. This athermal reversion competes with an isothermal stabilization of the α″ plates. Further heating results in the isothermal precipitation of α, which seems to reproduce the preferred orientations of the previous α″, and is thus also accompanied by a shape memory effect; an isothermal shape memory effect in the opposite direction as the preceeding athermal shape change. The precipitation of α occurs on two microstructural scales, extremely fine α nucleating in the β matrix on extant isothermal ω particles, and the direct transformation of the unreverted α″ plates to α. Finally, when heating is continued to the β-transus temperature, the α begins to dissolve back to the parent β. This final dissolution is accompanied by a textural loss in the α phase, and consequently gives rise to several final “shape adjustments”, the nature of which have not been fully explained.