Effect Of Stacking Fault Energy Research Articles

Stacking fault energy (s.f.e.) and grain size effects (d) on the tensile behaviour of f.c.c. polycrystalline alloys at 300 K: Back stress and effective stress evolutions

The aim of this work is to provide experimental results to understand grain size and stacking fault energy effects (γ/μb) on tensile hardening f.c.c. alloys. The hardening rate is discussed in terms of back stress (X) and effective stress (Σ ef ) evolutions. Irrespective of the material studied, tensile hardening behaviour before necking is divided into three stages (I, II, and III). These stages were previously discussed using qualitative and semiquantitative TEM observations [I]. In particular, we have shown that intergranular back stress evolution relates the hardening rate in stage I, where single and planar slip are observed in most of the grains. In the other stages, latent hardening and intragranular back stress are the main parts of the hardening rate in relation with the formation of heterogeneous dislocation structures. An increase of grain size and/or a decrease of stacking fault energy favour planar slip and then stage I, in terms of plastic strain. The transition between stage II and stage III seems to be less dependent on grain sizes irrespectively of s.f.e., The classical Hall-Petch relation is discussed in terms of back and effective stresses for different plastic strain levels. If these two components verify the Hall-Petch relation, however, effective stress is less dependent on grain size than back stress. This last dependence increases in stage I, where intergranular back stress is the main pan of hardening and decreases in the other stages where this component decreases and intragranular back stress increases. The grain size effect on effective stress is well explained in terms of mean length path using dislocation modelling.

Estimation of interaction energies Me-(C, N) in f.c.c. iron-based alloys using thermo-calc thermodynamic database

The tensile behavior of austenitic stainless steels with different concentration of nitrogen was studied by analyzing deformation microstructure and calculating short-range ordering (SRO) as well as stacking fault energy (SFE). Increasing nitrogen concentration increased SFE and SRO, which affected the tensile behavior. From the early to intermediate deformation stages, the nitrogen addition induced slip planarity and the delay of dynamic recovery, which were brought by the increase of SRO. As a result, work hardening rate (WHR) rose with the nitrogen content at the end of the intermediate deformation stage. On the other hand, the SFE effect was noticeable at higher strain levels. With increasing nitrogen concentration, the formation of slip lines was suppressed and dynamic recovery was promoted. Hence, the reduction in WHR was accelerated with the nitrogen addition. When the overall dynamic recovery behavior was assessed with dislocation-density based constitutive modeling, it was found to depend mainly on SRO rather than on SFE. Therefore, the alloy with higher nitrogen content could achieve higher dislocation density and flow stress.

Effect of stacking-fault energy on the development of a dislocation substructure, strain hardening, and plasticity of fcc solid solutions

Effect of stacking-fault energy on the development of a dislocation substructure, strain hardening, and plasticity of fcc solid solutions

In-situ observations of microvoid coalescence: Stacking fault energy effects

Although it has been well established that microvoid coalescence occurs during static or quasi-static fracture in ductile materials, the exact mechanism for microvoid formation is still unclear. It has been argued that microvoids initiate and grow from second phase particles. However this argument cannot be used to explain the existence of microvoids on the fracture surfaces of "pure" materials. An alternative mechanism for their formation in "pure" materials is that they initiate and grow along dislocation cell walls. If this premise is true; then the nature and extent of microvoid coalescence should be related to the stacking fault energy (SFE) of the material since the latter is a controlling parameter in the formation of dislocation cells. The relationship between microvoid coalescence and stacking fault energy may have some basis since absolute cell dimensions are of the same magnitude as the observed dimple sizes. The present study examines the effect of dislocation cell structures on the formation of microvoids as a function of the stacking fault energy of a given material through direct observation of the void formation and growth process within the TEM. The fundamental aspects of the work is to correlate the dislocation substructures, void initiation, growth, and coalescence to the resulting fracture surfaces.

The influence of stacking fault energy and adhesion on the wear of copper and aluminum bronze

The subsurface deformation induced in pure copper and a series of Cu-Al alloys has been characterized using scanning electron microscopy after prolonged dry sliding contact against an Al 2O 3 cylinder. Scanning electron microscopy was found to be a very useful technique for examining the deformation in depth. The strength of the adhesive bond between copper and Al 2O 3 was found to be less than the cohesive strength of either material. Therefore true transfer layers were not formed. Owing to dynamic recrystallization occurring near the surface, extremely large strains ( i.e. greater than 12) were sustained and resulted in very low wear rates in agreement with the results of Bill and Wisander. A mechanically mixed layer, which is continuous with the substrate and incorporates oxide particles, was observed. Two mechanisms were identified for the production of wear debris. The layered structure proposed by Heilmann to explain the random misorientation of subgrains was confirmed. The adhesive strength of the Cu-Al alloys examined with Al 2O 3 was greater than the cohesive strengths of the alloys. Therefore metal transfer to the slider occurred. The subsurface deformation behavior of aluminum bronze was found to be very dependent on both the stacking fault energy (SFE) and the orientation of individual grains and is related to the wear rate. In the case of aluminum bronze sliding against A1 2O 3, adhesion effects overshadow the influence of SFE. However, in other systems, it is expected that SFE effects will play a dominant role in determining wear response.

High Temperature Creep of Ni-Cr-Co Alloys and the Effect of Stacking Fault Energy

High Temperature Creep of Ni-Cr-Co Alloys and the Effect of Stacking Fault Energy

Creep and dislocation substructure

The elevated-temperature creep of pure metals and class II solid-solution alloys is reviewed with particular reference to dislocation-climb controlled creep. The phenomenological relationship between externally controlled variables such as stress and temperature are discussed in the context of easily measurable microstructural parameters such as grain size, crystal structure, and diffusivity. The role played by more subtle substructural parameters, e.g. dislocation density, distribution of dislocations inside subgrains and in subboundaries, interaction between dislocation networks, effect of stacking-fault energy, and dependence of subgrain misorientation angle on strain are emphasized. The currently available models of dislocation creep that specifically emphasize the substructural evolution during creep are reviewed. Finally, the effect of thermomechanical processing in altering the microstructure and the concomittant changes in creep properties are discussed and illustrated with recent creep data on pure Mo and a Mo-5W alloy.

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.

The Effect of Stacking-Fault Energy on the Stress/Strain Curve of Dispersion-Hardened Ni-Co Alloys

AbstractThe 0.1% proof stress and strain-hardening rates of four Ni-Co-2½ vol.-% Al2O3 alloys, which varied in matrix stacking-fault energy from 24 × 10−2 to 1 × 10−2 J/m2, have been studied in the temperature range 77–1000 K. The proof stress of the 0, 30, and 60% Co alloys was shown to be independent of temperature and strain rate below 600 K. However, that of the 67% Co alloy was temperature-sensitive, as a result of the temperature-dependence of stacking-fault energy which markedly affects the stability of Orowan loops at low fault energies. At temperatures above 600 K the strain-hardening rate of all the alloys became temperature- and strain-rate dependent, because of a recovery process that is dependent on stacking-fault energy.

Effect of stacking-fault energy on steady-state creep rates in copper-base solid solutions

Effect of stacking-fault energy on steady-state creep rates in copper-base solid solutions

Dislocation structure and the effect of stacking-fault energy in high temperature creep of Cu-Al solid solutions

Abstract Cu-Al solid solutions (containing 2·5, 5·5, 11 and 16 at. % Al) with different stacking-fault energies were subjected to high temperature creep tests and their dislocation structure was investigated by means of transmission electron microscopy. The comparison of the structures observed in different solid solutions resulted in the conclusion that at high temperatures their stacking-fault energies are probably less different than at low temperatures. The results of the dislocation density evaluation correspond with the macroscopic creep characteristics. The effect of the stacking-fault energy on the high temperature creep behaviour has been discussed.

Effect of stacking-fault energy on thermal activation of attractive junctions in change-in-stress creep of FCC metals

Change-in-stress creep experiments involving increments as well as decrements of stress have been conducted on Ag, Au, Cu, Ni, Pb, Pd and Pt at temperatures at and below room temperature. The activation volumes (v) evaluated from the flow parameter,B (given by $B=({\Delta}ln \hspace {2mm}{\epsilon}/{\Delta}{\tau})=v/kT$ obtained by making stress increments, were found to be in the range $10^2$ to $10^4b^3$, indicating the rate-controlling mechanism to be the intersection of glide and forest dislocations. The value of the flow parameter was found to be higher for creep following stress decrements than increments, the difference being a decreasing function of stress and temperature. The results were analysed in terms of the model, earlier suggested for aluminium, based on the formation of attractive junctions on stress decrement which causes a change in the number of elements participating in the activation event. The stacking-fault widths and energies evaluated for these metals based on the above model are found to be in good agreement with the reported values.

Creep Mechanisms in Niobium-Stabilized Austenitic Steels

The steady-state creep rates in the range 550–775° C and 1600–40,000 lb/in2 have been determined for five niobium-stabilized austenitic steels with stacking-fault energies in the range 5–35 ergs/cm2. No effect of stacking-fault energy was observed. Analyses of the stressand temperature-dependence suggest that at strain rates > ∼ 10−8/sec (stresses near or above the yield strength, ∼ 10,000 lb/in2) the creep is controlled by a cross-slip recovery process, while at lower strain rates creep is controlled by the diffusion of solute attached to dislocations. These interpretations are consistent with recovery measurements, electron metallographs, and calculations of climb rates. The consequences as regards the design of creep-resisting alloy steels are discussed.