Low Stacking Fault Energy Materials Research Articles

Effect of starting grain size on the evolution of microstructure and texture during thermo-mechanical processing of CoCrFeMnNi high entropy alloy

The effect of starting grain size on the formation of microstructure and texture in heavily deformed equiatomic CoCrFeMnNi high entropy alloy (HEA) was investigated. For this purpose two alloys with average grain size ∼7 μm (fine grained starting material or FGSM) and 200 μm (coarse grained starting material or CGSM) were cold-rolled to 90% and 95% reduction in thickness and isochronally annealed for one hour at temperatures ranging from 700 °C to 1200 °C. Development of strong brass type texture, typical of low stacking fault energy (SFE) materials was noticed in both FGSM and CGSM. Near ultrafine microstructure formation was observed after annealing at 700 °C but extensive grain growth was observed only after annealing at 1200 °C. The FGSM showed finer grain size as compared to CGSM after heavy deformation and annealing. The annealing twin boundary fraction increased with increasing annealed grain size. The annealing textures of both FGSM and CGSM did not show strong dominance of the {236<385> or {113}<332> components unlike low SFE brass. Furthermore, the volume fractions of different annealing texture components of the HEA were not significantly affected by starting grain size or annealing temperature in stark contrast to low SFE brass. These differences could be explained on the basis of homogenously deformed matrix and sluggish diffusion in HEA which greatly reduced the preferential nucleation and growth of components, respectively.

Analysis of microstructure and microtexture during grain growth in low stacking fault energy equiatomic CoCrFeMnNi high entropy and Ni–60 wt.%Co alloys

The evolution of microstructure, texture and fraction of annealing twin boundaries is studied in Ni–60wt.%Co and equiatomic CoCrFeMnNi high entropy alloy (HEA) with similar low stacking fault energy (SFE). The two materials are cold-rolled to 90% reduction in thickness and subjected to different annealing treatments to achieve very similar average grain size. The twin boundary fraction in the two materials was found to be similar for the same average grain size. This indicated that twin boundary fraction depended on the final grain size. The two alloys revealed a strong brass type deformation texture typical of low SFE materials. However, the texture after grain growth in the two alloys was strikingly dissimilar. The Ni–Co alloy showed strong α-fiber (ND//〈110〉) components, namely, G ({110}〈011〉) and G/B ({110}〈115〉) due to the preferential growth of these components. In contrast, the microstructure of the HEA showed no preferential grain growth and texture components in HEA revealed only minor changes. These sharply different microstructure and texture formation mechanisms in the two alloys could be explained on the basis of sluggish diffusion in multicomponent HEA which greatly diminished grain boundary mobility and restricted preferential grain growth.

Microstructural development of high temperature deformed AZ31 magnesium alloys

Due to their significant role in automobile industries, high temperature deformation of Mg–Al–Zn alloys (AZ31) at constant stress (i.e. creep) were studied at a wide range of stresses and temperatures to characterize underlying deformation mechanism, dynamic recrystallization (DRX) and dislocation density evolution. Various microstructures (e.g. grain growth & DRX) are noted during steady-state creep mechanisms such as grain boundary sliding (GBS), dislocation glide creep (DGC) and dislocation climb creep (DCC). Although a combination of DRX and grain growth is characteristic of low stacking fault energy materials like Mg alloys at elevated temperatures, observation reveals grain growth at low strain-rates (GBS region) along with dynamic recovery (DRV) mechanism. X-Ray Diffraction (XRD) analysis revealed a decrease in dislocation density during GBS region while it increased under dislocation based creep mechanisms which could be related to the possible DRV and DRX respectively. Scanning Electron Microscopic (SEM) characterization of the fracture surface reveals more inter-granular fracture for large grains (i.e. GBS region with DRV process) and more dimple shape fracture for small grains (i.e. DGC & DCC region with DRX).

Development of grain structure during friction-stir welding of Cu–30Zn brass

High-resolution electron backscatter diffraction was employed to study the evolution of microstructure during friction stir welding of a typical low stacking fault energy material (Cu–30Zn brass) over a wide range of welding temperatures. In all cases, microstructure evolution was found to be governed by discontinuous recrystallization that occurred via grain-boundary bulging and subsequent recrystallization nucleation. This promoted drastic grain refinement and significant strengthening. The crystallographic textures that evolved were typically dominated by a simple shear texture.

Martensitic Transformation in Micron-Sized Fcc Single Crystals

Detailed transmission electron microscopy examinations verified that α′-martensite formed in micron-sized pillars is nearly dislocation free, surprisingly different than its counterpart in bulk samples, which usually contains a high dislocation density. Furthermore, the martensite was found to nucleate at the intersection between two packets of stacking faults in this low stacking fault energy material. A corresponding mechanism for the nucleation and growth of martensite in micron-sized pillars was proposed.

SCC growth rate measurements and surface oxide film characterization on high SFE austenitic stainless steels in a simulated BWR water environment

ABSTRACTStress Corrosion Cracking (SCC) has been detected in Boiling Water Reactors (BWRs) on core shrouds and primary water re-circulation piping made of low carbon stainless steels. Material hardening strongly affects SCC propagation behavior, and SCC growth rates increase with increasing hardness of austenitic stainless steels caused by cold work or neutron irradiation.Research work has been conducted in the authors’ laboratories with the aim of improving SCC resistance using chemical composition control of stainless steels. It has been previously reported that high stacking fault energy (SFE) materials showed better SCC resistance than low SFE materials due to hardening being suppressed in high SFE materials. In the present study, SCC growth rate (CGR) tests were performed using 15% cold worked Types 316L and 25Cr-20Ni stainless steels in a simulated BWR water environment. The 25Cr-20Ni stainless steel used has high SFE value due to chemical composition control and measured SCC growth rates were lower than those of low SFE stainless steels.However, oxidation behavior is one of the more important factors influencing SCC of austenitic stainless steels in addition to material hardening behavior, and the influence of the chemical composition control necessary to increase SFE on oxidation behavior in BWR primary coolants is still unclear. In this study, therefore, immersion tests using Types 316L and 25Cr-20Ni stainless steel specimens were also conducted in the simulated BWR water environment. The surface oxide films on the specimens were then analyzed with micro-Raman spectroscopy and glow discharge optical emission spectroscopy in order to help clarify the oxidation behavior.The results of these tests and analyses showed that the NiFe2O4 content of the outer oxide layers on the high SFE stainless steels was higher than that on the low SFE stainless steels. The inner oxide film on the 25Cr-20Ni stainless steel also had a high chromium content.Based on the above results, SCC resistance and oxidation behavior of high SFE austenitic stainless steels in a simulated BWR water environment will be discussed.

Surface Engineering of Stainless Steels: Role of Surface Mechanical Attrition Treatment (SMAT)

Surface mechanical attrition treatment (SMAT) technique has became popular to develop a nanostructured surface layer on metallic materials for upgrading their overall properties and performance. In this paper, we have presented the SMATing behavior of low stacking fault energy material like AISI 304 using optical microscopy, SEM, microhardness measurement and XRD analysis. SMATing was performed for 15, 30, 45, 60, 75, 90 min by using hardened bearing-steel balls (size: 5.7 mm diameter, hardness: 500HV0.1) at 50 Hz vibrating frequency. XRD analysis indicated the lowest grain-size of about 8.6 nm in the surface region of specimen SMATed for 60 min. In comparison with the non-SMATed specimen, 17 times increase in the dislocation density and 4 times increase in the micro-strain were observed in this SMATed specimen. Improvement in the surface-hardness due to the SMAT was almost two times hardness before SMAT was 190 HV0.1 and after SMAT it was 400 HV0.1. There is a gradual decrease in the hardness value across the cross-section of the specimen, and core-hardness value was reached after 300 μm depth below the surface. XRD results indicated the possibility of martensitic phase transformation at the surface during SMATing of AISI 304 steel. SMATed AISI 304 specimens showed good thermal stability at 550°C for 6 h which was confirmed by microhardness measurement

Dependence of equilibrium stacking fault width in fcc metals on the γ-surface

A phase field dislocation dynamics model that can model widely extended dislocations is presented. Through application of this model, we investigate the dependence of equilibrium stacking fault width (SFW) on the material γ-surface in fcc metals. This phase field model includes a direct energetic dependence on a parametrization of the entire γ-surface, which is directly informed by density functional theory. A wide range of materials are investigated and include both very low stacking fault energy (SFE) materials, such as silver, and high SFE materials, such as palladium. Additionally, analysis shows that by accounting for the unstable stacking fault energy, γU, we can better describe the material dependence of the equilibrium SFW rather than using only the intrinsic SFE, γI. Specifically, we see a direct dependence of the stable SFW between partial dislocations on the energy difference (γU − γI), which describes the energy barrier that partial dislocations must overcome in order to widen the stacking fault.

Influence of Dislocation Separation on Dynamic Strain Aging in a Fe&amp;ndash;Mn&amp;ndash;C Austenitic Steel

ential slip system of trailing partials is not simply determined by Schmid factors of the trailing partials, but is also affected by the Schmid factors of the corresponding leading partials. In spite of these significant differences among the motions of perfect, leading partial and trailing partial dislocations, the effect of the separation of dislocations has not been considered for discussing the dynamic strain aging behavior in low stacking fault energy materials so far. In order to discuss and correlate the effect of the dislocation separation with experimental facts, we selected aF e­17Mn­0.3C alloy (mass%) as an austenitic steel with low stacking fault energy. The temperature and strain rate dependence of dynamic strain aging behavior was investigated. We found that the deformation temperature and the critical strain for the onset of serrations could not be expressed as a simple relationship, i.e., the critical strain decreased in the relatively-low temperature region increased in the middle range temperature region, and decreased again in the relatively-high temperature region with increasing deformation temperature. The characteristic dynamic strain aging behavior may have been caused by the separation of dislocations. This paper is intended to share the importance of the dislocation separation and its experimental facts in dynamic strain aging of low stacking fault energy materials.

Texture Evolution during Recrystallisation of Cold Rolled TWIP Steel

A TWinning Induced Plasticity (TWIP) steel was cold rolled to 42% thickness reduction followed by isochronal annealing for 300 s between 600-850 °C. Bulk texture evolution during recrystallisation was investigated by X-Ray Diffraction. While the development of the α-fibre after cold rolling is typical of low stacking fault energy materials, anomalously higher intensities were noted for the Goss ({110}) compared to Brass ({110}) orientations. Upon recrystallisation, the main rolling texture components were retained and ascribed to nucleation at orientations close to those of the deformed matrix followed by annealing twinning which leads to crystallographically identical variants. Unlike previous texture investigations on austenitic steels, the relatively homogeneous deformation microstructure and uniform distribution of subsequent nucleation sites led to the retention of the F ({111}) orientation. Moreover, the firsthand observation of the Rotated Copper ({112}) orientation in TWIP steel is attributed to the second order twinning of the A ({110}) orientation.

On the Onset of Dynamic Recrystallization in Steels

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanisms available for the control of microstructure. This mechanism occurs during hot forming operations over a wide range of metals and alloys and it is known to be as a powerful tool which can be used to the control of the microstructure and properties of alloys. Therefore is important to know, particularly in low stacking fault energy (SFE) materials, the precise time for which DRX is available to act. At constant strain rate such time is defined by a critical strain, εc. Unfortunately this critical value is not directly measurable on the flow curve; as a result different methods have been developed to derive it. Focused on steels, in the present work the state of art on the critical strain for the initiation of DRX is summarized and a review of the different methods and expressions for determining εc is included. The collected data is suitable to feeding constitutive models.

Structural response of superaustenitic stainless steel to friction stir welding

Electron backscattering diffraction was employed to study grain structure development and texture evolution during friction stir welding (FSW) of a low stacking fault energy material, S31254 superaustenitic stainless steel. Formation of the final stir zone (SZ) microstructure was deduced to be primarily governed by discontinuous recrystallization occurring during the FSW cooling cycle. The textural pattern formed in the SZ was interpreted in the terms of {1 1 1}〈 u v w〉 and { h k l}〈1 1 0〉 partial simple shear fiber textures.

Texture evolution of cold rolled and annealed Fe–24Mn–3Al–2Si–1Ni–0.06C TWIP steel

The microstructure and texture evolution of 42% cold-rolled Fe–24Mn–3Al–2Si–1Ni–0.06C TWinning Induced Plasticity (TWIP) steel is investigated during isochronal annealing at temperatures between 600 and 850 °C. In the cold rolled condition, bulk texture returned the distinctive α-fibre for low stacking fault energy materials, with higher intensities for Goss ({1 1 2}〈 0 1 1 〉) compared to Brass ({1 1 1}〈 1 1 2 〉). A comparison between bulk and micro-textures, showed a significant slip contribution to the development of the Brass orientation, along with a possible role for micro-shear banding. Annealing twins contribute to recrystallisation from the early stages of nucleation and participate in generating new orientations thereafter. Unlike texture studies on other austenitic steels, the F ({1 1 1}〈 0 1 1 〉) and Rotated Copper ({1 1 2}〈 0 1 1 〉) orientations were detected in this work. The former is due to a more homogeneous distribution of nucleation sites, while the latter can be ascribed to second order twinning and the preferred-growth 30° 〈1 1 1〉 relation with the Brass rolling component. Based on the microstructural parameters from Electron Back-Scattering Diffraction (EBSD), the modified Hall–Petch (H–P) relation was successfully applied to the 0.2% proof stress.

Nanotwinned microstructures from low stacking fault energy brass by high-rate severe plastic deformation

High-rate severe plastic deformation by machining is explored in a prototypical low stacking fault energy material, brass, as a route for generating a high density of nanometer-scale twins amidst dislocation structures. This nanotwinned system shows a greatly improved strength compared to the bulk microcrystalline state. The twin densities are found to be retained even after modest thermal agitation, which is also seen to engender a small “hardening by annealing” effect in the nanotwinned material.

Microstructure Evolution and Microstructure-Property Relationships in Friction Stir Processing of NiAl Bronze

Friction stir processing (FSP) has been employed for localized modification and control of microstructures in NiAl bronze materials, which are widely utilized for marine components. The thermomechanical cycle of FSP results in homogenization and refinement and the conversion of microstructures from a cast to a wrought condition within stir zones in the material. However, the direct measurement of stir zone temperatures, strains, strain rates and cooling rates is difficult due to steep gradients and transients in these quantities, and this is an impediment in the assessment of FSP-induced microstructures and properties. Quantitative microstructure analyses following FSP of cast NiAl bronze materials have been used to develop estimates of stir zone thermomechanical cycles. The estimation procedures will be reviewed and the microstructure-based estimates will be compared to results from computational models and embedded thermocouples measurements. Stir zone microstructures comprise a mixture of primary α grains and transformation products of the β that formed during processing. Recrystallization in the primary α occurred due to particle-stimulated nucleation in this low stacking fault energy material. Factors that influence the distribution of strength and ductility in the stir zone appear to include the mixture of microstructure constituents and gradients in microstructure due to gradients in processing conditions.

Study of Bauschinger Effect by Reverse Shearing of 70:30 Brass

Reverse shear experiments were conducted on a low stacking fault energy (SFE) material, namely 70:30 brass, and for comparison a high SFE material, namely commercial pure aluminum 1100, was chosen. Interesting results on the Bauschinger effect were observed. Appreciable Bauschinger effect was seen for 70:30 brass and this was explained through a new possible de-twinning theory, investigated here, which causes the fraction of twin to decrease during the reverse cycle leading to a decrease in the flow stress level. The other possible reason considered is the appreciable back stress developed due to the presence of obstacles (deformation twins) causing dislocation pile-ups, a theory previously investigated. The de-twining hypothesis was verified using deformation path change tests (changing deformation mode from simple shear to simple compression). It was also seen that the Bauschinger effect was negligible for aluminum (a high SFE material). Microscopic study was conducted on the sheared samples to study the microstructure evolution during forward and reverse shear deformation.

Simulation of Recrystallization Textures in fcc Materials Starting from Self-Consistent Modelling Results

Recrystallization (RX) textures in FCC materials have defied modelling for quite a long time despite the major micro-mechanisms influencing the texture development are currently considered understood. FCC materials are coarsely classified in high and low stacking fault energy materials with a rather continuous transition between them. Both extreme kinds will give rise, after rolling deformation and later RX, to what are called Cube and Brass texture patterns. Besides long discussions, not still settled, about the influence of twinning on deformation textures, attempts of simulating RX textures have been unsuccessful. The current contribution will show SelfConsistent (SC) plus RX modelling taking into account accumulated deformation energy and misorientation angles between neighbouring crystals. Starting from grains characterized by previous SelfConsistent simulations, nucleation is probabilistically allowed on each crystal whenever the accumulated energy is larger than certain threshold. Grain boundary mobility is commanded by a probability function of the misorientation angles also calculated by the SC model. The modelling is able to predict the right trend for many of the literature experimental data without resorting to other more sophisticated variables.

Deformation induced Nanostructure and texture in MP35N alloys

The macro-texture was related to the nano-platelets formed during the rolling deformation of a Co-Ni-Cr-Mo alloy (MP35N) with low stacking fault energy. The deformed materials showed {011}(533) texture but also had {011}(211) and {011}(100) texture components. The {011}(533) component reached the maximum at 74% reduction-in-area. Further deformation of the material to 80% decreased the intensity of the {011}(533) component. The cold deformation introduced platelets of a few atomic layer in thickness and less than 100 nm in diameters. The habit planes of the platelets were identified to be {111}, which were perpendicular to both the rolling and {011} crystallographic plane. Therefore, the tensile strain in the rolling direction assisted formation of the platelets, which were identified as stacking faults. A high density of nano-platelets and dislocations strengthened materials and influenced the plastic deformation behaviors and texture evolution. Thus, the MP35N developed slightly different textures from other low stacking fault energy materials. The maximum at {011}(533) was related the nanoplatelets and stacking fault energy. The {011}(112) and {110}(001)components could be linked to the low stacking fault energy.

Stacking fault energy and dynamic recovery: do they impact the indentation size effect?

Aluminum, a high stacking fault energy (SFE) material and α-brass, a low SFE material were tested for the indentation size effect (ISE) using a combination of microhardness (high load) and nanoindentation (low load). Four samples from each material were tested; annealed electropolished, annealed mechanically polished, work-hardened electropolished, and work-hardened mechanically polished. The micro and nano indentations were made using a Vickers and a Berkovitch diamond indenter tips with indentation loads between 0.1–3 (microindentations) and 10 −4–10 −2 N (nanoindentations), respectively. Based on areas measured using optical and scanning electron microscopy in addition to contact stiffness, it is found that the calculated nanohardness increases monotonically with decreasing load or depth of indentation. The work-hardened samples are harder than the annealed ones except for aluminum at shallow indents where it is not possible to distinguish between differences in hardness. All eight samples regardless of the SFE, rate and intensity of cross-slip and dislocation climbing rates (dynamic recovery), and level of prior work hardening of the material, explicitly exhibited an ISE. The magnitude of the ISE is not influenced by SFE, dynamic recovery, or prior level of work hardening. The data are found to behave linearly consistent with the Strain Gradient Plasticity model (SGP) for micro and deep nanoindentations; the shallow nanoindentation data deviated into a second linear behavior constituting what we term a ‘bilinear behavior’.

A metallographic and quantitative analysis of the influence of stacking fault energy on shock-hardening in Cu and Cu–Al alloys

This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.