Papers by Keyword: Stacking Fault Energy

Abstract: Percutaneous coronary intervention (PCI) is a minimally invasive treatment for ischemic heart disease, commonly supported by balloon-expandable stents to prevent arterial restenosis. Stent materials must combine high ultimate tensile strength, high ductility, low 0.2% proof strength, high corrosion resistance, high X-ray visibility, and magnetic susceptibility close to that of human soft tissues. The Co-20Cr-15W-10Ni (mass%) alloy, standardized as ASTM F90 and commonly referred to as L605 is widely employed for this purpose. Recently, the demand for stents with smaller diameters and thinner struts has grown, as these significantly lower restenosis risk. Alloys for thin-strut stents therefore require exceptional mechanical and physical performance. This paper reviews the microstructures and mechanical and physical properties of the carbon- and Pt-modified Co-Cr-W-Ni alloys developed by our group. The Co-20Cr-15W-10Ni-0.2C (mass%) alloy achieved an excellent strength–ductility balance and a low 0.2% proof strength owing to the γ-stabilizing effect of carbon. First-principles calculations further revealed that carbon addition increases stacking fault energies (SFEs) in Co-Cr alloys. Pt-modified Co-Cr alloys exhibited higher X-ray visibility than L605 and greater strength than Pt-Cr steel while maintaining comparable ductility. Collectively, these results indicate that carbon- and Pt-modified Co-Cr alloys are promising candidates for next-generation balloon-expandable stents, particularly thin-strut designs.

Abstract: High-throughput computational screening (HTCS) based on CALPHAD (Calculation of Phase Diagram) was employed to investigate potential chemical compositions within the medium manganese steel family for achieving desired austenite stability and stacking fault energy (SFE). The primary objective was to identify optimal alloy compositions that balance the complex effects of various alloying elements on retained austenite fraction and related mechanical properties. Utilising TC-Python Thermo-Calc software coupled with a custom-developed algorithm, two optimised compositions were determined: 0.35C, 9Mn, 1Mo, 3Al, 1Si, 0.05Nb, 0.3V (alloy 353), and 0.35C, 9Mn, 1Mo, 3Al, 1Si, 0.1Nb (alloy 310) in wt.% to be the best fited composition to our selected criteria. The alloys were subsequently produced via open-air induction furnaces, and the microstructure was analysed after the hot forging condition. The initial multiphase as-cast structure, primarily composed of lath martensite, δ-ferrite (34 vol.%), and retained austenite (RA, 5–7 vol.%), experienced notable grain refinement. Forging reduced δ-ferrite grain sizes from 39 µm to 12 µm (alloy 310) and from 46 µm to 9 µm (alloy 353), accompanied by increased RA content (28 vol.% for alloy 310 and 46 vol.% for alloy 353) and reduced RA grain sizes (1.2 µm and 1.9 µm, respectively). Non-metallic inclusions (NMIs) were analysed using field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, classifying inclusions primarily as AlN, MnS, (Mo,Nb)C, or their combinations. No significant differences in inclusion types were observed, but forged samples displayed reduced inclusion sizes. In summary, the results showed that HTSC effectively identified optimal compositions with a high fraction of retained austenite.

Abstract: Austenitic stainless steels are commonly used for hydrogen storage and transportation. These alloys have a high nickel (Ni) content, which increases alloy cost. In this study, high manganese (Mn) austenitic alloys were evaluated as potential lower cost alternatives. Two heats of high Mn alloys with different stacking fault energies (SFE) of ~29 mJ·m^-2 and 49 mJ·m^-2 were acquired. Additionally, a new vanadium (V)-microalloyed high Mn alloy was designed to achieve a SFE of 47 mJ·m^-2 to minimize planar slip deformation mechanisms. Post-processing via cold working in conjunction with aging was also performed on the V-microalloyed high Mn steel. Hydrogen embrittlement sensitivity was investigated using circumferential notch tensile specimens cathodically charged with hydrogen in a 0.05M NaOH electrolytic solution. The alloys were compared to a cold-worked 316L stainless steel, which exhibited no strength loss due to hydrogen. The high Mn alloys with SFE of ~29 mJ·m² and 49 mJ·m^-2 had notch strength losses of 11 and 6 pct, respectively. The V-microalloyed high Mn steel in the as-hot-rolled condition had a notch strength loss of 17 pct. The V-microalloyed high Mn steel in the cold worked and aged condition indicated no notch strength loss in hydrogen, which was comparable to the performance of the 316L stainless steel.

Abstract: Pure copper, pure silver, Cu-6.8at%Al, Cu-1at%Mn and Cu-5at%Ni (stacking fault energies (SFEs) are about 41, 22, 23, 43 and 105mJ/m², respectively) were processed by equal channel angular pressing (ECAP) at the same homologous temperatures to investigate the effect of SFE and solute atoms on microstructural evolutions. The final grain size of Cu-6.8at%Al after eight passes of ECAP was the smallest followed by that of Cu-1at%Mn with little difference. The former alloy reaches saturation for grain size and grain boundary misorientation in early passes of ECAP while the latter continues decreasing even after eight passes. The role of shear bands and deformation twins is predominant for grain fragmentation in the early stage for Cu-6.8at%Al and Ag with low SFE while evolution from cell walls to grain boundaries is main mechanism for Cu, Cu-1at%Mn and Cu-5at%Ni with medium or high SFE. Solute Mn atom of Cu-Mn with high atomic size misfit and may suppress the dynamic recovery which transforms cell walls to grain boundaries, and allow accumulation of higher dislocations and reduction of cell size to smaller scales.

Abstract: The uniaxial tensile tests were conducted at different temperatures to explore the coupled influence of stacking fault energy (SFE) and short-range clustering (SRC) on the plastic deformation behavior of Cu-Ni alloys. The results demonstrate that the ultimate tensile strength and uniform elongation decrease with increasing temperature due to the competitive influence of SFE and SRC. Dynamic strain aging (DSA) effect is observed at 200 and 250°C, and such an effect becomes more notable with increasing Ni content. The occurrence of DSA effect is thought to be caused by pinning of moving dislocations by SRC and diffusing solute atoms. The plastic deformation mechanisms for Cu-Ni alloys is mainly governed by wavy slip of dislocations at different temperatures, since the SFE of Cu-Ni alloys are very high especially at high temperatures, and the effect of SRC can be nearly ignored.

Abstract: Low-density medium-manganese steels offer a vast development prospect for industrial application due to their outstanding combination of mechanical properties and density reduction. The microstructural evolution following tensile deformation of cold-rolled and annealed Fe-10Mn-10Al-0.7C steels was investigated by means of SEM and TEM microstructure analysis and XRD measurements. Annealing in the range of 700-1100 °C led to an austenite-ferrite dual-phase microstructure that was characterized by tensile strength of 700-1100 MPa and elongation of 6-34%. κ-carbides were observed in steels annealed at relatively low temperatures (700-850 °C). The steel exhibited the optimum combination of tensile strength of 930 MPa and elongation of 34% after annealing at 900 °C for 0.5 h. The stacking fault energy was estimated to be 69mJ/m2 considering the difference between average constituent and practical constituent of austenite caused by the high ferrite fraction. The deformed microstructures of the austenite exhibited the typical planer glide characteristics in sequence of dislocation array, Taylor lattice, Taylor lattice domain and microband. And the wavy glide occurs in ferrite was manifested by tangled dislocation and dislocation cells.

Abstract: The effects of thermo-mechanical training on damping capacity of two types of stainless steels, Fe-18Cr-8Ni (SUS 304) and Fe-25Cr-20Ni (SUS 310S) stainless steels, are studied. The thermo-mechanical training with bending deformation is adopted, since vibration manner in internal friction measurement is bending mode. An anisotropic damping capacity as well as hardness of samples is studied. It is found that deformation induced ε-martensite is observed for trained SUS 304 sample, while deformation twins are formed in the trained SUS 310S sample. It is also found that internal friction of SUS 304 sample is larger than that of SUS 310S sample. Increase in number of training results in an increase in the internal friction and hardness. In addition, anisotropic damping capacity is observed in the samples subjected the thermo-mechanical training. To be concluded, the thermo-mechanical training is useful for enhancement of both damping capacity and strength of SUS 304 and SUS 310S stainless steels.

Abstract: Al addition in TWIP steel not only reduces the specific weight but also increases the stacking fault energy which strongly affects the deformation mechanisms. Hot rolled air cooled TWIP steel with low Al content (1.61 wt. %) reveals duplex microstructure comprising austenite with ferrite, whereas steel with higher content of Al (3.56 wt. %) reveals fully austenite microstructure. It is evident that nano-twins are formed within austenite grain after 50% cold deformation. TWIP steel with the duplex microstructure exhibits an excellent combination of strength and ductility. Hardness and tensile strength values of air cooled steel specimens increase with a concomitant lowering of total elongation with the application of cold deformation. However, steel with low Al content shows higher hardness and tensile strength along with lower elongation as compared to the TWIP steel having higher Al content.

Abstract: Cu and Cu-30wt.%Zn alloys with stacking fault energies (SFEs) of 78 mJ/m² and 14 mJ/m² were processed by surface mechanical attrition treatment (SMAT) at room temperature and liquid nitrogen (LN) temperature, respectively. The effect of SFE and deformation temperature on tensile properties of these samples was investigated. The tensile testing results indicated that the yield strength and uniform elongation of these samples enhanced simultaneously with decreasing SFE. Meanwhile, the LN-SMAT processed samples exhibited remarkably higher strength and slightly lower ductility compared to those processed at room temperature. The SFE affected the deformation mechanisms of metals greatly. X-ray diffraction (XRD) measurements indicated that the twin density increased while the average grain size decreased with SFE decreasing, and twinning became the dominant deformation mechanism. The relationship between microstructure and mechanical property is also discussed.

Abstract: Fatigue crack growth (FCG) behavior of three high manganese austenitic twin-induced plasticity (TWIP) steels with different stacking fault energy (SFE) was investigated, aiming at studying the correlation between the FCG resistance and the SFE of the steels. FCG tests were performed using three-point bending specimens at room temperature at stress ratio of 0.1 under the control of stress intensity factor range. Test results showed that the fatigue threshold values of these steels decrease with increasing the SFE. However, in the Paris regime, the crack growth rates of the steels do not appear to correlate directly with SFE. These results are discussed according to the degree of fatigue crack closure and the deformation mode of crack tip zone.