{"id":{"repo_id":"stellenbosch","oai_identifier":"oai:scholar.sun.ac.za:10019.1/135708"},"canonical_url":"https://search.dev.ndltd.org/etd/stellenbosch/oai:scholar.sun.ac.za:10019.1/135708","repository":{"repo_id":"stellenbosch","name":"Stellenbosch University","base_url":"https://scholar.sun.ac.za/server/oai/request"},"display":{"title":"Empirical Multiphase Model for Strength Evolution of High-Chromium Steel Alloys","abstract":"Duplex stainless steel 2205 (DSS2205) exhibits complex non-monotonic strength evolution during thermal ageing at elevated temperatures, limiting predictive capability for long-term component life assessment. This dissertation develops and validates the METALS (Microstructure Evolution and Thermal Ageing-Linked Strength) model. An empirical multiphase framework coupling phase-field microstructural predictions with mechanical property evolution through phenomenological energy-strength relationships. A comprehensive six-month isothermal ageing study at 500 °C established the experimental foundation, revealing characteristic strength evolution: 47.9% hardening (695 to 1028 MPa, 0-504 hours), 13.6% softening to 873 MPa (1680 hours), and a recovery to 1020 MPa (4368 hours). TEM/EELS analysis at six timepoints identified competitive Cr₂N/MoN precipitation as the governing mechanism, with late-stage Ti/V-containing phases contributing to recovery behaviour. The phase-field model incorporates a critical enhancement distinguishing compositional (deviatoric) and volumetric (dilatational) misfit strain components. Volumetric contributions, while representing a small fraction of total elastic energy during peak precipitation (~5% at 1680h), generating compressive hydrostatic stresses that significantly influence precipitate dissolution kinetics. The elastic energy grows substantially at late stages (reaching ~23% at 3700h), correlating with increased dissolution activity. GPU-accelerated spectral methods enable 4368-hour predictions in ~72 hours, demonstrating practical computational efficiency. Validation demonstrates stage-dependent accuracy: good early-stage agreement (RMSE = 6.18 MPa, 0.5% error for 0-672 hours), moderate intermediate performance (RMSE = 79.48 MPa, 7.79% error for 672-2520 hours), and reasonable late-stage predictions (RMSE = 71.59 MPa, 6.06% beyond 2520h). Parametric sensitivity analysis reveals MoN activation energy critically governs intermediate-stage behaviour, which provides a clear pathway for model refinement. The systematic late-stage underprediction identifies the need for explicit TiN/VN phase incorporation. The validated framework enables quantitative strength predictions for DSS2205 components during service exposure, supports alloy composition optimisation through computational screening, and establishes a transferable methodology applicable to other high-chromium steel systems. The research demonstrates that empirically-calibrated phenomenological coupling within thermodynamically-consistent microstructural frameworks provides actionable predictive capability even when complete mechanistic understanding remains incomplete.","abstract_html":"Duplex stainless steel 2205 (DSS2205) exhibits complex non-monotonic strength evolution during thermal ageing at elevated temperatures, limiting predictive capability for long-term component life assessment. This dissertation develops and validates the METALS (Microstructure Evolution and Thermal Ageing-Linked Strength) model. An empirical multiphase framework coupling phase-field microstructural predictions with mechanical property evolution through phenomenological energy-strength relationships. A comprehensive six-month isothermal ageing study at 500 °C established the experimental foundation, revealing characteristic strength evolution: 47.9% hardening (695 to 1028 MPa, 0-504 hours), 13.6% softening to 873 MPa (1680 hours), and a recovery to 1020 MPa (4368 hours). TEM/EELS analysis at six timepoints identified competitive Cr₂N/MoN precipitation as the governing mechanism, with late-stage Ti/V-containing phases contributing to recovery behaviour. The phase-field model incorporates a critical enhancement distinguishing compositional (deviatoric) and volumetric (dilatational) misfit strain components. Volumetric contributions, while representing a small fraction of total elastic energy during peak precipitation (~5% at 1680h), generating compressive hydrostatic stresses that significantly influence precipitate dissolution kinetics. The elastic energy grows substantially at late stages (reaching ~23% at 3700h), correlating with increased dissolution activity. GPU-accelerated spectral methods enable 4368-hour predictions in ~72 hours, demonstrating practical computational efficiency. Validation demonstrates stage-dependent accuracy: good early-stage agreement (RMSE = 6.18 MPa, 0.5% error for 0-672 hours), moderate intermediate performance (RMSE = 79.48 MPa, 7.79% error for 672-2520 hours), and reasonable late-stage predictions (RMSE = 71.59 MPa, 6.06% beyond 2520h). Parametric sensitivity analysis reveals MoN activation energy critically governs intermediate-stage behaviour, which provides a clear pathway for model refinement. The systematic late-stage underprediction identifies the need for explicit TiN/VN phase incorporation. The validated framework enables quantitative strength predictions for DSS2205 components during service exposure, supports alloy composition optimisation through computational screening, and establishes a transferable methodology applicable to other high-chromium steel systems. The research demonstrates that empirically-calibrated phenomenological coupling within thermodynamically-consistent microstructural frameworks provides actionable predictive capability even when complete mechanistic understanding remains incomplete.","abstract_has_math":false,"creators":["Cupido, Llewellyn Heinrich"],"institution":"Stellenbosch : Stellenbosch University","degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Mahomed, Nawaz"],"committee_chairs":[],"committee_members":[],"year":2026,"date_issued":"2026-03","date_published":"2026-03","updated_at":"2026-07-24T04:40:12Z","subjects":[],"languages":["en"],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://scholar.sun.ac.za/handle/10019.1/135708","outbound_label":"Repository record","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Mahomed, Nawaz"]},{"key":"dc:contributor.other","label":"Dc Contributor Other","values":["Stellenbosch University. Faculty of Engineering. Dept. of Mechanical & Mechatronic Engineering."]},{"key":"dc:creator","label":"Author","values":["Cupido, Llewellyn Heinrich"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2026-04-08T09:57:19Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2026-04-08T09:57:19Z"]},{"key":"dc:date.issued","label":"Date","values":["2026-03"]},{"key":"dc:publisher","label":"Institution","values":["Stellenbosch : Stellenbosch University"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://scholar.sun.ac.za/handle/10019.1/135708"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["Thesis (PhD)--Stellenbosch University, 2026.","Cupido, L. H. 2026. Empirical Multiphase Model for Strength Evolution of High-Chromium Steel Alloys. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/a5813faa-72ba-45a5-b4b8-272db8eeac27"]},{"key":"dc:description.abstract","label":"Abstract","values":["Duplex stainless steel 2205 (DSS2205) exhibits complex non-monotonic strength evolution during thermal ageing at elevated temperatures, limiting predictive capability for long-term component life assessment. This dissertation develops and validates the METALS (Microstructure Evolution and Thermal Ageing-Linked Strength) model. An empirical multiphase framework coupling phase-field microstructural predictions with mechanical property evolution through phenomenological energy-strength relationships. A comprehensive six-month isothermal ageing study at 500 °C established the experimental foundation, revealing characteristic strength evolution: 47.9% hardening (695 to 1028 MPa, 0-504 hours), 13.6% softening to 873 MPa (1680 hours), and a recovery to 1020 MPa (4368 hours). TEM/EELS analysis at six timepoints identified competitive Cr₂N/MoN precipitation as the governing mechanism, with late-stage Ti/V-containing phases contributing to recovery behaviour. The phase-field model incorporates a critical enhancement distinguishing compositional (deviatoric) and volumetric (dilatational) misfit strain components. Volumetric contributions, while representing a small fraction of total elastic energy during peak precipitation (~5% at 1680h), generating compressive hydrostatic stresses that significantly influence precipitate dissolution kinetics. The elastic energy grows substantially at late stages (reaching ~23% at 3700h), correlating with increased dissolution activity. GPU-accelerated spectral methods enable 4368-hour predictions in ~72 hours, demonstrating practical computational efficiency. Validation demonstrates stage-dependent accuracy: good early-stage agreement (RMSE = 6.18 MPa, 0.5% error for 0-672 hours), moderate intermediate performance (RMSE = 79.48 MPa, 7.79% error for 672-2520 hours), and reasonable late-stage predictions (RMSE = 71.59 MPa, 6.06% beyond 2520h). Parametric sensitivity analysis reveals MoN activation energy critically governs intermediate-stage behaviour, which provides a clear pathway for model refinement. The systematic late-stage underprediction identifies the need for explicit TiN/VN phase incorporation. The validated framework enables quantitative strength predictions for DSS2205 components during service exposure, supports alloy composition optimisation through computational screening, and establishes a transferable methodology applicable to other high-chromium steel systems. The research demonstrates that empirically-calibrated phenomenological coupling within thermodynamically-consistent microstructural frameworks provides actionable predictive capability even when complete mechanistic understanding remains incomplete."]},{"key":"dc:title","label":"Title","values":["Empirical Multiphase Model for Strength Evolution of High-Chromium Steel Alloys"]}]}],"canonical_facts":{"dc:contributor.advisor":["Mahomed, Nawaz"],"dc:contributor.other":["Stellenbosch University. Faculty of Engineering. Dept. of Mechanical & Mechatronic Engineering."],"dc:creator":["Cupido, Llewellyn Heinrich"],"dc:date.accessioned":["2026-04-08T09:57:19Z"],"dc:date.available":["2026-04-08T09:57:19Z"],"dc:date.issued":["2026-03"],"dc:description":["Thesis (PhD)--Stellenbosch University, 2026.","Cupido, L. H. 2026. Empirical Multiphase Model for Strength Evolution of High-Chromium Steel Alloys. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/a5813faa-72ba-45a5-b4b8-272db8eeac27"],"dc:description.abstract":["Duplex stainless steel 2205 (DSS2205) exhibits complex non-monotonic strength evolution during thermal ageing at elevated temperatures, limiting predictive capability for long-term component life assessment. This dissertation develops and validates the METALS (Microstructure Evolution and Thermal Ageing-Linked Strength) model. An empirical multiphase framework coupling phase-field microstructural predictions with mechanical property evolution through phenomenological energy-strength relationships. A comprehensive six-month isothermal ageing study at 500 °C established the experimental foundation, revealing characteristic strength evolution: 47.9% hardening (695 to 1028 MPa, 0-504 hours), 13.6% softening to 873 MPa (1680 hours), and a recovery to 1020 MPa (4368 hours). TEM/EELS analysis at six timepoints identified competitive Cr₂N/MoN precipitation as the governing mechanism, with late-stage Ti/V-containing phases contributing to recovery behaviour. The phase-field model incorporates a critical enhancement distinguishing compositional (deviatoric) and volumetric (dilatational) misfit strain components. Volumetric contributions, while representing a small fraction of total elastic energy during peak precipitation (~5% at 1680h), generating compressive hydrostatic stresses that significantly influence precipitate dissolution kinetics. The elastic energy grows substantially at late stages (reaching ~23% at 3700h), correlating with increased dissolution activity. GPU-accelerated spectral methods enable 4368-hour predictions in ~72 hours, demonstrating practical computational efficiency. Validation demonstrates stage-dependent accuracy: good early-stage agreement (RMSE = 6.18 MPa, 0.5% error for 0-672 hours), moderate intermediate performance (RMSE = 79.48 MPa, 7.79% error for 672-2520 hours), and reasonable late-stage predictions (RMSE = 71.59 MPa, 6.06% beyond 2520h). Parametric sensitivity analysis reveals MoN activation energy critically governs intermediate-stage behaviour, which provides a clear pathway for model refinement. The systematic late-stage underprediction identifies the need for explicit TiN/VN phase incorporation. The validated framework enables quantitative strength predictions for DSS2205 components during service exposure, supports alloy composition optimisation through computational screening, and establishes a transferable methodology applicable to other high-chromium steel systems. The research demonstrates that empirically-calibrated phenomenological coupling within thermodynamically-consistent microstructural frameworks provides actionable predictive capability even when complete mechanistic understanding remains incomplete."],"dc:identifier.uri":["https://scholar.sun.ac.za/handle/10019.1/135708"],"dc:language.iso":["en"],"dc:publisher":["Stellenbosch : Stellenbosch University"],"dc:title":["Empirical Multiphase Model for Strength Evolution of High-Chromium Steel Alloys"],"dc:type":["Thesis"]},"updated_at":"2026-07-24T04:40:12Z"}