Stacking faults, deformation-induced martensite and micromechanics of metastable austenite in steels studied by high-energy synchrotron X-ray diffraction

Austenitic stainless steels are known for their remarkable corrosion resistance and exhibit a very high ductility and toughness. They posses the face centered cubic crystal structure. Depending on the chemical composition of the austenite, the austenite may be metastable and during plastic deformation may undergo a deformation-induced phase transformation into α′-martensite - and that α′-martensite is partly responsible for the steel’s remarkable mechanical properties. In order to predict and control the occurrence and extent of the deformation-induced transformation into α′-martensite, it is crucial to have a profound knowledge of that transformation. Such knowledge is important to further improve austenitic stainless steels but also to contribute to the development of third generation advanced high strength steels that possess a multi-phase microstructure since the deformation behavior of the austenitic phase crucially influences their bulk deformation properties. Accordingly, in order to improve the current knowledge about the micromechanical behavior of steels and to predict the deformationbehavior of metastable austenite reliably, parameters affecting the austenite’s deformation behavior need to be described and quantified. This work contributes to such knowledge by investigating experimentally the effect of temperature, chemical composition, and grain orientationon the deformation behavior of austenite. The investigations were performed mainly with the aid of high-energy X-ray diffraction(HEXRD). HEXRD is a measurement method that allows to examine the materials’ response to plastic deformation in a bulk specimen non destructively. With HEXRD phases and their fraction evolution, latticestrains and stresses, and the stacking fault energy (γSF) can be followed in situ as the sample is subjected to load. Moreover, the high spatial resolution of HEXRD allows line profile analysis, i.e. to study the shape of diffraction peaks in order to quantify the formation of dislocations and stacking faults as well as their evolution during the course of deformation. Also, high-energy X-ray diffraction microscopy (HEDM) measurementwere conducted in order to resolve and follow the deformation behavior of individual grains embedded within the polycrystalline bulk during deformation. This is important to understand the effect of grain orientation, grain neighborhood, and grain morphology on the deformation behavior of individual grains and consequently the deformationbehavior of the bulk as a whole. The temperature effect on γSF was studied both on powders of three austenitic model alloys with different alloy compositions using an in situ temperature HEXRD experiment and on a commercial 301LN bulk specimen with the aid of an in situ tensile loading experiment. It was found that the γSF increases significantly with increasing temperature. Moreover, the temperature induced increase of γSF significantly influences the predominantly active deformation mechanism. At low temperatures, large fractions of stacking faults, ε- and α′-martensite formed, which also reflects on the properties of the steel by a high work hardening rate. With increasing temperature, and consequently increasing γSF, the formation of stacking faults, ε- and α′-martensite becomes less predominant. As a result a significant decrease in work hardening with increasing temperature was observed. Moreover, it was found, that at elevated temperatures, the dissociation of dislocation into partial dislocation occurs at significantly higher strain. In addition to temperature, grain orientation was found to affect the deformation behavior of austenitic steels substantially. Grains deformed along [100] form predominantly stacking faults, whereas grains deformed along [111] mainly deform via dislocation glide. Grain orientation also played a key role in the formation of deformation-induced phases. Crystalline austenitic regions oriented with their {111} at 45° to external load were found to transform preferentially into ε-martensite before further transforming into α′-martensite, whereas crystalline austenitic regions oriented with their {111} at 0° and 90° to the load, transformed directly into α′-martensite, without transforming into ε-martensite first.The knowledge acquired by studying single phase austenitic steel was expanded to medium Mn steels (MMnS), possessing a multi-phase microstructure. It was found that the average bulk deformation behavior of medium Mn steels is crucially affected by the interdependencies between the micromechanical deformation behavior and the stability of the austenite, which can be controlled by tuning microstructure and austenite composition.The contribution of this work is to increase the knowledge of the deformation-induced martensitic phase transformations of metastable austenite, its dependence with γSF, temperature, and the correlation with parameters affecting the deformation behavior in the bulk which are not considered in the γSF.Austenitiska rostfria stål är kända för sitt goda korrosionsmotstånd och mekaniska egenskaper med mycket hög duktilitet och seghet. De har den ytcentrerade kubiska kristallstrukturen och beroende på den specifika legeringens kemiska sammansättning kan austeniten vara metastabil och genomgå en deformationsinducerad fasomvandling till α′-martensit - under plastisk deformation. α′-martensiten är delvis ansvarig för stålets goda mekaniska egenskaper. För att förutsäga och kontrollera förekomsten och omfattningen av den deformationsinducerade omvandlingen till α′-martensit, är det avgörande att ha en djup kunskap om den fasomvandlingen. Sådan kunskap är viktig för att ytterligare förbättra austenitiska rostfria stål men också för att bidra till utvecklingen av tredje generationens avancerade höghållfasta stål som har en flerfasmikrostruktur, men där deformationsbeteendet hos den austenitiska fasen har avgörande betydelse för materialets generella deformationsegenskaper. Följaktligen, för att förbättra den nuvarande kunskapen om det mikromekaniska beteendet hos stål och för att på ett tillförlitligt sätt förutsäga deformationsbeteendet hos metastabil austenit, måste parametrar som påverkar austenitens deformationsbeteende beskrivas och kvantifieras. Detta arbete bidrar till sådan kunskap genom att experimentellt undersöka effekten av temperatur, kemisk sammansättning och kornorientering på austenitens deformationsbeteende. Undersökningarna har utförts huvudsakligen med hjälp av högenergiröntgendiffraktion (HEXRD). HEXRD är en mätmetod som gör det möjligt att undersöka materialens svar på plastisk deformation i ett bulkprov på ett icke-förstörande sätt. Med HEXRD kan faser och deras fraktionsutveckling, gittertöjningar och spänningar samt staplingsfelenergi (γSF) studeras in-situ när provet utsätts för belastning. Dessutom tillåter den höga spatiella upplösningen hos HEXRD linjeprofilanalys, dvs. att studera formen på diffraktionstopparna, för att kvantifiera bildandet av dislokationer och staplingsfel samt deras utveckling under deformationsförloppet. Vidare genomfördes högenergiröntgendiffraktionsmikroskopi (HEDM) mätningar för att studera deformationsbeteendet hos individuella korn inbäddade i den polykristallina bulken under deformation. Detta är viktigt för att förstå effekten av kornorientering, kornomgivning och kornmorfologi på deformationsbeteendet hos enskilda korn och följaktligen deformationsbeteendet för det polykristallina materialet. Temperatureffekten på γSF studerades både på pulver av tre austenitiska modelllegeringar med olika legeringssammansättningar genom användandet av ett in-situ HEXRD-experiment under termisk behandling samt på ett kommersiellt 301LN bulk prov under in-situ dragprovsbelastning. Det visade sig att γSF ökar avsevärt med stigande temperatur. Dessutom påverkar den temperaturinducerade ökningen av γSF signifikant den dominerande aktiva deformationsmekanismen hos austeniten. Vid låga temperaturer bildas stora fraktioner av staplingsfel, ε- och α′-martensit, vilket också inducerar ett högt deformationshårdnande hos stålet. Med ökande temperatur, och följaktligen ökande γSF, blir bildningen av staplingsfel, ε- och α′-martensit mindre dominerande. Som ett resultat observerades en signifikant minskning av deformationshårdnandet med ökande temperatur. Dessutom fann man att vid förhöjda temperaturer sker dissociationen av dislokationer till partiella dislokationer vid betydligt högre töjning. Förutom temperaturen visade sig kornorienteringen påverka deformationsbeteendet hos austenitiska stål avsevärt. Korn som deformeras längs [100] bildar övervägande staplingsfel, medan korn som deformeras längs [111] huvudsakligen deformeras via dislokationsglidning. Kornorientering spelade också en nyckelroll i bildandet av deformationsinducerade faser. Korn orienterade med {111} vid 45◦ mot den pålagda belastningen visade sig företrädesvis omvandlas till ε-martensit innan de vidare omvandlas till a′-martensit, medan korn orienterade med 111 vid 0◦ och 90◦ mot belastningen omvandlas direkt till α′-martensit utan att först omvandlas till ε-martensit. Kunskapen som förvärvats genom att studera enfasiga austenitiska stål utvidgades sedan till ”Medium manganese stål (MMnS)” som hade en flerfasig mikrostruktur. Det visade sig att det genomsnittliga bulkdeformationsbeteendet för MMnS påverkas avgörande av stålets mikromekansiska deformationsbeteende och austenitens deformationsbeteende och stabilitet, vilken kan styras av austenitens sammansättning. Bidraget från denna avhandling är att öka kunskapen om deformationsinducerade martensitiska fasomvandlingar i stål med metastabil austenit och beroendet av γSF, temperatur och andra parametrar som påverkar deformationsbeteendet i bulken som inte beaktas av γSF

Formation of Dislocations and Stacking Faults in Embedded Individual Grains during In Situ Tensile Loading of an Austenitic Stainless Steel

The formation of stacking faults and dislocations in individual austenite (fcc) grains embedded in a polycrystalline bulk Fe-18Cr-10.5Ni (wt.%) steel was investigated by non-destructive high-energy diffraction microscopy (HEDM) and line profile analysis. The broadening and position of intensity, diffracted from individual grains, were followed during in situ tensile loading up to 0.09 strain. Furthermore, the predominant deformation mechanism of the individual grains as a function of grain orientation was investigated, and the formation of stacking faults was quantified. Grains oriented with [100] along the tensile axis form dislocations at low strains, whilst at higher strains, the formation of stacking faults becomes the dominant deformation mechanism. In contrast, grains oriented with [111] along the tensile axis deform mainly through the formation and slip of dislocations at all strain states. However, the present study also reveals that grain orientation is not sufficient to predict the deformation characteristics of single grains in polycrystalline bulk materials. This is witnessed specifically within one grain oriented with [111] along the tensile axis that deforms through the generation of stacking faults. The reason for this behavior is due to other grain-specific parameters, such as size and local neighborhood

Stacking faults, deformation-induced martensite and micromechanics of metastable austenite in steels studied by high-energy synchrotron X-ray diffraction

Austenitic stainless steels are known for their remarkable corrosion resistance and exhibit a very high ductility and toughness. They posses the face centered cubic crystal structure. Depending on the chemical composition of the austenite, the austenite may be metastable and during plastic deformation may undergo a deformation-induced phase transformation into α′-martensite - and that α′-martensite is partly responsible for the steel’s remarkable mechanical properties. In order to predict and control the occurrence and extent of the deformation-induced transformation into α′-martensite, it is crucial to have a profound knowledge of that transformation. Such knowledge is important to further improve austenitic stainless steels but also to contribute to the development of third generation advanced high strength steels that possess a multi-phase microstructure since the deformation behavior of the austenitic phase crucially influences their bulk deformation properties. Accordingly, in order to improve the current knowledge about the micromechanical behavior of steels and to predict the deformationbehavior of metastable austenite reliably, parameters affecting the austenite’s deformation behavior need to be described and quantified. This work contributes to such knowledge by investigating experimentally the effect of temperature, chemical composition, and grain orientationon the deformation behavior of austenite. The investigations were performed mainly with the aid of high-energy X-ray diffraction(HEXRD). HEXRD is a measurement method that allows to examine the materials’ response to plastic deformation in a bulk specimen non destructively. With HEXRD phases and their fraction evolution, latticestrains and stresses, and the stacking fault energy (γSF) can be followed in situ as the sample is subjected to load. Moreover, the high spatial resolution of HEXRD allows line profile analysis, i.e. to study the shape of diffraction peaks in order to quantify the formation of dislocations and stacking faults as well as their evolution during the course of deformation. Also, high-energy X-ray diffraction microscopy (HEDM) measurementwere conducted in order to resolve and follow the deformation behavior of individual grains embedded within the polycrystalline bulk during deformation. This is important to understand the effect of grain orientation, grain neighborhood, and grain morphology on the deformation behavior of individual grains and consequently the deformationbehavior of the bulk as a whole. The temperature effect on γSF was studied both on powders of three austenitic model alloys with different alloy compositions using an in situ temperature HEXRD experiment and on a commercial 301LN bulk specimen with the aid of an in situ tensile loading experiment. It was found that the γSF increases significantly with increasing temperature. Moreover, the temperature induced increase of γSF significantly influences the predominantly active deformation mechanism. At low temperatures, large fractions of stacking faults, ε- and α′-martensite formed, which also reflects on the properties of the steel by a high work hardening rate. With increasing temperature, and consequently increasing γSF, the formation of stacking faults, ε- and α′-martensite becomes less predominant. As a result a significant decrease in work hardening with increasing temperature was observed. Moreover, it was found, that at elevated temperatures, the dissociation of dislocation into partial dislocation occurs at significantly higher strain. In addition to temperature, grain orientation was found to affect the deformation behavior of austenitic steels substantially. Grains deformed along [100] form predominantly stacking faults, whereas grains deformed along [111] mainly deform via dislocation glide. Grain orientation also played a key role in the formation of deformation-induced phases. Crystalline austenitic regions oriented with their {111} at 45° to external load were found to transform preferentially into ε-martensite before further transforming into α′-martensite, whereas crystalline austenitic regions oriented with their {111} at 0° and 90° to the load, transformed directly into α′-martensite, without transforming into ε-martensite first.The knowledge acquired by studying single phase austenitic steel was expanded to medium Mn steels (MMnS), possessing a multi-phase microstructure. It was found that the average bulk deformation behavior of medium Mn steels is crucially affected by the interdependencies between the micromechanical deformation behavior and the stability of the austenite, which can be controlled by tuning microstructure and austenite composition.The contribution of this work is to increase the knowledge of the deformation-induced martensitic phase transformations of metastable austenite, its dependence with γSF, temperature, and the correlation with parameters affecting the deformation behavior in the bulk which are not considered in the γSF.Austenitiska rostfria stål är kända för sitt goda korrosionsmotstånd och mekaniska egenskaper med mycket hög duktilitet och seghet. De har den ytcentrerade kubiska kristallstrukturen och beroende på den specifika legeringens kemiska sammansättning kan austeniten vara metastabil och genomgå en deformationsinducerad fasomvandling till α′-martensit - under plastisk deformation. α′-martensiten är delvis ansvarig för stålets goda mekaniska egenskaper. För att förutsäga och kontrollera förekomsten och omfattningen av den deformationsinducerade omvandlingen till α′-martensit, är det avgörande att ha en djup kunskap om den fasomvandlingen. Sådan kunskap är viktig för att ytterligare förbättra austenitiska rostfria stål men också för att bidra till utvecklingen av tredje generationens avancerade höghållfasta stål som har en flerfasmikrostruktur, men där deformationsbeteendet hos den austenitiska fasen har avgörande betydelse för materialets generella deformationsegenskaper. Följaktligen, för att förbättra den nuvarande kunskapen om det mikromekaniska beteendet hos stål och för att på ett tillförlitligt sätt förutsäga deformationsbeteendet hos metastabil austenit, måste parametrar som påverkar austenitens deformationsbeteende beskrivas och kvantifieras. Detta arbete bidrar till sådan kunskap genom att experimentellt undersöka effekten av temperatur, kemisk sammansättning och kornorientering på austenitens deformationsbeteende. Undersökningarna har utförts huvudsakligen med hjälp av högenergiröntgendiffraktion (HEXRD). HEXRD är en mätmetod som gör det möjligt att undersöka materialens svar på plastisk deformation i ett bulkprov på ett icke-förstörande sätt. Med HEXRD kan faser och deras fraktionsutveckling, gittertöjningar och spänningar samt staplingsfelenergi (γSF) studeras in-situ när provet utsätts för belastning. Dessutom tillåter den höga spatiella upplösningen hos HEXRD linjeprofilanalys, dvs. att studera formen på diffraktionstopparna, för att kvantifiera bildandet av dislokationer och staplingsfel samt deras utveckling under deformationsförloppet. Vidare genomfördes högenergiröntgendiffraktionsmikroskopi (HEDM) mätningar för att studera deformationsbeteendet hos individuella korn inbäddade i den polykristallina bulken under deformation. Detta är viktigt för att förstå effekten av kornorientering, kornomgivning och kornmorfologi på deformationsbeteendet hos enskilda korn och följaktligen deformationsbeteendet för det polykristallina materialet. Temperatureffekten på γSF studerades både på pulver av tre austenitiska modelllegeringar med olika legeringssammansättningar genom användandet av ett in-situ HEXRD-experiment under termisk behandling samt på ett kommersiellt 301LN bulk prov under in-situ dragprovsbelastning. Det visade sig att γSF ökar avsevärt med stigande temperatur. Dessutom påverkar den temperaturinducerade ökningen av γSF signifikant den dominerande aktiva deformationsmekanismen hos austeniten. Vid låga temperaturer bildas stora fraktioner av staplingsfel, ε- och α′-martensit, vilket också inducerar ett högt deformationshårdnande hos stålet. Med ökande temperatur, och följaktligen ökande γSF, blir bildningen av staplingsfel, ε- och α′-martensit mindre dominerande. Som ett resultat observerades en signifikant minskning av deformationshårdnandet med ökande temperatur. Dessutom fann man att vid förhöjda temperaturer sker dissociationen av dislokationer till partiella dislokationer vid betydligt högre töjning. Förutom temperaturen visade sig kornorienteringen påverka deformationsbeteendet hos austenitiska stål avsevärt. Korn som deformeras längs [100] bildar övervägande staplingsfel, medan korn som deformeras längs [111] huvudsakligen deformeras via dislokationsglidning. Kornorientering spelade också en nyckelroll i bildandet av deformationsinducerade faser. Korn orienterade med {111} vid 45◦ mot den pålagda belastningen visade sig företrädesvis omvandlas till ε-martensit innan de vidare omvandlas till a′-martensit, medan korn orienterade med 111 vid 0◦ och 90◦ mot belastningen omvandlas direkt till α′-martensit utan att först omvandlas till ε-martensit. Kunskapen som förvärvats genom att studera enfasiga austenitiska stål utvidgades sedan till ”Medium manganese stål (MMnS)” som hade en flerfasig mikrostruktur. Det visade sig att det genomsnittliga bulkdeformationsbeteendet för MMnS påverkas avgörande av stålets mikromekansiska deformationsbeteende och austenitens deformationsbeteende och stabilitet, vilken kan styras av austenitens sammansättning. Bidraget från denna avhandling är att öka kunskapen om deformationsinducerade martensitiska fasomvandlingar i stål med metastabil austenit och beroendet av γSF, temperatur och andra parametrar som påverkar deformationsbeteendet i bulken som inte beaktas av γSF

Formation of Dislocations and Stacking Faults in Embedded Individual Grains during In Situ Tensile Loading of an Austenitic Stainless Steel

The formation of stacking faults and dislocations in individual austenite (fcc) grains embedded in a polycrystalline bulk Fe-18Cr-10.5Ni (wt.%) steel was investigated by non-destructive high-energy diffraction microscopy (HEDM) and line profile analysis. The broadening and position of intensity, diffracted from individual grains, were followed during in situ tensile loading up to 0.09 strain. Furthermore, the predominant deformation mechanism of the individual grains as a function of grain orientation was investigated, and the formation of stacking faults was quantified. Grains oriented with [100] along the tensile axis form dislocations at low strains, whilst at higher strains, the formation of stacking faults becomes the dominant deformation mechanism. In contrast, grains oriented with [111] along the tensile axis deform mainly through the formation and slip of dislocations at all strain states. However, the present study also reveals that grain orientation is not sufficient to predict the deformation characteristics of single grains in polycrystalline bulk materials. This is witnessed specifically within one grain oriented with [111] along the tensile axis that deforms through the generation of stacking faults. The reason for this behavior is due to other grain-specific parameters, such as size and local neighborhood

In Situ Bulk Observations and Ab Initio Calculations Revealing the Temperature Dependence of Stacking Fault Energy in Fe–Cr–Ni Alloys

The dependence of stacking fault energy ($γ_{SFE}$) on temperature in austenitic Fe–Cr–Ni alloy powders was investigated by in situ high energy synchrotron X-ray diffraction and ab initio calculations in the temperature range from − 45 °C to 450 °C. The X-ray diffraction peak positions were used to determine the stacking fault probability and subsequently the temperature dependence of $γ_{SFE}$. The effect of temperature on the diffraction peak positions was found to be mainly reversible; however, recovery of dislocations occurred above about 200 °C, which also gave an irreversible contribution. Two different ab initio-based models were evaluated with respect to the experimental data. The different predictions of the models can be explained by their respective treatment of the magnetic moments for Cr and Ni, which is critical for the alloy compositions investigated. Ab initio calculations, taking longitudinal spin fluctuations (LSF) into consideration within the quasi-classical phenomenological model, predict a temperature dependence of $γ_{SFE}$ in good agreement with the experimentally evaluated trend of increasing $γ_{SFE}$ with increasing temperature: $|Δγ_{SFE}/ΔT|$=0.05mJm$^{−2}$/K. The temperature effect on $γ_{SFE}$ is similar for all three investigated alloys: Fe–18Cr–15Ni, Fe–18Cr–17Ni, Fe–21Cr–16Ni (wt pct), while their room temperature γSFE are evaluated to be 22, 25, 20 mJ m$^{−2}$, respectively

Cu precipitation-mediated formation of reverted austenite during ageing of a 15–5 PH stainless steel

A Cu precipitation-mediated austenitic transformation during ageing treatment of a 15–5 PH stainless steel is revealed through atom probe tomography, in situ synchrotron X-ray diffraction and computational thermodynamics and kinetics. The austenitic transformation is proposed to occur through the pathway: Cu precipitation at the martensite/retained austenite interfaces or at martensite lath boundaries → partitioning of austenite stabilizing elements towards interfaces of the Cu precipitates → reverted austenite formation