Stainless steels are very important engineering materials in a variety of applications such as in the food industry and nuclear power plants due to their combination of good mechanical properties and high corrosion resistance. However, ferrite-containing stainless steels are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation of the ferrite phase, where it decomposes into Fe-rich ferrite (α) and Cr-rich ferrite (α'). The phase separation is accompanied with a severe loss of toughness. Therefore, the upper service temperature of ferrite-containing stainless steels in industrial applications has been limited to around 250°.

In the present work, Fe-Cr based steels were mainly investigated by atom probe tomography. A new method based on the radial distribution function (RDF) was proposed to quantitatively evaluate both the wavelength and amplitude of phase separation in Fe-Cr alloys from the atom probe tomography data. Moreover, a simplified equation was derived to calculate the amplitude of phase separation. The wavelength and amplitude was compared with evaluations using the auto-correlation function (ACF) and Langer-Bar-on-Miller (LBM) method, respectively. The results show that the commonly used LBM method underestimates the amplitude of phase separation and the wavelengths obtained by RDF shows a good exponential relation with aging time which is expected from the theory. The RDF is also an effective method in detecting the phenomena of clustering and elemental partitioning.

Furthermore, atom probe tomography and the developed quantitative analysis method have been applied to investigate the influence of different factors on the phase separation in Fe-Cr based alloys by the help of mainly mechanical property tests and atom probe tomography analysis. The study shows that: (1) the external tensile stress during aging enhances the phase separation in ferrite. (2) Phase separation in weld bead metals decomposes more rapidly than both the heat-affected-zone metals and the base metals mainly due to the high density of dislocations in the welding bead metals which could facilitate the diffusion. (3) The results show that Ni and Mn can enhance the phase separation comparing to the binary Fe-Cr alloy whereas Cu forms clusters during aging. (4) Initial clustering of Cr atoms was found after homogenization. Two factors, namely, clustering of Cr above the miscibility gap and clustering during quenching was suggested as the two responsible mechanisms. (5) The homogenization temperatures significantly influence the evolution of phase separation in Fe-46.5at.%Cr.

The formation of Cr-rich and Fe-rich domains upon ageing of an initially homogeneous Fe-Cr alloy at elevated temperatures (300-600 ºC) is commonly referred to as phase separation. The behaviour originates from a miscibility gap in the Fe-Cr phase diagram. The boundary of the miscibility gap is denoted the binodal, and the line where the second derivative of the molar Gibbs energy w.r.t. composition is zero, the spinodal. In the region between the binodal and spinodal lines, the phase separation is said to occur by means of nucleation and growth. Inside the spinodal line, no thermally activated nucleation event is needed, and the initially homogeneous alloy decomposes "spinodally" into Cr-rich and Fe-rich regions. This type of phase transformation can be viewed as a continuous build up of Cr-rich regions, that also are interconnected, forming a microstructure characteristic for alloys decomposed spinodally. Phase separation has been of great interest within the metallurgical community as well as industry, due to its embritteling effect. Phase separation in Cr-rich ferritic steels, and thus embrittlement, sets a practical upper service temperature of ~300 ºC for Cr-containing ferrites. It is desirable to develop understanding and modelling capability for decomposing alloy systems, since such knowledge could be used to relieve the limitation in service temperature. The current work has been focused around the development and use of computer simulations, using thermodynamic and kinetic input from databases, in order to progress towards alloy design where decomposition is minimized. Simulations in this work are based on solving the so called Cahn-Hilliard equation, where an important parameter is the gradient energy, since it influences both the morphology and rate of decomposition in the simulations. An attempt at formulating a general model for the gradient energy coefficients in multi-component systems has been made, but has yet to be properly tried against experimental data. Improvements, and insights, to the initial state used in simulations has also been achieved. The combination of above mentioned efforts is a step towards a predictive tool for decomposition of complex alloys. Such a tool could not only be an aid in future alloy design, but also be used as an aid as a diagnosis tool in life time assessment of critical components already in use and thereby difficult to assess on site by means of in-destructive testing, typically components in nuclear power facilities.

Fasseparation i ferrit är uppdelningen av en, initialt homogen, Fe-Cr-legering i Cr-rika och Fe-rika domäner vid åldring vid förhöjd temperatur (300-600 ºC). Tendensen till fassepration tar sig termodynamiskt uttryck som en s.k. blandningslucka i det binära fasdiagrammet för Fe-Cr. Blandningsluckan, eller binodalen, och spinodalen - definierad som samlingen av punkter där andraderivatan av molära Gibbs energi är lika med noll, är centrala begrepp i teorin om fasseparation. I området mellan binodalen och spinodalen sker sönderfallet i form av termiskt aktiverad kärnbildning, tillväxt och förgrovning. Innanför spinodalen behövs ingen termisk aktivering och den initialt homogena legeringen sägs sönderfalla spinodalt, dvs. genom gradvis uppbyggnad av Cr-rika och Fe-rika områden. Fassepration genom spinodalt sönderfall ger upphov till en karaktäristisk mikrostruktur, bestående av kontinuerligt sammanhängande områden av Cr- och Fe-rika områden. Sönderfall av Fe-Cr-legeringar har rönt mycket intresse både inom akademien och industrin pga dess försprödande effekt, som därmed begränsar den maximala temperaturen vid vilken dessa material kan användas. Det är därför önskvärt att förbättra både förståelsen och möjligheterna att förutspå förekomsten av fassepration genom simulering, för att på så sätt utveckla material som är okänsliga för fassepration vid högre driftstemperaturer. Detta arbete fokuserar på utveckling och användning av simuleringar som utnyttjar termodynamiska och kinetiska databaser. Simuleringarna i detta arbete baseras på att lösa den s.k. Cahn-Hilliard ekvationen, kopplad till nämnda databaser. En viktig parameter i Cahn-Hilliard ekvationen är gradientenergikoefficienten, vilken påverkar såväl morfologi som sönderfallshastighet. I detta arbete har en generell formulering för gradientenergi i multikomponentsystem utvecklats, dock krävs mer arbete för att validera den framtagna modellen. Dessutom så har detta arbete lett fram till insikter vad gäller val av den initiala strukturen som används som startpunkt för simuleringarna. En sammanflätning av nämnda delförbättringar leder ett steg närmare ett funktionellt verktyg för förutsägelser om materials beteende gällande fasseparation vid åldring av komplexa legeringar. Ett sådant verktyg kan vara till hjälp vid framtida legeringsdesign, men även fungera som diagnosverktyg för komponenter som redan tagits i bruk och därmed är svåra att undersöka oförstörande på plats, t ex kritiska komponenter i kärnkraftverk.