Characterization of small precipitates is important for development of new alloys. One inherent  difficulty  in  characterisation  of  small  precipitates  with  electron microscopy techniques  while  the  particles  are  embedded  in  the  matrix,  is  that the surrounding bulk material will contribute to the analysed signal- limiting, for example, the quantification of the composition of particles.  In order to avoid the matrix contribution, the extraction replica technique was developed in the 1950's. Extraction replicas are made by extracting only the particles from a material onto a thin film for further analysis by scanning or transmission electron microscopy. Different types of particles can be examined by this technique, including various carbides, nitrides, oxides and borides, as well as many intermetallic phases.  In this work, direct replication techniques were compared to two-stage replication for low-alloyed steel, concluding that both are suitable for qualitative analysis, but direct replication is preferred for quantitative analysis.  Successful replication of the zirconium-based Zirlo and the stainless steel 254 SMO was performed, where the precipitated phases were isolated and the composition quantified.   Particle extraction was successful also for the zirconium-based Zircaloy-2 and two types of  aluminium  alloys,  but  no  quantification of  composition  could  be  made  due to different problems associated with etching of each alloy, showing that properchoice of etchant is crucial for the quality of the replicas. / Karaktärisering av små utskiljningar är viktigt för utveckling av nya legeringar. En svårighet  vid  karaktärisering  av  små  utskiljningar  med elektronmikroskopi när partiklarna  sitter  kvar  i  matrisen  är  att  matrisen kommer  ge  ett  bidrag till den analyserade signalen,  därför är t.ex.   möjligheterna för kvantifiering av sammansättning begränsade. För att undvika bidraget från matrisen utvecklades extraktionsrepliker  på  1950-talet.  Extraktionsrepliker  tillverkas  genom  att extrahera partiklar från ett material till en tunn film som sedan kan analyseras med hjälp av svep- eller transmissionselektronmikroskopi. Olika sorters partiklar kan  undersökas  med hjälp  av  denna  metod.     Dessa  inkluderar  olika  typer av  karbider, nitrider, borider, oxider  och  många  olika  sorters  intermetalliska partiklar. I  detta arbete  jämfördes direkta  metoder  med  tvåstegsrepliker  på låglegerat  stål. Slutsatsen  blev  att  båda metoderna  lämpar  sig  för  kvalitativ analys av små partiklar, men direkta metoder är att föredra för kvantitativ analys. Lyckade repliker tillverkades även av zirkoniumlegeringen Zirlo och det rostfria stålet  254  SMO,  där  utskiljningarna  kunde isoleras  och sammansättningen kvantifieras.   Extraktion  av  partiklar  var  lyckad  även  för zirkoniumlegeringen Zircaloy-2  samt  två  typer  av  aluminiumlegeringar, men ingen  kvantifiering  av sammansättningen  kunde  göras  på  grund  av  olika problem  relaterade  till  den valda etsmetoden för varje legering, vilket visar att valet av etsmedel ar avgörande för kvaliteten av replikerna.

η (eta) nitride, Cr3Ni2SiN, is a precipitate found in high temperature austenitic stainless steel and is not yet included in Thermo-Calc steel database TCFE7. The aim of this thesis is to collect thermodynamic data to enable the addition of η nitride in the databases. Three laboratory melts with varying levels of silicon, chromium and nickel have been aged at 700-1000 °C for 75 h, 300 h and 1200 h and examined by Light Optical Microscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Wavelength Dispersive Spectroscopy, Electron Backscattered Diffraction and X-ray Powder Diffraction. η nitride is in the studied alloys an equilibrium phase stabilized with nitrogen. Presence of η nitride was confirmed by Energy Dispersive Spectroscopy and X-ray Powder Diffraction. It was found to precipitate in four different ways, at primary grain boundaries, intragranularly, as a "skeleton-like" precipitate and as a border around the occurring Cr2N precipitates. The area fraction of η nitrides increases with longer aging times and is favored by silicon and nickel. The composition of η nitride is not changing regardless of material composition, aging temperature and aging time. The composition of η nitride in all three materials are 8.7-9.7 wt.% silicon, 47-54 wt.% chromium, 1.4-4.1 wt.% iron and 33-36 wt.% nickel. The nitrogen content 2 determined by Wavelength Dispersive Spectroscopy is 2.8-3.2 wt.%. No complete equilibrium was achieved and together with incomplete mixing of the alloying elements during melting, the microstructure is difficult to evaluate. Other precipitates found are Cr2N, π nitride, σ phase and two unidentified phases, M and N. Of these phases at least Cr2N is not an equilibrium phase as it dissolves during aging. Further aging to achieve complete equilibrium is necessary.