The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughnessof the steel. However, too high austenitisation temperature may result inexcessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to marten site on cooling. Secondly, when tempering at 600°Cfor extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides.These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis.

The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression.

Today, there is an essential need for lightweight, energy-efficient, environmentally benign engineering systems, and this is the driving force behind the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges have motivated the use of magnesium alloys for lightweight structural systems. Magnesium has a density of 1.74 g/cm3, which is almost 30% less than that of aluminium, one quarter of steel, and almost identicalto polymers. The ease of recycling magnesium alloys as compared to polymers makes them environmentally attractive, but their poor mechanical performance is the primary reason for the limited adoption of these alloys for structural applications. The Mg-Al-Zn alloy AZ91D exhibits an excellent combination of strength, die-castability, and corrosion resistance. However, its mechanical performance with regard to creep strength, for example, at evaluated temperatures is poor. Moreover, very little is known about the correlation between its mechanical properties and microstructural features. This thesis aims to provide new knowledge regarding the role played by microstructure in the mechanical performance of the magnesium alloy. The properties/performance of the material in relation to process parameters became of great interest during the investigation. An exhaustive characterisation of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 was performed using optical and electron backscatter diffraction (EBSD). These microstructural parameters were correlated to the offset yield point (Rp0.2), fracture strength, and elongation to failure of the material. It was proposed that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy in a temperature range of room temperature to 190°C by forming a rigid network of intermetallic. The presence of this network was confirmed by studying the thermal expansion behaviour of samples of the alloy containing different amounts of Mg17Al12. A physically based constitutive model with a wide validity range was successfully adapted to describe the flow stress behaviour of AZ91D with various microstructures. The temperature-dependent variables of the model correlated quite well with the underlying physics of the material. The model was validated through comparison with dislocation densities obtained using EBSD. The influence of high-pressure die-cast parameters on the distortion and residual stress of the cast components was studied, as were distortion and residual stress in components after shot peening and painting. Interestingly, it was found that intensification pressure has a major effect on distortion and residual stresses, and that the temperature of the fixed half of the die had a slight influence on the component's distortion and residual stress. / Numera finns det ett väsentligt behov av lätta, energieffektiva och miljövänliga tekniksystem. Detta behov är drivkraften för utveckling av ett brett utbud av material för energigenerering, energilagring, framdrivning och transport. Dessa utmaningar motiverade användningen av magnesiumlegeringar för lättviktskonstruktioner. Magnesium har en densitet på 1,74 g/cm3, vilket är ca 30% lägre än för aluminium, en fjärdedel av densiteten för stål och nästan i nivå med många polymerer. Då magnesiumlegeringar dessutom är lätta att återvinna, jämfört med polymerer, gör det dem miljömässigt attraktiva. Låga mekaniska egenskaper är den främsta orsaken till begränsad användning av dessa legeringar för lastbärande tillämpningar. Mg-Al-Zn-legeringen AZ91D uppvisar en utmärkt kombination av styrka, gjutbarhet och korrosionsbeständighet. Dess mekaniska egenskaper vid förhöjd temperatur, som tex kryphållfasthet, är låga. Dessutom är korrelationen mellan mikrostruktur och mekaniska egenskaper oklar. Denna avhandling syftade till att ge ny kunskap om mikrostrukturens roll för magnesiumlegeringars mekaniska egenskaper. Slutligen var materialets egenskaper i förhållande till processparametrar vid tillverkningen av stort intresse. En omfattande karaktärisering av kornstorleks-, sekundära dendritarmavstånds (SDAS)-fördelning och fraktion av Mg17Al12 utfördes med hjälp av optisk mikroskopering och diffraktion av bakåtspridda elektroner (EBSD). Mikrostrukturen korrelerades till sträckgränsen (Rp0.2), brottstyrkan och brottförlängningen. Det föreslogs att den intermetalliska fasen, Mg17Al12, spelar en viktig roll vid bestämning av legeringens mekaniska och fysikaliska egenskaper vid temperaturintervall från rumstemperatur upp till 190°C genom att bilda ett styvt nätverk av intermetaller. Uppkomsten av ett sådant nätverk stöddes genom en studie av den termiska expansionen av legeringen för olika fraktioner av Mg17Al12. En fysikalisk konstitutiv modell med ett brett giltighetsområde användes framgångsrikt för att beskriva det plastiska flytbeteendet hos AZ91D för olika mikrostrukturer. De temperaturberoende variablerna i modellen korrelerade ganska väl med materialets underliggande fysik. Modellen validerades genom att jämföra dislokationstätheten som predikterades av modellen och den med EBSD uppmätta dislokationstätheten. Påverkan av pressgjutningsparametrar på geometrisk tolerans och restspänning hos de gjutna komponenterna studerades. Vidare studerades geometrisk tolerans och restspänning av komponenter efter pening och målning. Intressant nog hade eftermatningsfasen en stor effekt på geometrisk tolerans och restspänningar. Dessutom hade temperaturen på den fasta formhalvan av verktyget även ett visst inflytande på komponentens geometriska tolerans och restspänning.

The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression.:Inhaltsverzeichnis 1 Einleitung 1 2 Grundlagen 9 2.1 Titan und Titan-Legierungen. . . . . . . . . . . . . . 9 2.2 Das binäre System Ti-Fe. . . . . . . . . . . . . .11 2.3 Phasendiagramm, Gleichgewichts-/ Nichtgleichgewichtsprozesse. . . . . . . . . . . . . .11 2.3.1 Kristallstrukturen der eutektischen Phasen . . . . . . . . . . . . . . 14 2.3.2 Klassifizierung von Phasengrenzflächen. . . . . . . . . . . . . .15 2.3.3 Eigenschaften intermetallischer Phasen mit B2- Struktur. . . . . . . . . . . . . . 17 2.4 Erstarrung von Schmelzen . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.5 Das eutektische System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.5.1 Metastabile Legierungen . . . . . . . . . . . . . . . . . . . . . . . . 24 2.5.2 Keimbildung von eutektischen Systemen . . . . . . . . . . . . . . . 26 2.5.3 Klassifizierung eutektischer Gefüge. . . . . . . . . . . . . . . . . . 27 2.5.4 eutektische Systeme . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.5.5 Bestimmung der Erstarrungsgeschwindigkeit nach Jackson und Hunt. . . . . . . . . . . . . . 31 2.6 Einfluss des Gefüges auf die Verformungsmechanismen . . . . . . 32 2.7 Prozessrouten zur Herstellung nanostrukturierter/ultrafeinkörniger (ns/ufk) Materialien . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.8 Duktilität und Festigkeit ns/ufk Materialien (Stand der Forschung) . . . . 39 3 Werkstoffauswahl und Probenherstellung. . . . . . . . . . . . . . 46 3.1 Werkstoffauswahl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.2 Probenherstellung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2.1 Herstellung der Vorlegierung im Lichtbogenofen . . . . . . . . 48 3.2.2 Herstellung der Legierungen nach der Bridgeman-Technik . . 49 3.2.3 Herstellung der Ti-Fe- bzw. Ti-Fe-Sn-Legierungen in verschiedenen Rascherstarrungsanlagen . . . . . . . . . . . . . 50 3.2.3.1 Stabherstellung Kalttiegelanlage . . . . . . . . . . . . . . 52 3.2.3.2 Stabherstellung Kipptiegelanlage . . . . . . . . . . . . . . 52 3.2.3.3 Stabherstellung Differenzdruckgussanlage . . . . . . . 53 4 Charakterisierungsmethoden. . . . . . . . . . . . . . 55 4.1 Chemische Analytik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.1.1 Nasschemische Analyse . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.1.2 Nichtmetallanalyse . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2 Röntgendiffraktometrie (XRD) . . . . . . . . . . . . . . . . . . . . . . . 56 4.3 Mikroskopische Untersuchungen . . . . . . . . . . . . . . . . . . . . . . 57 4.3.1 Lichtmikroskopie (LM) . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.3.2 Rasterelektronenmikroskopie (REM) . . . . . . . . . . . . . . . . 59 4.3.3 Transmissionenelektronenmikroskopie (TEM) . . . .. . . . . . 61 4.4 Mechanische Eigenschaften . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.4.1 Härte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.4.2 Druckversuch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.4.3 Zugversuch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.4.4 In situ Druck- und in situ Zugversuch . . . . . . . . . . . . . 64 4.5 Ultraschallmessung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.6 Dilatometermessung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5 Einphasige betafi-Ti(Fe)- und TiFe IP-Legierungen. . . . . . . . . . . . . 68 5.1 Die fibeta-Ti(Fe)-Legierung . . .. . . . . . . . . . . . . . . . . . . . . . . . 69 5.2 Die intermetallische Phase TiFe . . . . . . . . . . . . . . . . . . . . . . . . 82 6 Gerichtet erstarrte Ti70,5Fe29,5-Legierung . . . . . . . . . . . . . .92 7 Rasch erstarrte Ti70,5Fe29,5-Legierung . . . . . . . . . . . . . . 99 7.1 Gefüge der rasch erstarrten Ti70,5Fe29,5-Legierung . . . . . . . . 99 7.2 Mechanische Charakterisierung der rasch erstarrten Ti70,5Fe29,5- Legierung . ..120 7.2.1 Druckversuche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 7.2.2 Bestimmung der elastischen Konstanten . . . . . . . . . . . . 128 7.2.3 Zugversuche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 7.2.4 In situ Druck- und in situ Zugversuche . . . . . . . . . . . . . 134 8 Rasch erstarrte Ti-Fe-Sn-Legierung . . . . . . . . . . . . . .138 8.1 Gefüge der Ti-Fe-Sn-Legierung . . . . . . . . . . . . . . . . . 139 8.2 Mechanische Eigenschaften der Ti-Fe-Sn-Legierung . . . . . . . . . 143 9 Zusammenfassung und Ausblick . . . . . . . . . . . . . . 146 Abbildungsverzeichnis I Tabellenverzeichnis VIII Literaturverzeichnis X Anhänge XXII A Das Ti-Fe-Phasendiagramm nach [1] XXII B Dilatometermessung XXIII C Die elastischen Konstanten der Ti-Fe- und Ti-Fe-Sn-Legierung XXIV D XRD-Messungen (Transmission) XXV E Bestimmung des Fe-Gehaltes in Abhängigkeit von der Gitterkonstanten a0 XXVIII Eidesstattliche Erklärung XXIX Danksagung