The aim of the work presented was to investigate the effects of upper bainite on impact toughness in quench and tempered low alloy plate steels. The experimental research included construction of CCT diagrams by dilatometry, verification of phases by optical microscopy (OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and finally impact testing of Charpy specimens with mixed bainite:martensite microstructures. Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model. Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels respectively which indicated linear growth with a considerable lengthening rate of laths and negligible thickening. The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model and the higher hardenability with reference to its CCT diagram, suggested a strong Mo alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute drag like effect. The effect of upper bainite in a tempered martensitic matrix was investigated for the following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total absorbed impact energy by an adverse effect on crack nucleation energy and crack propagation energy. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / Unrestricted

Abstract not available

Abstract not available

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 220-225).

The most widely used titanium alloy in the hot sections of a jet engine is Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and with the technological advancements additive manufacturing (AM) of Ti-6242 is of great interest. Although, there is not much published related to additively manufactured Ti-6242. The project aimed to provide an experimental input for simulations of additively manufactured,laser metal deposited–wire (LMD-w) of Ti-6242. The main part of this project has been to experimentally study the effect of isothermal temperatures and holding times on the microstructural changes in as-built Ti-6242 on Ti-64 base-plate. The evaluation of micro-structural changes with isothermal temperature, holding time and cooling rates on Ti-6242 are quantified in terms of alpha laths, phase fraction by using MIPAR (image analysis software) and preliminary determine mechanical properties by performing hardness tests.The microstructures obtained through a process are highly dependent on the thermal history is it exposed to and the mechanical properties are determined by the microstructures present. In this thesis work the alpha lath thickness and the hardness of the material increases with increasing isothermal temperature. From the phase fractions obtained forthe heat treated samples an equilibrium phase diagram and a TTT diagram is plotted for as-built microstructure.

Haynes 282 is a gamma prime (𝛾′)-strengthened nickel base superalloy developed in 2005, exhibiting a good combination of high temperature properties and fabricability. Microstructural features such as 𝛾′ and carbides play an important role in deriving the mechanical properties of the alloy during heat treatment. As Haynes 282 is a relatively new alloy with insufficient literature availability, the present thesis is aimed at studying the evolution of microstructure for different heat treatment times and temperature with a special focus on 𝛾′ phase precipitation kinetics with different initial conditions for the material. The study is divided into two sections with objectives which are focused on the different ends to the heat-treatment time scales. The first objective of this study was to investigate γ' precipitation at short heat treatment times and develop Time-Temperature Precipitation (TTP) and Hardness (TTH) diagrams for Haynes 282 using a novel arc heat treatment. In this technique, a steady state temperature gradient, covering room temperature to liquidus, was created using stationary TIG arc on a disc mounted on a water-cooled chamber. Aged and solutionized samples were arc heat treated for 1.5 minutes, 30 minutes and 4 hours. The study was complemented with temperature modelling, thermodynamic calculations, and 𝛾′ precipitation simulation. A unique graded microstructure formed, consisting of dendritic region in fusion zone; dissolution area of all phases including MC carbides, grain boundary carbides, and 𝛾′; grain boundary carbide zone, 𝛾′ band; and base metal. 𝛾′ precipitate size increased with increasing time and temperature. 𝛾′ precipitation simulation model was developed, and it showed very good agreement with experimental results. Finally, the results were summarized in TTH and TTP diagrams. The second objective in this work was to study understand the coarsening behaviour of 𝛾′ phase with an initial pre-heat-treated GKN heat treatment using furnace heat treatment. Isothermal heat treatments for temperatures from 800°C to 1120°C and times from 30 seconds to 96 hours were performed. Morphological changes in 𝛾′ phase, particle size distribution, grain sizes and hardness on these isothermal heat-treated states are presented in this work. Additionally, A TC PRISMA precipitation model was evaluated to predict 𝛾′sizes and compare it with the measurements. It was concluded that complex initial microstructure, containing bimodal distribution of 𝛾′ precipitates, caused deviations between predicted and measured values, while the model, in the previous objective, predicted the sizes in close approximation to the experimental values. Therefore, further understanding and development of precipitation kinetics with the software should be done to achieve closer results to the experiment. / Haynes 282 är ett gamma prime (𝛾′) - förstärkt superlegering av nickelbas som utvecklades 2005 och uppvisar en god kombination av högtemperaturegenskaper och tygbarhet. Mikrostrukturella egenskaper såsom 𝛾′ och karbider spelar en viktig roll för att få de mekaniska egenskaperna hos legeringen under värmebehandling. Eftersom Haynes 282 är en relativt ny legering med otillräcklig litteraturtillgänglighet syftar den aktuella avhandlingen till att studera utvecklingen av mikrostruktur för olika värmebehandlingstider och temperatur med ett särskilt fokus på 𝛾′ fasutfällningskinetik med olika initiala förhållanden för materialet. Studien är uppdelad i två sektioner med mål som är inriktade på de olika ändarna på värmebehandlings tidsskalorna. Det första syftet med denna studie var att undersöka 𝛾′nederbörd vid korta värmebehandlingstider och utveckla Time-Temperature Precipitation (TTP) och Hardness (TTH) diagram för Haynes 282 med användning av en ny bågvärmebehandling. I denna teknik skapades en jämn temperaturgradient, som täcker rumstemperatur till liquidus, med användning av stationär TIG-båge på en skiva monterad på en vattenkyld kammare. Åldriga och lösningsbara prover bågvärmebehandlades under 1,5 minuter, 30 minuter och 4 timmar. Studien kompletterades med temperaturmodellering, termodynamiska beräkningar och 𝛾′utfällningssimulering. En unik graderad mikrostruktur bildad, bestående av dendritisk region i fusionszon; upplösningsområde för alla faser inklusive MC-karbider, korngränsande karbider och 𝛾′; korngränsen karbidzon, 𝛾′ band; och oädel metall. 𝛾′utfällningsstorlek ökade med ökande tid och temperatur. 𝛾′utfällningssimuleringsmodell utvecklades, och den visade mycket bra överensstämmelse med experimentella resultat. Slutligen sammanfattades resultaten i TTH- och TTP-diagram. Det andra syftet med detta arbete var att studera förstå det förgrovning beteendemönster hos 𝛾′ fasen med en initial förvärmebehandlad GKN-värmebehandling med ugnsvärmebehandling. Isotermiska värmebehandlingar för temperaturer från 800 ° C till 1120 ° C och gånger från 30 sekunder till 96 timmar utfördes. Morfologiska förändringar i 𝛾′fasen, partikelstorleksfördelning, kornstorlekar och hårdhet på dessa isotermiska värmebehandlade tillstånd presenteras i detta arbete. Dessutom utvärderades en TC PRISMA-nederbördsmodell för att förutsäga 𝛾′ storlekar och jämföra den med mätningarna. Det drogs slutsatsen att komplex initial mikrostruktur, innehållande bimodal fördelning av 𝛾′-fällningar, orsakade avvikelser mellan förutspådda och uppmätta värden, medan modellen i det tidigare målet förutspådde storleken i nära anpassning till experimentvärdena. Därför bör ytterligare förståelse och utveckling av utfällningskinetik med programvaran göras för att uppnå närmare resultat till experimentet.

Les évolutions de microstructures du nouvel alliage chinois Ti B19 ont été étudiées au cours de divers traitements thermiques et des relations entre microstructures et propriétés mécaniques ont été établies. Les cinétiques de transformations isothermes ont été établies par des mesures de résistivité électrique. Les structures et microstructures ont été caractérisées par DRX synchroton, Microscopies Optiques et Electroniques (MEB, MET). Les cinétiques de transformation et les caractéristiques microstructurales. Les cinétiques de transformation isothermes ont été obtenues, analysées (loi JMAK), et cet ensemble de résultats a conduit à l'établissement du diagramme TTT de l'alliage TiB19. Enfin il a été montré que la vitesse de chauffage a une très forte influence sur les transformations mises en jeu, et qu'une pré déformation plastique accélère les cinétiques de transformation (introduction de nouveaux sites de germination). Enfin les cinétiques de transformation ont été caractérisées en refroidissement continu depuis le domaine monophasé bêta. Une première approche de la modélisation des cinétiques de transformation a été menée en utilisant le loi JMAK et le principe d'additivité de Scheil. Enfin les relations entre microstructures et propriétés sont discutés / The phase transformations and microstructure evolutions has been characterized for different thermal treatments, and the relationships between final microstrures and properties have been investigated in the new metastable Ti-B19 alloy. The isothermal phase transformation kinetics and the influence of different heat treatment phaths have been establisheb by using in-situ electrical Resistivity. The structures have been determined by synchrotron X-Ray Diffraction and the microstructures were observed by SEM and TEM. The results show that phase transformation kinetics and microstructure characteristics are strongly dependent on the aging temperature (ranging from 300 to 700°C). The global isothermal phase transformation phase transformation kinetics has been got and anallyzed with JMAK equation, and the TTT diagram of Ti-B19 alloy has been established. We have also shown that the heating rate has remarkable influence on the isothermal phase transformation behaviors and the pre-deformation accelerates the transformation kinetics. The microstructure evolutions during cooling are obviously dependent on the cooling rates. A first attempt has been made to calculate the transformation kinetics during cooling using JMAK law and Scheil principle. Finally, the relationship between mechanical properties and microstructure has been discussed