Electron Beam Powder Bed Fusion manufacturing of components is an additive manufacturing process that is complex and has widespread advantages for aerospace and many industrial processes. It reduces costs and has a larger powder particle size requirement. This gives the benefit of a higher mass deposition rate and thus faster production time compared to Laser-Powder Bed Fusion process. Powder bed manufacturing processes often lead to columnar grain structure formation along the build direction, resulting in components that have anisotropic physical and mechanical properties. This is a major problem that limits the applications of this technique. In order to promote equiaxed grains, as well as refine the columnar morphology and eliminate anisotropic properties, the roles of process conditions and presence of inoculants or heterogeneous nucleating sites are considered. In this study, the addition of titanium nitride inoculants is used to promote columnar to equiaxed grain transition in ferritic stainless steel with the use of melting strategies and variable process parameters. It has been found that the thermal gradient (G) to solidification rate (R) ratio (G/R ratio) controls grain morphology and texture: a low G/R ratio has been shown to promote the formation of equiaxed grains. The process conditions for this transition were investigated. The samples were analyzed after printing single line tracks in the Freemelt One machine, and thereafter studied with the aid of optical microscopy to ascertain the combination of machine parameters that results in a successful transition from columnar grains to equiaxed. The study concluded that there was an increase in the fraction of equiaxed grains under these conditions; a low thermal gradient, high scanning velocity and low area energy. Ultimately, further investigation will be needed to establish the exact process parameters that will promote the transition from columnar to equiaxed grains in ferritic stainless steel. The findings from this study can be used by future researchers to create solidification maps for this steel grade and assist industry to tailor specific textures in ferritic stainless steel to achieved desired microstructures and mechanical properties. / Electron  Beam  Powder  Bed  Fusion  (E-PBF)  tillverkning  av  komponenter  är en  additiv tillverkningsprocess  som  är  komplex  och  har  omfattande fördelar  för  flyg  och  många industriella processer. Det minskar kostnaderna och har ett större krav på pulverpartikelstorlek. Detta ger fördelen av en högre massavsättningshastighet och därmed snabbare produktionstid jämfört med Laser-Powder Bed Fusion-processen. Tillverkningsprocesser för pulverbädd leder ofta  till  att en kolumnformig  kornstruktur  bildas  längs  byggriktningen, vilket resulterar  i komponenter  som  har  anisotropa  fysikaliska  och mekaniska egenskaper.  Detta  är  ett  stort problem som begränsar tillämpningarna av denna teknik. För att främja likaxliga korn, samt förfina den kolumnära morfologin och eliminera anisotropa egenskaper, övervägs rollerna för processbetingelser och närvaron av ympmedel eller heterogena kärnbildningsställen. I denna studie  används  tillsatsen  av  inokulanter  av  titannitrid  för  att  främja  kolumnär  till  likaxlig kornövergång  i  ferritiskt rostfritt  stål  med  användning  av  smältstrategier  och  variabla processparametrar.  Det  har  visat  sig  att  förhållandet  mellan  termisk gradient  (G)  och stelningshastighet   (R)   (G/R-förhållande)   styr   kornmorfologi   och   textur:   ett   lågt   G/R- förhållande har visat sig främja bildningen av likaxliga korn. Processförhållandena för denna övergång undersöktes. Proverna analyserades efter att ha skrivit ut spår med en rad i Freemelt One-maskinen  och  studerades  därefter  med  hjälp  av  optisk mikroskopi  för  att  fastställa kombinationen   av   maskinparametrar   som   resulterar   i   en   framgångsrik   övergång   från kolumnära  korn till  likaxliga.  Studien  drog  slutsatsen  att  det  fanns  en  ökning  av andelen likaxliga korn under dessa förhållanden; en låg termisk gradient, hög avsökningshastighet och låg areaenergi. I slutändan kommer ytterligare undersökningar att behövas för att fastställa de exakta processparametrarna som kommer att främja övergången från kolumnära till likaxliga korn i ferritiskt rostfritt stål. Resultaten från denna studie kan användas av framtida forskare för att skapa stelningskartor för denna stålkvalitet och hjälpa industrin att skräddarsy specifika texturer i ferritiskt rostfritt stål för att uppnå önskade mikrostrukturer och mekaniska egenskaper.

Additive  manufacturing  technologies  are  widely  used  in  aerospace,  space,  and  turbine industries. Parts can be manufactured directly by selectively adding materials layer-by-layer. A key aspect that is critical to the quality of the final component being manufactured is the powder characteristics. The prevailing powder characterisation techniques help in predicting the flowability of powders but do not relate to the spreading nature of the powder. To create high-quality thin layers of metal powder, it is essential to understand powder spreadability in powder bed-based additive manufacturing processes. The objective of this study was to create spreadability metrics using image analysis, mass analysis, and density analysis. A lab-scale experimental setup was constructed to replicate the powder bed-based additive manufacturing process. The impact of spreading speed and layer thickness on five different steel powders were studied using the suggested metrics. The metrics obtained powder rheometry and revolution powder analysis. The flowability parameters were compared to the spreadability analysis. Image analysis was shown to be efficient to predict the spreading nature of the powder when the processing parameters are varied. One metric, the convex hull ratio, was found to be high for  free-flowing  powders.  The  spread  area  of  free-flowing  powders  was  higher  than  the powders with poor flow properties. A mass-based analysis procedure shows that the ratio of mass deposited to the theoretical mass fluctuated in a systematic manner as a function of testing parameters  and  for  different  powders,  suggesting  that  the  mass  analysis  might  be  another potential   metric   to   assess   spreadability.   The   density-based   analysis   was   effective   in differentiating the layer density of different powders under various experimental conditions. It   is   expected   that   the   proposed   metrics   will   be   a   beginning   for  developing   further characterisation techniques. For example, the layer thickness could be studied by creating a homogenous  layer.  We  anticipate  these  metrics  to  be  used  to  develop  standardisation techniques for defining and quantifying powder spreadability, and thereby improve quality ofadditive manufacturing processes. / Additiv  tillverkning  är  teknologier  som  har  stor  uträckning  inom  flyg-,  rymd  och  turbin industrier. Delar kan bli tillverkade direkt genom att lagervis addera material på varandra. En nyckelaspekt som är kritisk till kvalitén av den slutgiltiga komponenten är egenskaperna hos pulvret. De allmänna teknikerna för pulverkarakterisering hjälper till att förutspå flytförmågan hos pulver men relaterar ej till dess spridningsförmåga. För att kunna skapa högkvalitativa skikt av  metallpulver  är  det  nödvändigt  att  förstå  pulvrets  spridningsförmåga  inom  pulverbädds baserade additiva tillverkningsprocesser. Målet  med denna studie var  att skapa ett mått för spridningsförmågan  genom  bild-  och  massanalys.  Ett  experimentellt  upplägg  i  labbskala konstruerades för att efterlikna en pulverbädds baserad additiv tillverkningsprocess. Effekten av bladets hastighet och lagrets tjocklek på fem olika pulver studerades genom användandet av de  föreslagna  mätetalen.  De  framtagna  mätetalen  jämfördes  sedan  med  existerande  pulver karakteriseringsmetoder  såsom  FT-4  Rheometer  och  pulver  analys  med  hjälp  av  roterande trumma. Slutligen så jämförs flytbarhets parametrarna med spridbarhets mätetalen. Det visar sig att bildanalysen är tillräckligt bra på att förutspå spridningsförmågan hos pulvret när  processparametrarna  låtes  vara  varierande.  Mer  specifikt  så  var  förhållandet  mellan pulvrets yta och det konvexa höljet stort för pulver som visar bra spridning. De framtagna procent  värden  från  massanalysdiagrammen  fluktuerar  vid  olika  processparametrar  hos  de olika  pulvren,  vilket  kan  betyda  att  massanalys  kan  vara  ett  potentiellt  sätt  för  att  mätta spridningsförmågan hos pulver. Det är förväntat att dessa föreslagna mätetal kommer vara början för utveckling av ytterligare karakteriseringstekniker. Till exempel, för att studera densiteten och tjockleken hos ett lager skulle man kunna skapa homogena lager. Vi förutser att dessa mätetal kommer att bli använda för att skapa standardiseringstekniker för att definiera och kvantifiera spridningsförmågan hos ett pulver och genom detta förbättra kvaliteten av den additiva tillverkningsprocessen.

Cast iron, taking the advantages of the advanced castability forming components of complex geometries and favorable mechanical properties, is employed in engine components in truck industries. Compacted graphite iron (CGI) integrates both merits of lamellar graphite iron (LGI) and spheroidal graphite iron (SGI) such as good machinability and high thermal conductivity from LGI, high ultimate tensile strength (UTS), good fatigue resistance, high elastic modulus, and high ductility from SGI, thus is now becoming a competitive alternative of traditional LGI in cylinder blocks and heads. Due to the shape complexity of cast components, residual stresses arise accordingly. Normal methods for measuring stresses have various practical difficulties that affect accuracy. For example, in strain gauge measurements such as hole drilling and cutting, casting skins need to be polished as the attachment of strain gauge requires a smooth surface condition for precise detection, though any mechanical treatment would change the residual stress state. On the other hand, electropolishing applied in XRD measurement for extracting depth profile causes no release of stresses, nevertheless, there is no dissolution reaction on graphite particles. This would retard further polishing and form a rough surface instead of flat extraction. A visual strain detection system relies on a stable and clean surface condition, therefore, when it is combined with the drilling technique, the drilling chips could be a vital problem for repeatability when they block the view of drilling edges. Ultrasonic measurement, in theory, has lower precision by averaging the stresses within a certain volume beneath surfaces. A number of methods have been developed to measure residual stresses, ranging from destructive to non-destructive according to the removal amount of materials. In this thesis work, several measurement methods are implemented on cylinder heads and the results are compared with simulation to develop a suitable method of measuring residual stresses in cast engine components. It is found that longer shakeout time lowers the tensile stresses and develops more compressive stresses in the surface layer. Cutting is a suitable method compared with others. Incremental center-hole drilling technique is not suitable to measure cast components as the surface grinding before stain gauge mounting causes high deviation. Hole drilling with visual strain detection provided high errors within the first 0.1 mm as the strains were too weak to be visualized at the beginning of drilling. The electropolishing process was also found retarded by graphite particles, and the XRD results are more trustworthy with more tilt angles. Ultrasonic measurement is rather rough due to the influence of graphite on the traveling velocity of ultrasound. / Tack vare sin utmärkta gjutbarhet, som möjliggör gjutning av komplexa geometrier, samt goda mekaniska egenskaper är gjutjärn första valet i många motorkomponenter inom lastbilsindustrin. Kompaktgrafitjärn (CGI) kombinerar fördelarna med lamellärt grafitjärn (LGI) och sfäroidalt grafitjärn (SGI) såsom god bearbetbarhet och hög värmeledningsförmåga från LGI, hög draghållfasthet (UTS), hög utmattningshållfasthet, hög elastisk modul (E-modul) och hög duktilitet från SGI. Detta gör kompaktgrafitjärn till konkurrenskraftigt alternativ till traditionell LGI i cylinderblock och huvud. På grund av formkomplexiteten hos gjutkomponenter uppstår restspänningar. Normala metoder för att mäta spänningar har olika praktiska svårigheter som påverkar noggrannheten. Vid töjningsmätningar genom hålborrning och sågning måste t ex gjutskinn slipas bort eftersom fästning av töjningsmätare kräver en jämn yta för exakt detektion, även om prepareringen kan påverka spänningstillstånd. Å andra sidan orsakar elektropolering som appliceras i XRD-mätning för extrahering av djupprofil inte att spänningar frigörs, däremot finns det inget etsmedel för grafitpartiklar. Detta gör att poleringen skapar en ojämn yta istället för en platts yta. Systemet för visuell detektering av förlängning förlitar sig på en stabil och ren yta. Därför kan borrspånen, när visuell detektering kombineras med borrteknik, vara ett viktigt problem för repeterbarheten. Ultraljudsmätning har i teorin lägre precision genom att den endast mäter medelvärdet av spänningarna i en viss volym under ytan. Ett antal metoder har utvecklats för att mäta restspänningar, både förstörande och oförstörande. I denna examensarbetesrapport implementeras flera mätmetoder på motorcylinderhuvuden och resultaten jämförs med simulering för att utveckla en lämplig metod för att mäta restspänningar i gjutna motorkomponenter. Det visas i arbetet att längre urslagstid kan sänka dragspänningarna och utveckla mer tryckspänningar i ytskiktet. Sågning är en lämplig metod jämfört med de andra. Inkrementell hålborrningsteknik är inte lämplig för att mäta restspänningar på gjutkomponenter eftersom ytslipningen före montering av töjningsmätare orsakar hög avvikelse. Hålborrning med visuell töjningsdetektering gav höga fel inom den första 0,1 mm från ytan eftersom töjningen är för liten för att kunna visualiseras i början av borrningen. Det är hänt ofta att elektropoleringsprocessen före XRD-mätningen blir fördröjd av grafitpartiklar, och XRD-resultaten är mer pålitliga med fler lutningsvinklar. Ultraljudsmätning är grov på grund av grafitens påverkan på ultraljudets hastighet.

At the royal institute of technology (KTH) in the department of applied process metallurgy, a novel modelling approach has been developed which allows a dynamic coupling between the commercial thermodynamic software Thermo-Calc and the commercial computational fluid dynamic (CFD) software Ansys Fluent, only referred to as Fluent in the study. The dynamic coupling approach is used to provide numerical CFD-models with thermodynamic data for the thermo-physical properties and for the fluid-fluid chemical reactions occurring in metallurgical processes. The main assumption forthe dynamic coupling approach is the existence of local equilibrium in each computational cell. By assuming local equilibrium in each computational cell it is possible to use thermodynamic data from thermodynamic databases instead of kinetic data to numerically simulate chemical reactions. The dynamic coupling approach has been used by previous studies to numerically simulate chemical reactions in metallurgical processes with good results. In order to validate the dynamic coupling approach further, experimental data is required regarding surface reactions. In this study, a graphiteand metallurgical coke oxidation experimental setup was suggested in order to provide the needed experimental data. With the experimental data, the ability of the dynamic couplings approach to numerically predict the outcome of surface reactions can be tested.By reviewing the literature, the main experimental apparatus suggested for the oxidationexperiments was a thermo-gravimetric analyzer (TGA). The TGA can provide experimental data regarding the reaction rate, kinetic parameters and mass loss as a function of both temperature and time. An experimental setup and procedure were also suggested.In order to test the ability of Fluent to numerically predict the outcome of surface reactions, without any implementation of thermodynamic data from Thermo-Calc, a benchmarking has been conducted. Fluent is benchmarked against graphite oxidation experiments conducted by Kim and No from the Korean advanced institute of science and technology (KAIST). The experimental graphite oxidation rates were compared with the numerically calculated graphite oxidation rates obtained from Fluent. A good match between the experimental graphite oxidation rates and the numerically calculated graphite oxidation rates were obtained. A parameter study was also conducted in order to study the effect of mass diffusion, gas flow rate and the kinetic parameters on the numerically calculated graphite oxidation rate. The results of the parameter study were partially supported by previous graphite oxidation studies. Thus, Fluent proved to be a sufficient numerical tool for numerically predicting the outcome of surface reactions regarding graphite oxidation at zero burn-off degree.

Slopping in BOS-processes poses many problems, most significantly to work environmentand process effectiveness. Due to the current weaknesses in slopping prediction systems afoaming slag suppression system with immediate effect is needed to prevent slopping.This project aims primarily to be a proof of concept for pressurized oxygen gas blowing as amean of foam steady-state height suppression to prevent slopping and secondarily toappreciate the viability of this concept economically.Five nozzles were designed and used to blow pressurized air onto foam made of silicon oil, atdifferent air flows. It was then determined which nozzle was the most effective by comparingheight difference with airflow. The airflow was compared to a life scale scenario to determinethe real flow rate which was used to determine economic viability.Results show that a nozzle with a circular small hole is the most effective nozzle requiring20 [ln min-1] to reduce the foam height by 69.6%. The real flow to achieve this would be0.605 [m3 s-1], however due to the cold models limitations this is not the true value. Theslopping suppression technique shows promise as a concept both economically andpractically. / Överkok i BOF-processen skapar många problem, mest i arbetsmiljön och minskar så väl produktionens effektivitet som takt. Eftersom de kontrollsystem som finns att tillgå idag     har vissa begränsningar så behövs det ett system för att motverka skumtillväxt med direkt inverkan för att hindra överkokning.Det här projektet ämnar huvudsakligen till att bevisa att ett pålagt flöde av syrgas kan trycka ned det skummande slaggets höjd och på så sätt förhindra överkokning och sekundärt till att bedöma konceptets ekonomiska rimlighet. Fem  munstycken designades  och användes till  att  blåsa tryckluft, med olika flöden, på skum bestående av   silikonolja.   Effektiviteten   hos   munstyckena   utvärderades   genom  att jämföra höjdskillnaden mot det pålagda luftflödet. Luftflödet jämnfördes sedan mot ett scenario i industriell skala och det verkliga luftflödet kunde därefter beräknas. Med detta  som  bakgrund gjordes en ekonomisk analys. Resultat visar att munstycket med ett litet cirkulärt håll är mest effektivt då det krävdes ett flöde på 20 [ln min-1] för att reducera skumhöjden med 69,6%. Det verkliga flödet beräknades till 0,605 [m3 s-1], men eftersom den kalla modellen har vissa begränsningar så är detta värde inte  sant.  Den  här  tekniken  för   att  förhindra   överkokning  ser   lovande   utbåde  ur  ett   ekonomiskt men också ur ett praktiskt perspektiv.

Hadfield steel is a material used in different industrial application, when a good resistance to severe impact loads and abrasion is expected. Its high ductility, high toughness, good work hardening capacity and high fatigue resistance make it especially suitable for theses conditions. But in order to reach those high mechanical properties, Hadfield steel must undergo a heat treatment. This step is considered as a crucial, because the as cast material contains different types of carbides, some beneficial, and some detrimental to the mechanical properties. Therefore, there is a strong interest in understanding the effect of HT parameters on the properties of Hadfield Steel. The goal of the project is to study the HT and quenching parameters used for the manufacturing of Hadfield steel, such as HT temperatures, holding time and quenching media. The microstructure such as: grain size and grain boundaries will be analyzed using an optical microscope (OM) and a Scanning Electron Microscope (SEM) before and after HT and quenching. A hardness tester will be used to measure the impact of the HT on the mechanical properties.

Artefacts known as amulet rings, a kind of Viking age jewellery, have often been encountered during archaeological excavations of Viking Age sites. The rings do not appear to have any practical use, but are rather considered to be of cultural or religious importance. Their exact function is however unclear, as are the details regarding the production and material choices for the rings.This study mainly examines the microstructure of samples from Viking Age amulet rings found in Dalecarlia, Sweden. To be able to conclude how much effort that was put into the rings a comparison with medieval cutting tools was conducted. Thus, the knives were made with more care in order to obtain the material properties required to give the knives the sharpness and hardness they need to function. The analysis shows that the amulet rings were made from heterogeneous low-carbon steel, indicating little or no welding and a poor-quality starting material. Were as the microstructure of the knives had homogeneous microstructures often involving perlite, showing decent carbon content.Thus, it appears that the Viking Age smiths forged the amulet rings without putting too much effort into the process.This study also involved the manufacturing of a new knife from low-carbon steel, this was done to see if it was possible to redeem carbon into the surface while forging. This however, was difficult when not having todays technical equipment at hand, thus the result was insufficient. / Historiska föremål, kallade amulett ringar vilket är ett slags vikingatida smycken, har hittats vid arkeologiska utgrävningar av vikingatida boplatser. Man har inte hittat något praktiskt användningsområde för ringarna, utan de tros vara av kulturell eller religiös betydelse. Deras funktion är dock oklar, likaså detaljerna angående tillverkningen och materialvalet för ringarna.Denna studie undersöker mikrostrukturen av amulett ringar från Dalarna i Sverige. För att kunna dra slutsatser om hur mycket arbete som lades på ringarna genomförs en jämförelse med medeltida eggverktyg. Eftersom eggverktyg gjordes med omsorg för att få rätt materialegenskaper för att vara vassa och hårda kan man jämföra dessa med ringarna. Analyserna visade att amulett ringarna var gjorda av lågkolhaltigt heterogent stål som indikerar enbart lite eller ingen vällning och ett dåligt utgångsmaterial. Tillskillnad från eggverktygen som uppvisade en homogen mikrostruktur som ofta innehöll perlit vilket tyder på relativt hög kolhalt. Följaktligen framstår det som att vikingatida smeder inte la ned särskilt mycket tid i smidesprocessen vid tillverkningen av amulett ringar.Denna studie innefattar också smidet av en ny kniv från ett lågkolhaltigt stål för att se om det är möjligt att lösa in kol i ytan genom smidet. Detta var dock svårt att göra utan dagens tekniska utrustning vilket gjorde att resultatet inte visade någon uppkolning.

The reduction of fuel consumption and carbon dioxide emissions are currently a key factor for the aviation industry due to major concerns about climate change and more restrictive environmental laws. One way to reduce both fuel consumption and CO2 emissions is by significantly decreasing the vehicle’s weight while increasing engine's efficiency. In order to meet these requirements, the European aero engine industry is continuously focusing on alternative manufacturing methods for load carrying structures in advanced materials, such as titanium and nickel-based superalloys. Alternatively to traditional large-scale single castings, new manufacturing methods involve sheet metal parts, small castings and forgings assembled by welding. These new manufacturing methods allow more flexible designs in which each part is made of the most suitable material state, leading to several advantages such as reduction of product cost and weight while increasing engine's efficiency. Nickel-based superalloys are widely used in the aero engine industry, typically constituting up to 50% of the total weight of the aircraft engine. Due to their excellent material properties at high temperatures in severe corrosive environments, these superalloys are employed most extensively in the hot sections of gas turbine engines for both military and civil aircrafts with running temperatures up to 650°C. In this thesis, a manufacturing process chain including forming and welding in the nickelbased superalloy 718 is studied. The main focus in the work lies on determining the thermomechanical properties, modelling and simulation of cold forming, study forming limits based on Nakazima tests for forming limit curves (FLC) and applying a damage and failure criterion. The work also comprises a brief study on hot forming. Finally, modelling of a subsequent welding procedure is included where residual stresses from the forming simulation are used to predict shape distortions due to the welding procedure. The results are compared with experimental observations. The cold forming procedure of a double-curved component made of alloy 718 is studied using FE-analyses and forming tests. The same geometry was used to produce a hot forming tool. During forming tests at room temperature, micro cracks and open cracks were observed in the draw bead regions, not indicated when formability is assessed using a forming limit curve (FLC). Standard material models such as von Mises or Barlat Yld2000-2D were not capable of accurately predict the behaviour of the material after the point of diffuse necking, making the prediction of damage and failure during forming a challenge. The GISSMO damage model was therefore calibrated and used to predict material failure in forming of alloy 718. Tensile, plane strain, shear and biaxial tests at room temperature are performed up to fracture and continuously evaluated using Digital Image Correlation (DIC) by ARAMIS™. In this work, the GISSMO damage model is coupled with the anisotropic Barlat Yld2000-2D material model for forming simulations in alloy 718 at room temperature using LS-DYNA. Numerical predictions are able to accurately predict failure on the same regions as observed during the experimental forming tests. Comparisons of the distribution of damage on one of the draw beads between simulations and damage measurements by acoustic emission indicate that higher damage values correspond to bigger micro cracks. Numerical FE-predictions of the cold forming and subsequent welding procedure shows that the welding procedure further increases the shape distortions. This was found to agree with experimental observations. / Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator

A systematic survey of morphology of bainite and proeutectoid ferrite was carried out in order to validate some old thoughts of bainite transformation mechanism. It is confirmed that there is no morphological evidence supporting a sharp change neither between Widmanstätten ferrite and the ferritic component of upper bainite, nor between upper and lower bainite. Both Widmanstätten ferrite and upper bainite start with precipitation of ferrite plates at a grain boundary while lower bainite starts with intragranular nucleation. In case of grain boundary nucleation, a group of parallel plates with same crystallographic orientation to the parent austenite grain forms. This process is followed by a second stage of decomposition of the austenitic interspace, which remained in between the primary ferrite plates. At high temperature, the austenitic interspace would either retain as thin slabs or transform into pearlite through a nodule originated from a grain boundary. At lower temperature, cementite precipitation starts to be possible and initiates simultaneous growth of ferrite. Generally, there are two modes of such eutectoid reactions operating in the second stage, i.e. a degenerate and a cooperative mode, which would lead to typical upper and lower bainite, respectively, in definition of carbides morphology. Both upper and lower bainite according to this definition are observed in a wide temperature range. A sharp temperature between the upper and lower bainite structures thus exists only when the definition is based on their nucleation sites, i.e. grain boundary nucleation for upper bainite and intragranular nucleation for lower bainite. Supposing that the first stage is a diffusionless process it should have a high growth rate to prevent carbon diffusion. This is not supported by lengthening rate obtained in current study as well as data from literature for Fe-C alloys. Finally, it is shown that the “subunits” play no role in the lengthening process of bainite. / <p>QC 20170523</p>

The main aim of this thesis was to design and develop new Aluminium based compositionally complex alloys (CCAs) using the high entropy alloy (HEA) concept, and to understand their evolution of microstructures during casting and also after the secondary process which is heat-treatment, and finally to evaluate their subsequent mechanical properties. Prior to the development of alloys, a computational technique ThermoCalc was used which helped in understanding the phase formation in various results. Use of thermodynamic physical parameters for predicting the stability of single-phase fields was done to assess their validity in predicting the compositional regions of the alloys developed. The first alloy developed is Al73.6Mg18Ni1.5Ti1.9Zr1Zn4 in at% (NiTiZrZn) CCA. The microstructure consists of the FCC as a primary phase with ~49% volume fraction along with β-AlMg and intermetallic (IM) phases including Al3Ni, Al3Ti, and Al3Zr. After casting, the microstructure showed some presence of eutectic structures. The Al3Ti, and Al3Zr IM phases seemed to precipitate early which led to less homogenization of Ti and Zr, causing deviation in the amount of these elements in the matrix. Further, the CCA was heat-treated at 375 oC for 24hrs and 48hrs and the evolution of microstructure along with its hardness and phase transformation characterisation was investigated. The second developed alloy was quaternary Al65.65Mg21.39Ag10.02Ni2.94 in at% (AgNi) CCA. In the as-cast state, the main phase (matrix) was FCC with ~64 % volume fraction along with BCC, β-AlMg and Al3Ni IM phases. There was a good level homogenization of all elements in the alloy. They were further heat- treated at 400 oC for 24 hrs and 48 hrs and were studied for any change in microstructure along with its hardness and thermal stability. This CCA had the highest hardness value from all developed CCAs. Lastly, in order to check how Ni affects the microstructure and properties of (AgNi) CCA, a ternary Al67.2Mg22.09Ag10.7 in at% (Ag) CCA was developed. The composition was kept such that it is exactly 97% by excluding the Ni. During the development of this alloy, the cast was cooled in two ways first being the normal cooled just like other CCAs and second being a fast cooling method. Both of these alloys consists of the FCC phase as a primary phase with 72% volume fraction along with BCC and β-AlMg. Both of them were also heat treated at 400 oC for 24 hrs and 48 hrs to evaluate any changes in microstructure and also to assess its hardness and thermal stability.