The complete and accurate thermodynamic and kinetic description of any systemis crucialfor understanding and predicting its properties. A particular interest is in systemsthat are used for some practical applications and have to be constantly improved usingmodification of their composition and structure. This task can be quite accuratelysolved at a fundamental level by density functional theory methods. Thesemethods areapplied to two practically important systems Fe-Cr and TiC-ZrC.The elastic properties of pure iron and substitutionally disordered Fe-Cr alloy are investigatedas a function of temperature and concentration using first-principles electronicstructurecalculations by the exact muffin-tin orbitals method. The temperature effectson the elastic properties are included via the electronic, magnetic, and lattice expansioncontributions. It is shown that the degree of magnetic order in both pure iron andFe90Cr10 alloy mainly determines the dramatic change of the elastic anisotropy of thesematerials at elevated temperatures. A peculiarity in the concentration dependence ofthe elastic constants in Fe-rich alloys is demonstrated and related to a change in theFermi surface topology.A thermodynamic model for the magnetic alloys is developed from first principles andapplied to the calculation of bcc Fe-Cr phase diagram. Various contributions to the freeenergy (magnetic, electronic, and phonon) are estimated and included in the model. Inparticular, it is found that magnetic short range order effects are important just abovethe Curie temperature. The model is applied for calculating phase equilibria in disorderedbcc Fe-Cr alloys. Model calculations reproduce a feature known as a Nishizawahorn for the Fe-rich high-temperature part of the phase diagram.The investigation of the TiC-ZrC system includes a detailed study of the defect formationenergies and migration barriers of point defects and defect complexes involvedin the diffusion process. It is found, using ab initio atomistic simulations of vacancymediateddiffusion processes in TiC and ZrC, that a special self-diffusion mechanism isoperative for metal atom diffusion in sub-stoichiometric carbides. It involves a noveltype of a stable point defect, a metal vacancy ”dressed” in a shell of carbon vacancies.It is shown that this vacancy cluster is strongly bound and can propagate through thelattice without dissociating. / <p>QC 20120604</p> / HERO-M

The aim of the project was to establish rate and mechanisms of solid state reduction of manganese ores. The project studied carbothermal reduction of manganese oxide MnO, two Groote Eylandt (Australian) and Wessels (South African) manganese ores in hydrogen, helium and argon atmospheres at temperatures up to 1400C for MnO and 1200C for manganese ores. Experiments were conducted in the fixed bed reactor with on-line off-gas analysis. The major findings are as follows. ?? Rate and degree of reduction of MnO and ores increased with increasing temperature. ?? Reduction of MnO and manganese ores at temperatures up to 1200C was faster in helium than in argon, and much faster in hydrogen than in helium. The difference in MnO reduction in hydrogen and helium decreased with increasing temperature to 1400C. ?? Addition of up to 7 vol% of carbon monoxide to hydrogen had no effect on MnO reduction at 1200C. ?? In the process of carbothermal reduction of ores in hydrogen at 1200C, silica was reduced. ?? Reduction of both GE ores was slower than of Wessels ore. This was attributed to high content of iron oxide in the Wessels ore. ?? Carbon content in the graphite-ore mixture had a strong effect on phases formed in the process of reduction; thus, in the reduction of Wessels ore with 12-16 wt% C, a-Mn and Mn23C6 were formed; when carbon content was above 20 wt%, oxides were reduced to carbide (Mn,Fe)7C3. ?? Kinetic analysis showed that mass transfer of intermediate CO2 from oxide to graphite in carbothermal reduction in inert atmosphere was a contributing factor in the rate control. ?? High rate of reduction of manganese oxide in hydrogen was attributed to formation of methane which facilitated mass transfer of carbon from graphite to oxide. Hydrogen was also directly involved in reduction of manganese ore reducing iron oxides to metallic iron and higher manganese oxides to MnO. Reduction of Wessels and Groote Eyland Premium Fines ores in the solid state is feasible at temperatures up to 1200C; while temperature for solid state reduction of Groote Eyland Premium Sands is limited by 1100C.

Orientador: Juno Gallego / Resumo: O desgaste abrasivo é uma das causas de falhas em equipamentos e responsável por prejuízos nos processos industriais. Uma técnica capaz de minimizar os efeitos deste fenômeno é a aplicação de revestimentos duros nas superfícies críticas. Recentemente foram desenvolvidos revestimentos contendo carbonetos de titânio formados pela fusão de misturas contendo cavacos das ligas ASTM F67 e ASTM F136 sobre peças de aço-carbono ASTM A-36. Após a aplicação do processo de soldagem GTAW obteve-se significativa fração volumétrica de TiC grosseiro com elevada dureza. No presente trabalho foi investigada a refusão dessas soldas pelo processo de soldagem a Laser Nd:YAG pulsado. A microestrutura das amostras foi analisada por difração de raios-X (DRX), microscopia eletrônica de varredura (MEV) e microdureza Vickers. A caracterização microestrutural mostrou que houve refinamento dos carbonetos de titânio (TiC) na matriz ferrítica refundida com o Laser, cujo principal efeito foi um endurecimento do metal de solda. A caracterização mecânica demonstrou um aumento na microdureza da superfície do material. Este comportamento favorece o uso da refusão a Laser para a melhoria da qualidade das superfícies que demandam maior resistência ao desgaste abrasivo. / Mestre

One of the most important additives in High Speed Steels (HSS) is cobalt, mainly for its effect on the hot properties. Based on statistic data about the increased price of cobalt and its negative effect on human health, an ethical and financial barrier in the steel industry have occurred. In order to solve the problem, it is of great importance to examine the future cobalt price and accessibility, as well as examine the possibility of finding alternative substitutes to cobalt. The purpose of this project was therefore to examine alternatives to cobalt as an alloying element in HSS. A qualitative literature study was performed by analyzing the economy of cobalt, studying the main reasons for cobalts tendency to improve the hot properties of the steel and finding alternative elements to replace, or at least reduce, cobalt in HSS without degrading the hot properties. Cobalt is used both in the chemical and metallurgical business. But the demand of cobalt is largely driven by chemical purposes with the focus on its rechargeable battery applications. The analysis shows that there is nothing pointing at a significant decrease of the price of cobalt. Lithium ion batteries stands for about 50% of current cobalt supply, which is why the price has surged the recent years. The market for electric vehicles and rechargeable batteries has skyrocketed. To decrease the price of cobalt, a substitute for cobalt in rechargeable batteries would need to be found, which is not very likely for the time being. The effect of cobalt in HSS is mainly on the red hardness and tempering resistance. Cobalt increases the bonding strength in the steel matrix and changes the microstructure of the finer secondary carbides. Also the growth rate and coalescence rate of the carbides decreases. This causes the red hardness and the tempering resistance to increase. To replace cobalt, several alternative alloying elements have been researched. Among the most promising are niobium, nitrogen and aluminium, where niobium were found to be of most interest, due to the broad support of relevant articles in the field of powder metallurgical processing. The positive effect of niobium could be regarded as three-fold. The first contribution is the refinement of grain size and homogeneity of the primary carbides, which increases the overall hardness. The second effect is that the addition of niobium shifts the phase equilibria in such a way that the precipitation of primary carbides mainly will be in the form of hard and stable NbC. The majority of the other alloying elements will hence be precipitated as secondary carbides during tempering. The final effect is an increase in secondary hardness, as a consequence of the large amounts of vanadium and smaller amounts of niobium that is being precipitated during tempering to the secondary carbides. This enables a high matrix hardening potential in the optimal state of tempering.

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