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This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides. In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials. Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb. A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions. Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.

No presente trabalho foi avaliado o efeito da microestrutura e da capacidade de suportar carregamento de camadas nitretadas produzidas em aços ferramenta AISI H13 e AISI D2 sobre a aderência e a resistência ao desgaste microabrasivo de revestimentos de TiN-PVD. Em cada um desses aços, foram produzidas camadas nitretadas de diferentes estruturas e espessuras, e foram determinadas experimentalmente as curvas potencial início de formação de camada branca, para a nitretação a 520oC. Para o aço ferramenta AISI H13, o emprego de tempos de pré-tratamento de nitretação mais prolongados ( aproximadamente 11 h) foi necessário para aprofundar a camada nitretada e, conseqüentemente, aumentar a capacidade de suportar carregamento dos revestimentos, evitando a formação de bordas que provocam o lascamento e a escamação das camadas de TiN. Observou-se que esse tipo de falha persiste se a zona de endurecimento for pouco profunda, uma vez que a transição de propriedades mecânicas da camada de TiN para o núcleo não nitretado continua abrupta e a capacidade de suportar carregamento da camada nitretada ainda é baixa. Por outro lado, curtos tempos de nitretação (aproximadamente 42 min.) foram suficientes para aumentar a aderência das camadas de TiN ao aço ferramenta D2, pois o núcleo não nitretado desse aço possui uma capacidade de suportar carregamento razoável. Observou-se que a resistência ao desgaste microabrasivo e a aderência dos revestimentos são prejudicadas com a presença de uma camada preta na interface camada de TiN/camada nitretada. Quando a superfície dos revestimentos é carregada, falhas do tipo “casca de ovo" facilmente ocorrem. / In this work, the influence of both the microstructure and the load-bearing capacity of nitrided layers, formed on top of AISI D2 and AISI H13 tool steels, on adhesion and wear resistance of PVD-TiN coatings was studied. The threshold nitriding potential curves for the above mentioned steels and the optimum conditions of the pre-treatments which increased the adhesion as well as the wear resistance of the PVD-TiN were determined experimentally. By using longer nitriding times (about 11 h) and lower nitrogen contents in the gas mixture (about N2-5%vol.), it was possible to minimize the pile-up degree of the TiN/H13 nitrided substrates and, consequently, the occurrence of coatings chipping. This flaw persists when the nitrided layer is thin, due to an abrupt transition of mechanical properties at the TiN coating / steel core interface. Shorter nitriding times (about 42 min.) and lower nitrogen contents (about N2-5%vol.), on the other hand, are sufficient to guarantee a better adhesion of TiN coatings on AISI D2 tool steel, as the core of such steel possesses relatively better load-bearing capacity than the AISI H13 tool steel. The presence of a black layer at the TiN/nitrided layer interface was observed in all coatings deposited over nitrided layers produced above the threshold nitriding potential curves. This layer affects adversely the wear resistance and the adhesion of the TiN coatings. When higher loads are applied on the coated surface, “egg shell" type flaws easily occur.

No presente trabalho foi avaliado o efeito da microestrutura e da capacidade de suportar carregamento de camadas nitretadas produzidas em aços ferramenta AISI H13 e AISI D2 sobre a aderência e a resistência ao desgaste microabrasivo de revestimentos de TiN-PVD. Em cada um desses aços, foram produzidas camadas nitretadas de diferentes estruturas e espessuras, e foram determinadas experimentalmente as curvas potencial início de formação de camada branca, para a nitretação a 520oC. Para o aço ferramenta AISI H13, o emprego de tempos de pré-tratamento de nitretação mais prolongados ( aproximadamente 11 h) foi necessário para aprofundar a camada nitretada e, conseqüentemente, aumentar a capacidade de suportar carregamento dos revestimentos, evitando a formação de bordas que provocam o lascamento e a escamação das camadas de TiN. Observou-se que esse tipo de falha persiste se a zona de endurecimento for pouco profunda, uma vez que a transição de propriedades mecânicas da camada de TiN para o núcleo não nitretado continua abrupta e a capacidade de suportar carregamento da camada nitretada ainda é baixa. Por outro lado, curtos tempos de nitretação (aproximadamente 42 min.) foram suficientes para aumentar a aderência das camadas de TiN ao aço ferramenta D2, pois o núcleo não nitretado desse aço possui uma capacidade de suportar carregamento razoável. Observou-se que a resistência ao desgaste microabrasivo e a aderência dos revestimentos são prejudicadas com a presença de uma camada preta na interface camada de TiN/camada nitretada. Quando a superfície dos revestimentos é carregada, falhas do tipo “casca de ovo” facilmente ocorrem. / In this work, the influence of both the microstructure and the load-bearing capacity of nitrided layers, formed on top of AISI D2 and AISI H13 tool steels, on adhesion and wear resistance of PVD-TiN coatings was studied. The threshold nitriding potential curves for the above mentioned steels and the optimum conditions of the pre-treatments which increased the adhesion as well as the wear resistance of the PVD-TiN were determined experimentally. By using longer nitriding times (about 11 h) and lower nitrogen contents in the gas mixture (about N2-5%vol.), it was possible to minimize the pile-up degree of the TiN/H13 nitrided substrates and, consequently, the occurrence of coatings chipping. This flaw persists when the nitrided layer is thin, due to an abrupt transition of mechanical properties at the TiN coating / steel core interface. Shorter nitriding times (about 42 min.) and lower nitrogen contents (about N2-5%vol.), on the other hand, are sufficient to guarantee a better adhesion of TiN coatings on AISI D2 tool steel, as the core of such steel possesses relatively better load-bearing capacity than the AISI H13 tool steel. The presence of a black layer at the TiN/nitrided layer interface was observed in all coatings deposited over nitrided layers produced above the threshold nitriding potential curves. This layer affects adversely the wear resistance and the adhesion of the TiN coatings. When higher loads are applied on the coated surface, “egg shell” type flaws easily occur.

<p>Proučavane su koroziona postojanost i tendencija lepljenja različitih<br />materijala u kontaktu sa tečnom Al&ndash;Si&ndash;Cu legurom. Ispitivanjem su<br />obuhvaćeni čelik za rad na toplo, plazma nitrirani čelik i dupleks<br />slojevi sa CrN, TiAlN, TiAlSiN i CrAlN prevlakama, različitog nivoa<br />površinske hrapavosti. Za ispitivanja pomenutih fenomena<br />primenjena je metoda izvlačenja, koja je unapređena kako bi se povećale<br />njena tačnost i verodostojnost simulacije procesa livenja. Korozioni<br />efekti su pojačani tako što su uzorci osim kratkog kontakta sa odlivkom<br />zadržavani i u dužim periodima u kontaktu sa tečnom legurom (5 i 20<br />min). Uprkos opštim stavovima, za ispitivane materijale je<br />ustanovljeno da su sile izvlačenja uzoraka iz Al&ndash;Si&ndash;Cu odlivaka<br />nezavisne od njihovog hemijskog sastava. Uticaj hrapavosti je izražen<br />kod uzoraka sa prevlakama kod kojih pri smanjenju hrapavosti dolazi do<br />povećanja sile izvlačenja. Sve ispitane prevlake su sklone mehaničkom<br />lepljenju Al&ndash;Si&ndash;Cu legure za svoje površine, ali sa aspekta korozije u<br />tečnom metalu značajno prevazilaze performanse čelika i plazma<br />nitriranog sloja. Duži kontakt livene legure sa površinama prevlaka<br />uzrokovao je niže vrednosti sila izvlačenja, što je posledica<br />oksidacije površina prevlaka. Ustanovljeno je da su ispitivane<br />prevlake inertne ka tečnoj leguri aluminijuma. Međutim, dolazi do<br />oksidacije i korozije materijala podloge kroz greške rasta koje su<br />prisutne u prevlakama. Stečena znanja o identifikovanim<br />mehanizmima habanja i propadanja zaštitnih slojeva prevlaka<br />poslužiće daljem razvoju dupleks slojeva namenjenih za zaštitu alata<br />za livenje pod pritiskom.</p> / <p>Corrosion resistance and soldering tendency of different materials in molten<br />Al&ndash;Si&ndash;Cu alloy were studied. Hot-working tool steel, plasma nitrided steel and<br />duplex layers with CrN, TiAlN, TiAlSiN and CrAlN top coatings, which were<br />produced to various degree of surface roughness, were covered by the study.<br />An ejection test was employed for investigation of the concerned phenomena.<br />The ejection test was improved in order to increase its accuracy and the<br />reliability of process simulation. Samples were examined in both short and<br />extended periods of contact (5 and 20 min) with liquid casting. Casting<br />solidification was extended in order to intensify the corrosion effects. Contrary<br />to common findings, it was found that the ejection force of the investigated<br />materials does not depend on their chemical composition. For the coated<br />samples, a pronounced dependence of the ejection force on the surface<br />roughness was found. The ejection force increases with decrease in surface<br />roughness. All investigated coatings are prone to mechanical soldering by Al&ndash;<br />Si&ndash;Cu alloy. Still, their corrosion resistance substantially exceeds the corrosion<br />resistance of steel and plasma nitrided layer. Longer exposure of coated<br />samples to cast alloy induced lower ejection forces, which is a consequence<br />of coatings oxidation. It was found that the investigated coatings are inert to<br />liquid aluminium. However, the underlying material undergoes oxidation and<br />corrosion through coating growth defects. The findings concerning the wear<br />mechanisms of protective layers support further development of duplex layers<br />intended for die casting tools protection.</p>