Metal Matrix Composites Prepared by Powder Metallurgy Route
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Vysoké učení technické v Brně. Fakulta strojního inženýrství
Abstract
Ve všeobecnosti, poznatky o design slitin, jejich výrobě a výběru legujúcich prvků sú omezené na slitiny s jedním základním prvkem. Tento fakt ale výrazně limituje možnosti a volnost výběru prvků pro dosáhnuti speciálních vlastností a mikrostruktur. V poslední dekádě se ukázalo, že materiálová věda a inženýrství nejsou ještě zdaleka prozkoumané v důsledku objevu nové třídy materiálů nazvané vysoko entropické slitiny (HEA high entropy alloys). Jejich objev upoutal pozornost vědecké komunity. Základní koncept pro jejich design je, že namísto jednoho, nebo dvou základních prvků obsahují minimálně 5 prvků v podobných atomových koncentracích. V posledních letech se objevila skupina materiálů odvozená od HEA, nazvaná slitiny so střednou entropii (MEA medium entropy alloys). Na rozdíl od HEA ale obsahují 3, nebo 4 prvky. Táto práce je věnovaná studiu přípravy a charakterizaci HEA, MEA a jejich kompozitů s pomocí metod práškové metalurgie. V této práci byli dohromady zkoumány tři kompozice: AlCoCrFeNiTi0.5, Co1.5Ni1.5CrFeTi0.5 a CoCrNi, kompozity s kovovou matricí (MMC metal matrix composites) vyztužené částicemi B4C s CoCrNi jako matricí. Hloubková mikrostrukturní a mechanická analýza těchto materiálů byla provedena pomoví metod rastrovací a transmisní elektronové mikroskopie spojené s tahovými a ohybovými zkouškami. V průběhu celé studie se objevovaly problémy s kontaminací kyslíkem, co se projevilo vznikem značného množství oxidů v připravených materiálech. U Slitiny AlCoCrFeNiTi0.5 byla naměřena tvrdost přesahující 800 HV. Její houževnatost ale byla velice omezena. V její mikrostruktuře byly identifikovány částice in-situ TiC v důsledku přítomnosti organického, anti-aglomeračního činidla (metanolu) v mlecí misce. Tato reakce může být použita v budoucnu k přípravě MMC se záměrnou disperzí TiC. Na druhé straně, slitina CoCrNi ukázala vysoké hodnoty tažnosti (26%) a meze pevnosti přes 1000 MPa. Mikrostruktura obsahovala majoritní FCC fázi s BCC precipitáty. Tahle slitina byla z důvodu vysoké tažnosti zvolena pro přípravu kompozitu s výztuží B4C. V průběhu slinování ale došlo k reakci mezi přítomným Cr a B4C, které výsledkem byl Cr5B3 borid. Tento kompozit mel pevnost v tahu 1400 MP a extrémne jemnozrnnou strukturu. Celková tažnost ale klesla na 1.9 %. Slitina AlCoCrFeNiTi0.5, která mela strukturu složenou jen z FCC tuhého roztoku dosáhla nejlepší kombinaci mechanických vlastností s pevností přesahující 1300 MPa a dostatečnou tažností 4%. Prášková metalurgie se ukázala jako vhodná metoda pro přípravu HEA a MEA slitin a jejich kompozitů, s dobrou kombinací pevnosti a tažnosti. Tato metoda dovoluje měnit mikrostrukturní parametry připravených materiálů jednoduchou úpravou parametrů procesu.
Conventionally, the alloy design, alloy production, and alloy selection are almost strictly confined to single element or one compound concept. Consequently, this alloy concept imposes a significant limit to the degrees of freedom in alloy’s composition and thus limits the development of special microstructure and properties. In the last decade, it has become particularly obvious that materials science and alloy engineering are still not fully explored due to an appearance of new class of alloys – usually called high entropy alloys (HEA). This exclusively new class of alloys caught significant scientific attention for the novelty of its approach to alloy design, as they do not contain a single base element, but rather at least 5 elements in very close atomic portions. In the recent years medium entropy alloys (MEA) appeared as a variant of HEAs with only three or four elements. The work is contributed to the research of feasibility of production of HEA and MEA alloys and composites by utilization of powder metallurgy (PM) manufacturing route, the combination of mechanical milling (MA) of elementary powders, followed by pressure or field assisted densification. Altogether three compositions have been studied: AlCoCrFeNiTi0.5, Co1.5Ni1.5CrFeTi0.5 and CoCrNi, as well as B4C metal matrix composite (MMC) with CoCrNi as matrix phase. Deep microstructural and mechanical analyses including transmission electron microscopy and tensile testing have been performed. During the whole study, the problems with the contamination of powders with oxygen have been observed, however the oxides formed relatively homogenous dispersion in all manufactured materials and they did not impair significant mechanical property reduction. AlCoCrFeNiTi0.5 exhibited relatively high hardness over 800 HV, but rather low ductility. The attempt has been made to improve the ductility with heat treatment procedure, but to no avail. The formation of in-situ TiC dispersion has been recorded, due to the utilization of carbon containing methanol as a process control agent during milling, that reacted with the present elemental Ti. In this manner metal matrix composite has been effectively produced. Additionally, the same procedure, the milling in the controlled amount of carbon containing medium, may be used also to produce other advanced composites with dispersion of in-situ formed TiC. On the other hand, CoCrNi alloy possessed very high tensile ductility (26%) and ultimate strength over 1000 MPa. Microstructure was composed of major FCC phase and BCC precipitates. The CoCrNi alloy has been due to the high ductility chosen as the best candidate for the subsequent production of metal matrix composites. The introduction of B4C resulted in the displacement reaction of Cr element with B4C, resulting in the formation of Cr5B3 boride phase. The composite possessed nano-grained microstructure and high tensile strength over 1400 MPa. However, the tensile ductility decreased to 1.9%. The AlCoCrFeNiTi0.5 alloy achieved the best combination of tensile ductility (4%) and remarkable strength over 1300 MPa, bearing pure FCC microstructure with extremely fine grain size. Therefore, the PM production route has proven to be a feasible way for the production of HEAs and MEAs, as well as HEA and MEA based metal matrix composites with remarkable combination of mechanical properties.
Conventionally, the alloy design, alloy production, and alloy selection are almost strictly confined to single element or one compound concept. Consequently, this alloy concept imposes a significant limit to the degrees of freedom in alloy’s composition and thus limits the development of special microstructure and properties. In the last decade, it has become particularly obvious that materials science and alloy engineering are still not fully explored due to an appearance of new class of alloys – usually called high entropy alloys (HEA). This exclusively new class of alloys caught significant scientific attention for the novelty of its approach to alloy design, as they do not contain a single base element, but rather at least 5 elements in very close atomic portions. In the recent years medium entropy alloys (MEA) appeared as a variant of HEAs with only three or four elements. The work is contributed to the research of feasibility of production of HEA and MEA alloys and composites by utilization of powder metallurgy (PM) manufacturing route, the combination of mechanical milling (MA) of elementary powders, followed by pressure or field assisted densification. Altogether three compositions have been studied: AlCoCrFeNiTi0.5, Co1.5Ni1.5CrFeTi0.5 and CoCrNi, as well as B4C metal matrix composite (MMC) with CoCrNi as matrix phase. Deep microstructural and mechanical analyses including transmission electron microscopy and tensile testing have been performed. During the whole study, the problems with the contamination of powders with oxygen have been observed, however the oxides formed relatively homogenous dispersion in all manufactured materials and they did not impair significant mechanical property reduction. AlCoCrFeNiTi0.5 exhibited relatively high hardness over 800 HV, but rather low ductility. The attempt has been made to improve the ductility with heat treatment procedure, but to no avail. The formation of in-situ TiC dispersion has been recorded, due to the utilization of carbon containing methanol as a process control agent during milling, that reacted with the present elemental Ti. In this manner metal matrix composite has been effectively produced. Additionally, the same procedure, the milling in the controlled amount of carbon containing medium, may be used also to produce other advanced composites with dispersion of in-situ formed TiC. On the other hand, CoCrNi alloy possessed very high tensile ductility (26%) and ultimate strength over 1000 MPa. Microstructure was composed of major FCC phase and BCC precipitates. The CoCrNi alloy has been due to the high ductility chosen as the best candidate for the subsequent production of metal matrix composites. The introduction of B4C resulted in the displacement reaction of Cr element with B4C, resulting in the formation of Cr5B3 boride phase. The composite possessed nano-grained microstructure and high tensile strength over 1400 MPa. However, the tensile ductility decreased to 1.9%. The AlCoCrFeNiTi0.5 alloy achieved the best combination of tensile ductility (4%) and remarkable strength over 1300 MPa, bearing pure FCC microstructure with extremely fine grain size. Therefore, the PM production route has proven to be a feasible way for the production of HEAs and MEAs, as well as HEA and MEA based metal matrix composites with remarkable combination of mechanical properties.
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Citation
MORAVČÍK, I. Metal Matrix Composites Prepared by Powder Metallurgy Route [online]. Brno: Vysoké učení technické v Brně. Fakulta strojního inženýrství. 2017.
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Document version
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Language of document
en
Study field
Fyzikální a materiálové inženýrství
Comittee
prof. RNDr. Miroslav Liška, DrSc. (předseda)
Ing. Juraj Lapin, DrSc. (člen)
doc. Ing. Kateřina Skotnicová, Ph.D. (člen)
doc. Ing. Vít Jan, Ph.D. (člen)
doc. RNDr. Jiří Sopoušek, CSc. (člen)
Date of acceptance
2017-12-15
Defence
Oponenti označili výsledky DDP za vynikající. Rovněž v diskusi doktorand prokázal výborný přehled o řešené problematice. Výsledky práce byly publikovány v recenzovaných časopisech.
Teze jsou v souladu s předpisy.
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práce byla úspěšně obhájena
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