Karakterizacija in optimizacija lastnosti aluminijeve zlitine utrjene s kvazikristali

Štrekelj, Neva (author), Markoli, Boštjan (mentor), Zupanič, Franc (co-mentor)

V doktorski disertaciji je potekal razvoj treh skupin kvazikristalnih aluminijevih zlitin. Sintezi posamezne zlitine je sledilo gravitacijsko ulivanje v bakreno kokilo z obliko nateznega preizkušanca s premerom 5,5 mm. Načrtovanje zlitin je potekalo na podlagi znanih vplivov elementov berilija in silicija na povečano nagnjenost k tvorbi ikozaedrične kvazikristalne faze (iQC-faze) Mackayevega tipa v sistemu Al-Mn(-Cu) pri razmerah strjevanja v bakreni kokili. Z različnimi metodami so bile ugotavljane mikrostrukturne značilnosti zlitin glede na njihovo povprečno kemijsko sestavo. Ugotavljane so bile mehanske lastnosti, pridobljene predvsem z izvedbo tlačnih preizkusov pri sobni temperaturi in temperaturah 300 ter 400 °C. Z uvedbo različnih toplotnih obdelav je bila nadalje preiskovana možnost izboljšanja mehanskih lastnosti vseh treh skupin zlitin. V nobenem primeru zlitinskega sistema se trdnostne lastnosti niso povišale s toplotno obdelavo T6, medtem ko direktno staranje na 160 ali 180 °C ni imelo vpliva na spremembo mikrostrukture in trdoto vzorcev. Potrjen je bil stabilizacijski učinek silicija na iQC-fazo v toplotnoobdelanih zlitinah, tako v sistemu zlitin z berilijem kot v sistemu zlitin brez berilija. Pozitiven vpliv so imeli že nizki dodatki silicija, 0,7–1,2 at. %, kljub eventualno manjšim deležem mangana v nekaterih zlitinah (pod 2 at. % Mn). Primarna iQC-faza je pričela med toplotno obdelavo (žarjenje na 510 °C) transformacijo na svojih robovih, a v notranjosti je bila njena struktura še vedno kvazikristalna. Prav tako so nizki dodatki silicija povečali tendenco zlitin v litem stanju k izoblikovanju primarne iQC-faze brez tvorbe morebitnih preostalih primarnih faz. V primeru zlitin brez silicija, se med toplotno obdelavo T6 primarna iQC-faza ni ohranila in je kazala na postopno transformacijo preko dekagonalne kvazikristalne faze v kristalno fazo. Po drugi strani, so dodatki berilija v zlitinah onemogočili tvorbo izločkov faze θ'-Al2Cu med toplotno obdelavo, ki bi prispevali k nadaljnjemu utrjevanju zlitin. Izločki, ki bi potencialno izboljšali trdnostne lastnosti, so nastali le v vzorcih zlitinskega sistema Al-Mn-Cu-Si, kjer je bila ugotovljena največja tlačna napetost tečenja pri primerjavi vzorcev v stanju T6 različnih zlitinskih sistemov. Upoštevaje mikrostrukturo in mehanske lastnosti, je bilo ugotovljeno, da ima prisotnost iQC-faze vpliv na povišanje napetosti tečenja in hkrati na zmanjšanje sposobnosti preoblikovanja zlitin pri temperaturah 25, 300 in 400 °C. S stališča trdnostnih lastnosti so zlitine v litem stanju boljše kot v stanju T6, med katerimi je imel največjo napetost tečenja vzorec zlitine Al-Mn-Cu-Si (247 MPa), in največjo trdoto (HV 1 = 120) vzorec iz sistema Al-Mn-Be-Cu-Si.

• Language: Slovenian
• Keywords: Kvazikristali, Aluminijeve zlitine, Mikrostruktura, Mehanske lastnosti, Metoda EBSD
• Work type: Doctoral dissertation
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2017
• PID: 20.500.12556/RUL-92556
• COBISS.SI-ID: 1683295
• Publication date in RUL: 13.06.2017

Secondary language

• Language: English
• Title: Characterization and optimization of properties of aluminium based alloy reinforced with quasicrystals

Abstract

In this doctoral thesis three groups of quasicrystalline aluminium alloys were developed and synthesized. Synthesis of each alloy was followed by gravitational casting in a copper mould with a shape of a tensile-test specimen with diameter of 5,5 mm. Development of the alloys involved known effects of beryllium and silicon on the increased forming ability of Mackay-type icosahedral quasicrystalline phase (iQC-phase) in the Al-Mn(-Cu) system during casting into a copper mould. Alloys’ microstructural characterization was performed via several observing techniques with respect to their average chemical composition. Mechanical properties of the individual alloy were obtained mainly through compression testing at room temperature, 300 and 400 °C. By additional various heat treatments, further potential for improving the mechanical properties of alloys from all three groups was studied. Heat treatment T6 did not increase strength properties in any of the alloys, while the direct artificial aging (T5) at 160 °C or 180 °C did not affect the microstructure and consequently the hardness of the Al-Mn-Be-Cu-alloys. In heat-treated state of the Al-Mn-Be-Cu-Si and Al-Mn-Cu-Si-alloys, the stabilizing effect of silicon on the iQC-alloys was confirmed. Smaller additions of the silicon (0,7–1,2 at. %) had already positive effect, despite possibly lower manganese content in some alloys (less than 2 at. % Mn). During the heat treatment (solutioning at 510 °C) edges of the primary iQC-phase begins to transform, but it’s inner structure remained quasicrystalline. Low additions of the silicon also increased the tendency to the formation of iQC-phase in as-cast state, without the formation of any other primary phases. During the heat treatment of the Al-Mn-Be-Cu alloys, primary iQC-phase gradually transformed to the decagonal quasicrystalline phase and finally into the crystalline phase. On the other hand, the additions of beryllium into the alloys prevented the formation of the hardening precipitates (θ’-Al2Cu) during the heat treatment. Precipitates with a potential hardening effect formed only in the Al-Mn-Cu-Si-alloys, which showed maximum compressive yield strength when comparing all three groups of investigated alloys in T6-state. Considering the microstructure and mechanical properties, it has been found that the presence of iQC-phase increased compressive yield strength, but at the same time reduced the formability at room temperature of all the alloys and at 300 and 400 °C in the case of Al-Mn-Be-Cu and Al-Mn-Be-Cu-Si-alloys. When the strength properties are considered, the alloys in as-cast state are better than in T6-state, the highest compressive yield strength had Al-Mn-Cu-Si-alloy’s sample (247 MPa), whereas the maximum hardness (HV 1 = 120) was achieved in the sample of Al-Mn-Cu-Si-alloy.

• Keywords: Quasicrystals, Aluminium alloys, Microstructure, Mechanical properties, EBSD technique