Modeliranje faznih ravnotežij in kinetike homogenizacije zlitin Al-Fe in Al-Fe-Si

Arbeiter, Jože (author), Medved, Jože (mentor), Kugler, Goran (co-mentor)

Doktorska disertacija obravnava proces homogenizacije izbranih aluminijevih zlitin iz sistemov Al-Fe in Al Fe Si. Raziskani so bili vplivi homogenizacijskega žarjenja na transformacijo metastabilnih faz, ki nastanejo ob neravnotežnem strjevanju. Preverjali smo, kako na transformacijo vpliva temperatura, čas in koncentracija silicija ter kako se spremembe teh parametrov odražajo na lastnostih obravnavanih zlitin. V okviru disertacije smo raziskovali štiri različne zlitine, od tega tri laboratorijsko izdelane (AlFe1, AlFe1Si0,1 in AlFe1Si0,5), s kontroliranimi parametri litja ter eno industrijsko zlitino (AlFe1Si0,1). Za analizo zlitin smo uporabili termodinamične simulacije s programskim orodjem Thermo-Calc, diferenčno vrstično kalorimetrijo, meritve električne upornosti ter svetlobno in vrstično elektronsko mikroskopijo. Rezultati kažejo, da ima dodatek silicija k zlitini AlFe1 vpliv na porazdelitev, delež in morfologijo metastabilnih faz, ki nastanejo med strjevanjem zlitine. Z dodatkom 0,1 mas. % Si se zmanjša vsebnost faze Al6Fe v primerjavi z zlitino AlFe1, dodatek 0,5 mas. % Si pa povzroči nastanek faze AlmFe, kjer je m ? 4,0–4,4. Definirali smo zaporedje poteka transformacije metastabilne faze Al6Fe, ki se začne z raztapljanjem elementov metastabilne in nukleacijo stabilne faze na območju med matrico ?Al in evtektičnim področjem. Zaradi daljšanja difuzijskih poti se transformacija po 4 h nadaljuje znotraj evtektičnega področja. Z dodatkom 0,1 mas. % Si skrajšamo čas, ki je potreben za transformacijo, z 8 h na 4 h pri 600 °C. Izdelan je bil model, ki na osnovi klasične teorije nukleacije in predpostavke o termodinamičnem ravnotežju na meji faza–matrica ter na osnovi Stefanove predpostavke o kvazistacionarnem spreminjanju koncentracijskega profila, omogoča simulacije kinetike transformacije faze Al6Fe v Al13Fe4. V modelu je bilo predpostavljeno, da na hitrost transformacije odločilno vpliva hitrost difuzije železa. Narejena je bila analiza vpliva temperature, površinske napetosti in načina nukleacije na hitrost transformacije faze Al6Fe v fazo Al13Fe4, ki je pokazala, da se z variacijo teh parametrov ni mogoče približati eksperimentalnim rezultatom. Z uporabo efektivnega difuzijskega koeficienta, ki vključuje poleg difuzije železa tudi samodifuzijo aluminija, se rezultati simulacij kinetike transformacije zelo dobro ujemajo z eksperimentalnimi rezultati.

• Language: Slovenian
• Keywords: zlitine Al-Fe, homogenizacija, fazne transformacije, modeliranje faznih transformacij, diferenčna vrstična kalorimetrija
• Work type: Doctoral dissertation
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2021
• PID: 20.500.12556/RUL-126837
• COBISS.SI-ID: 62520835
• Publication date in RUL: 06.05.2021

Secondary language

• Language: English
• Title: Modelling of phase equilibrium and homogenization kinetics of Al-Fe and Al-Fe-Si alloys

Abstract

The main focus of the doctoral thesis is the analysis of the homogenization process of selected Al-Fe and Al Fe Si aluminium alloys. During the non-equilibrium solidification of alloys, metastable phases are formed. The effects of the homogenization annealing on metastable phase transformation were observed. The impact of temperature, time and additions of silicon on phase transformation were defined. Three of the analysed alloys were produced in a controlled laboratory environment, while the fourth was industrially produced. These alloys were AlFe1, AlFe1Si0.1 and AlFe1Si0.5, and the industrial alloy was AlFe1Si0.1. Thermodynamic calculations with the program Thermo-Calc, differential scanning calorimetry, electrical resistivity measurements and optical and scanning electron microscopy were used to analyze the alloys. Results show that the addition of silicon to AlFe1 alloy had a significant impact on the distribution, amount and morphology of phases formed during the solidification process. By adding 0.1 wt.% of silicon to the AlFe1 alloy, the amount of the metastable Al6Fe phase was reduced. Adding 0.5 wt.% of silicon resulted in the formation of the AlmFe phase instead of the Al6Fe phase. The sequence of the transformation was defined, starting with the dissolution of the metastable phase and the nucleation of the stable phase Al13Fe4 on the boundary between the αAl matrix and the eutectic area. Due to increased diffusion lengths of iron atoms, the transformation within the eutectic area occured after 4 h of homogenization at 600 °C. With the addition of 0.1 wt.% of silicon the homogenization time required for the transformation of the Al6Fe metastable phase was shortened from 8 to 4 h at 600 °C. A model based on classical nucleation theory and on the assumption of thermodynamic equilibrium at the matrix-phase boundary was proposed. The concentration gradients at the nonstationary boundary were derived considering the Stefan pseudo-stationary assumption. The model was used to simulate the transformation of Al6Fe to Al13Fe4. The influence of temperature, diffusion coefficient, surface tension and the size of the energy barrier for heterogeneous nucleation on transformation rates was determined. An effective diffusion coefficient was used, which combines the diffusion coefficient of iron and the self-diffusion coefficient of aluminium and the results were in agreement with experimental data.

• Keywords: Homogenization, aluminium alloys, phase transformation, modelling, differential scanning calorimetry