Transmission electron microscopy studies of dispersoids and constituent phases in Al-Mn-Fe-Si alloys

Abstract

3xxx aluminium alloys are used among others in packaging, architectural applications and heat exchangers. Both dispersoids and constituent phases are very important in this alloy system. The size and distribution of the dispersoids have a strong influence on the deformation, the recovery and recrystallisation behaviour and final mechanical properties of the 3xxx Al alloys. Also the constituent phases influence recrystallisation, grain size, texture and mechanical properties. In this thesis dispersoids and constituent phases in 3xxx Al alloys were studied, using transmission electron microscope (TEM) techniques as the main tools for the investigations. The alloys studied here were direct chill-cast 3xxx aluminium alloys with varying material composition. They were studied after various low temperature homogenisations.

Constituent phases extracted from the Al matrix were studied with respect to type, lattice parameters, chemical composition and morphology in alloys with various composition. Al6(Fe,Mn) was found to be the most prominent constituent phase in the alloy with a low Si content. Orientation relationships (ORs) of constituent particles with relation to Al matrix were investigated for Al6(Fe,Mn) and α-Al(Fe,Mn)Si constituents. The OR between the Al matrix and the Al6(Fe,Mn) constituent was determined to be [-1-1-2]c // [-11-1]Al , (33-3) c // (0-2-2)Al . This OR is consistent with the OR of Al6(Fe,Mn) dispersoids. α-Al(Fe,Mn)Si constituent particles in the Si rich alloy were found to have various possible orientations.

The main focus of this thesis is the study of α-Al(Fe,Mn)Si dispersoids. How the precipitation of α-Al(Fe,Mn)Si dispersoids influences the hardness and tensile strength in alloys were systematically investigated. The evolution of density, size and volume fraction of the α-Al(Fe,Mn)Si dispersoids when varying the annealing time, the temperature and the alloy composition was quantitatively studied by conventional TEM and electrical measurements. A hardening effect from the dispersoids was revealed. The hardening effect increases with increasing Mn and Si contents in the alloys. A high number density of relatively small dispersoids are beneficial for the hardness. For some applications of the 3xxx alloys this hardening during low temperature annealing is of significant importance for the further process.

The α-Al(Fe,Mn)Si dispersoid phase is a cubic icosahedral quasi crystal approximation phase. Investigations of quasi crystal approximant phases are important for better understanding of quasi crystals. Diffraction studies verified that the most commonly observed OR for the α-Al(Fe,Mn)Si dispersoids is [1-11] α // [1-11]Al , (5-2-7)α // (011)Al . This orientation may be explained by the assumption that the Mackay icosahedron internal in the α-phase has a fixed orientation in relation to Al, similar to that of the icosahedral quasi crystals existing in this alloy system. High angle annular dark field scanning transmission electron microscopy (HAADF STEM) tomography and diffraction studies were combined to study the α-Al(Fe,Mn)Si dispersoids in the 3xxx Al alloys. Most dispersoids were found to have a plate shaped morphology after low temperature homogenisation at 450 °C for 24 hours. The largest proportion of the dispersoids follows the commonly observed OR with the Al matrix. A methodology was established, which connects information about the morphology, the OR and the habit plane of crystalline particles, not restricted only to the dispersoid phase studied in this thesis.

Aluminiumslegeringer

Produksjon av aluminium er en energikrevende prosess. Norges store vannkraftressurser gjør produksjon av aluminium i Norge både bærekraftig og lønnsom. Ved å tilsette legeringselementer kan materialegenskapene til aluminium forandres. Dette gir aluminiumslegeringer en rekke bruksområder. Gode egenskaper er høy styrke, lav vekt, god termisk og elektrisk ledningsvevene, høy formbarhet og lav vekt.  Lav vekt er fordelaktig i et miljøperspektiv for eksempel i transportsektoren, da drivstofforbruket kan bli redusert.

Aluminium er resirkulerbart, og bare 5% av energien som brukes i primær aluminiumsproduksjon trengs for å resirkulere aluminium. Den kjemiske sammensetningen av legeringene endres når materialet resirkuleres, og dette endrer materialegenskapene og mikrostrukturen til materialet. Man må derfor forsøke å forstå disse endringene og forutse hvordan material egenskapene vil endres ved resirkulering.

3xxx aluminium legeringer brukes blant annet til varmevekslere, bygningsprodukter og matinnpakning. I disse legeringene tilsettes jern, mangan og silisium. Konstituente faser blir dannet under størkningen av aluminium, og dispersoidene felles ut under homogenisering. Disse fasene er veldig viktige fordi de påvirker og bestemmer hvordan materialegenskapene endrer seg i legeringene. I denne avhandlingen har dispersoide faser og konstituente faser blitt studert med Transmisjonselektronmikroskop teknikker som hovedverktøy.

Hovedfokuset for avhandlingen har vært å studere α-Al(Fe,Mn)Si dispersoider. Hvordan disse dispersoidene påvirker hardhet og spenninger i materialet ble systematisk studert. Det ble funnet en hardhetseffekt i legeringene med et høyt Si og Mn innhold, som er viktig for noen anvendelser. En metodologi for å koble tomografi og diffraksjonstudier av α-Al(Fe,Mn)Si dispersoider ble også utviklet for å kunne gi et komplett bilde av dispersoidenes morfologi, fasetter og orientering.

α-Al(Fe,Mn)Si dispersoide fasen er en kubisk ikosaeder kvasi krystallinsk approksimasjon fase. Å undersøke slike kvasikrystallinske approksimasjons faser er viktig for å forså kvasikrystaller. Diffraksjonstudier bekreftet og verifiserte orienteringen av α-Al(Fe,Mn)Si dispersoidene i aluminium. En forklaringsmodell som knyttet kvasikrystallenes orientering og dispersoideorienteringen ble laget.