Investigation of the phases present in Al-7Si-0.3Mg alloy with the addition of transition metals

Moghadam Arya, Samira ORCID: 0000-0003-1084-3004 (2024). Investigation of the phases present in Al-7Si-0.3Mg alloy with the addition of transition metals. University of Birmingham. Ph.D.

Preview

MoghadamArya2024PhD.pdf
Text - Accepted Version
Available under License All rights reserved.
Download (53MB) | Preview

Abstract

This research aimed to investigate the effects of the addition of the transition metals molybdenum, tungsten, and vanadium to Al-7Si-0.3Mg (2L99) and the mechanism by which their addition can improve the tensile properties of 2L99. Although the addition of certain transition metals is thought to enhance the properties of Al alloys, research has been limited on this subject.
It is known that the presence of entrapped double oxide film defects (bifilms) in the aluminium alloy has a detrimental effect on its mechanical and corrosion resistance properties. As part of this study, various alloys of 2L99 + 0.4 wt.% Mo, W and V were made using the sand-casting method. It was observed that the addition of Mo and W improved the Ultimate Tensile Strength (UTS), %Elongation and yield strength of 2L99, whilst the addition of V showed a negligible increase in these parameters. Statistical analysis showed a significant increase in the Weibull moduli of the measured tensile properties of the alloys due to the addition of all three transition metals.
Subsequent to tensile testing, SEM and EDX analyses were carried out on various alloys to study the phases of oxides, nitrides and intermetallics on each alloy's polished and fractured surfaces. Characterisation of the fractured surfaces of 2L99 alloy in as-cast condition discovered for the first time AlN patches with a different "nodular” morphology of approximately
15 μm length, which were considered to be impermeable. This discovery reinforces Nyahumwa's hypothesis on the utilisation of internal gases, indicating that the presence of AlN is likely a result of increased consumption of entrapped air within the bifilms. This process begins with converting oxygen into spinel, followed by nitrogen transformation into AlN. Furthermore, impermeable nodular AlN morphologies observed in this study are considered to prevent hydrogen diffusion into the bifilms and prevent their expansion, ultimately leading to the reduction in bifilms’ size.
Sedimentation testing was carried out on 2L99 and 2L99 + Mo, W and V alloys by holding the melt at 800°C for 1 hour, followed by air cooling of the samples. The results showed that the addition of transition metals to 2L99 encouraged the settlement of a large portion of the intermetallics and bifilms to the base of the melt. This is thought to be due to the suitability of bifilms as a potent substrate for the nucleation of most intermetallics and the higher density of the intermetallics compared to liquid Al.
Nitrogen growth test was carried out on CP-AL, 2L99, 2L99 + 0.6 wt.% Mo and 0.2 wt.% W alloys. Alloy melts were held at 850°C in a nitrogen atmosphere for various holding times. The test showed the presence of different aluminium nitride morphologies such as rod, whisker, nodular, and feather-shaped.
Phase analysis also indicated that the reduction in the amount of bifilms resulted in the improvement of tensile properties of the 2L99 alloy, likely due to a combination of the following factors: (i) Substantial settling of double oxide films to the base of the melt due to nucleation of intermetallics on bifilms, (ii) The presence of more frequent AlN patches and morphologies suggesting an increase in the rate of consumption of the internal gases inside the bifilms and subsequent reduction of their size, and (iii) Alteration of the β- Al5FeSi morphology or its type to an α-compound intermetallic phase in 2L99, which is considered less detrimental to mechanical properties of the alloy due to its differing morphology and/or size.
The findings of this research contribute to understanding the mechanism by which the mechanical properties of 2L99 alloy are improved by the addition of Mo, W and V. Furthermore; it upholds previous research undertaken by Nyahumwa for the consumption of gases within double oxide films and the existence of nitride in the process, as well as providing evidence of different AlN morphologies in Al-Si-Mg alloys.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):