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# University of Melbourne Advanced Materials Report

## Key words

$$\alpha$$ Nucleation, $$\beta$$ Titanium Alloy, Strengthening, Alloying elements

## Abstract

This aim of this report is to discusses the formation of $$\alpha$$ particles in the $$\beta$$ matrix of titanium alloys with the assitance of athermal $$\omega$$ precipitates. It explores the different mechanisms that play a role in the nucleation and growth of $$\alpha$$ precipitates as well as further explores the implication of having distributed or clustered $$\alpha$$ particles in the solid solution of titanium alloys. This paper analyses the affect of $$\alpha$$ precipitates on strength, ductility and toughness and explains the phenomenon using research.

The main findings about the nucleation of $$\alpha$$ particles suggest that formation of distributed $$\alpha$$ precipitates is assisted by $$\omega$$ particles however there are a two different explanation that explain these cases. These speculated reasons are:

1. 1.

$$\alpha$$ is formed by a displasive method. This method suggests that upon annealing of the alloy containing only athermal $$\omega$$ and beta, $$\alpha$$ plates begin forming at the core of $$\omega$$ precipitates and consequentially displace them

2. 2.

Diffusional method, this suggests that due to rejection of alloying elements near $$\omega$$ precipitates present in titanium that are $$\alpha$$ stabiliser (i.e. Al) $$\alpha$$ particles form within close vicinity of the $$\omega$$ but not as the core. This phenomenon is used to explain the formation of $$\alpha$$ particle at $$\omega$$ /$$\beta$$ interface. According to the study evidence of both methods are quite prominent and therefore suggests the formation of $$\alpha$$ is a mix mode diffusional-displasive method.

## Introduction

Titanium is a light metal that is used in a range of application from aerospace industry to creating medical devices. Titanium possesses excellent strength and is a tough yet lightweight. It is widely used because of its resistance to corrosion and biocompatibility. In order to make titanium a more reliable metal to use in the industry alloys have been created in order to enhance the strength and toughness of titanium.

The paper chosen for study is called ” $$\omega$$ assisted nucleation and growth of $$\alpha$$ Particles”.The aim of this paper is to understand the different methods of $$\alpha$$ nucleation in titanium alloys as well as understating the effects of it appearance and morphology on its strength. It explores the changes in composition and the structure of the $$\beta$$ matrix according to the formation of $$\alpha$$ particle by varying temperature, aging times and different composition. There are a number of modern techniques used in this paper to identify these changes in the structure. They have mainly used:

• Scanning Electron Microscopy (SEM)

• Orientation Imaging Microscopy

• Transition Electron Microscopy (TEM)

• High Resolution Transition Electron Microscopy

The particular alloy under investigation in the paper is Ti-5553, which is more formally known as Ti-5 Al-5 Mo-5 V-3 Cr-0.5 Fe, it is a near $$\beta$$ titanium. This alloy exhibit the maximum strength of 1250 MPa and room temperature UTS to be as high as 1138-1172 MPa. Its properties include having high deep hardenability while still maintaining ductility and toughness. It is also able to endure high cycle fatigue, which makes it a very versatile and useful metal. However, one of the major disadvantages of this alloy is that the properties of Ti-5553 vary over wide range conditions, this is because the properties are critically dependent of the microstructure of the alloy. It is therefore the purpose of this study to determine nature of this alloy and to understand its properties as well as investigate methods to keep them consistent in order to ensure that it can be dependable material for application.

As mentioned before, the properties of this alloy heavily depend on the microstructure of titanium the study proposes that the nature of the Ti-5553 depends on the formation of $$\alpha$$ phase in the $$\beta$$ matrix. The volume fraction, morphology, size and distribution of $$\alpha$$ particle in the $$\beta$$ matrix are the main drivers of noticeable change in properties of Ti-5553. In order to alter these intrinsic features of $$\alpha$$ particles the heat treatment of the tittanium alloy or chemical composition may have to be changed. This object is mostly achieved by changing annealing temperatures or varying cooling rates as well as altering the time that the alloy in heated or cooled for.

Because Ti-5553 consist of 5 alloying elements, namely Al, Mo, V, Cr, and Fe, varying the heating temperature also affects the partitioning of these elements in the $$\beta$$ matrix, this pheno