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.

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 phenomenon is significant to the formation of \(\alpha\) particles and the overall properties of the alloy because the elements are either \(\beta\) stabilizer or \(\alpha\) stabilizers and therefore govern the nucleation of these phases at different sites. Moreover presence of oxygen and hydrogen in the alloy also play a significant role in the development of microstructures as well as the determining its strength.

According to the paper \(\alpha\) nucleates in the \(\beta\) phase at various locations mostly associated with defects, and grain boundaries namely pre-existing boundaries in \(\beta\) phase and \(\beta\)/\(\omega\) interface and \(\beta\)/\(\beta\)’interface. \(\alpha\) nucleation is also observed at dislocations as well as the boundaries between the alloying elements such as Al, Mo, V, Cr,O, H and Fe. This suggests that the number and the location of defects in the alloy would determine the density of \(\alpha\) nucleation and possible location.

Obtaining \(\alpha\) Through Quenching

The process of obtaining well-distributed \(\alpha\) particles is usually difficult because \(\alpha\) phase mostly nucleate at grain boundaries. To achieve a uniformly dispersed formation of \(\alpha\) particle titanium is solution treated above the \(\beta\) transus temperature and quenched to form athermal \(\omega\). The \(\omega\) sites are well distributed throughout the \(\beta\) matrix and when aged the \(\omega\) coarsens and act as heterogeneous sites for \(\alpha\) particles. As a result of distribution of \(\omega\) the \(\alpha\) forms is uniformly distributed in the \(\beta\) matrix and also exhibits morphologies that are distinctly different to \(\alpha\) formed at higher temperatures.

In this paper formation of \(\alpha\) particles have been studied in alloys with large misfits between \(\omega\) and \(\beta\) phase as well as alloys with lower \(\omega\)/\(\beta\) misfits. It is observed that for large misfits the \(\alpha\) nucleate on ledges and dislocations of \(\omega\)/\(\beta\) interface. Large misfits exhibits cube-shaped \(\omega\) precipitates. When coarsened the \(\omega\) loses coherency with the \(\beta\) matrix resulting in a nucleation site is \(\alpha\). In low misfit \(\omega\) is spherically or ellipsoid alloy sharped and these do not show significant evidence of being potential nucleation sites for \(\alpha\).(citation not found: nag_-assisted_2009) However, for alloys with low misfits locations a 2 stage aging process of these types of alloys have shown a \(\omega\)-assisted nucleation of \(\alpha\) particles.

There are two main studies that are used to explain the nucleation of \(\alpha\) particle. Neither of them have proven to be the one and only method of nucleation of \(\alpha\)

  • One study suggests that \(\alpha\) particles nucleate near \(\omega/\beta\) but at a certain distance. The argument in this paper is that alloying elements like Al are present in titanium alloys, are \(\alpha\) stabilizers while \(\omega\) destabilisers. SO the area around the \(\omega\) are rich in Al content which promotes the growth of \(\alpha\) particles in the vicinity of \(\omega\) due to rejection. (citation not found: nag_-assisted_2009)

  • There is another speculation that the \(\alpha\) plates form at the core of \(\omega\), this process is known as displacive transformation. This is proven by using TEM and HRTEM, as mentioned in the introduction.

It is however inconclusive as to which is mechanism is correct as although evidence for both techniques is present. If \(\omega\) /\(\beta\) did not play a significant role in the nucleation of \(\alpha\) then there should be noticeable difference in the quantity of \(\alpha\). However since alloys with low and high \(\omega\) /\(\beta\) boundaries show a significant difference in nucleation of \(\alpha\) their affect cannot be ignored.This paper specifically:

  1. 1.

    Studies the role of \(\omega\) the intragranular nucleation of \(\alpha\). Precipitates in Ti-5553 alloy containing the \(\beta\) as well as \(\alpha\) stabilizer. In this alloy Mo and V are \(\beta\) stabilizer and Al is \(\alpha\) stabilizer

  2. 2.

    Investigate growth of different variations of intragranualar \(\alpha\) precipitates and the changes in the microstructure

  3. 3.

    The partitioning of the alloying elements in the vicinity of \(\alpha\) and \(\beta\) particles

Result and Discussion

According to research it was observed that after heat treatment of \(\beta\) solution followed by quenching and aging the sample of titanium displayed micrographs of \(\beta\) grain surrounded by layer of \(\alpha\) and highly defined distributes nano intragranular \(\alpha\) precipitates. It was observed that slower cooling rate after heat treatment causes the \(\alpha\) particles to become coarser. SEM images also confirm that the \(\alpha\) precipitates nucleate on \(\beta\) grain boundaries and on intragranular locations

In order to study the formation titanium was \(\beta\) solutionised, quenched and aged and nucleation of \(\alpha\) interface was observed at \(\beta\) / \(\omega\) interfaces. , In their sample \(\omega\) precipitates are spherical in shape while \(\alpha\) particles exhibit a lenticular morphology upon cooling the precipitate is coarsened. It was observed the lenticular particles appear associated with \(\omega\) precipitates. It is speculated that these arise at \(\beta\) /\(\omega\) interface. It is however quite difficult to determine exact nucleation sites in this alloy as TEM still has low resolution.

When the ellipsoidal precipitates of \(\omega\) are present in the system it suggests low \(\omega\) /\(\beta\) misfits and therefore are considered less likely location for \(\alpha\) nucleation. The explanation of \(\alpha\) formation in this situation is analysed using 3DAP technology. The studies show the compositional profile during aging of the sample the region enriched in Cr regions show rejection of depletion of (V Mo Cr, \(\beta\) stabilizer and Al (\(\alpha\) stabilizer)) these suggest formation of \(\omega\) precipitates. This is illustrated in the image below The local piling of \(\alpha\) stabilizers in the \(\omega\) /\(\beta\) interface makes these sites viable for \(\alpha\) nucleation.

DAP results from the 350°2 h annealed Ti-5553 sample.

Figure 2 is a DAP results from the 350°2 h annealed Ti-5553 sample.It depicts (a) Compositional profiles for Ti, Al, V, Mo, Cr and Fe showing the composition of an \(\omega\) precipitate with an adjacent region of local Al enrichment corresponding to an \(\alpha\) precipitate. These profiles have been averaged across a cylinder of 5nm diameter shown in the three- dimensional reconstruction (inset). (b) Same compositional profiles as in (a), high-lighting the local Al enrichment corresponding to an \(\alpha\) precipitate. (c) Compositional profiles for Al, V, Mo, Cr and Fe showing the larger \(\alpha\) precipitate in the same sample.

However according to study of HRSTEM there is a clear connection of \(\alpha\) platelets forming at the core of the \(\omega\) precipitates, it is described as displasive transformation method. The studies show a mixed result of bot \(\omega\) assisted formation of \(\alpha\) particles at work, which suggest dual method of \(\alpha\) formation.Further investigation at annealing at higher temperature i.e. 400°Celsius suggests that \(\alpha\) particles coarsened and the \(\omega\) particles are greatly dissolves. HRTEM images shows well developed HCP structure of \(\alpha\) particles visible at \(\alpha\)/\(\beta\) interface. These images show a clear depletion of \(\beta\) stabilizers from \(\alpha\) regions and vice versa. At higher annealing temperature a greater coarsening of \(\alpha\) principates is observed

All in all the studies suggest that the \(\omega\) assisted nucleation of \(\alpha\) is a mix mode operation that involves both diffusional as well as displacive mechanisms. It suggests that while the phase may form due to displacement of \(\omega\) precipitate with \(\alpha\) the size and morphology of \(\alpha\) greatly depends on the heat treatment, the cooling rate heating temperate as well as presence of \(\alpha\) stabilisers and destabilisers. The \(\alpha\) particles have a plate like shape and form inside the \(\beta\) grain. The orientation of \(\alpha\) with \(\beta\) observes burgers orientation. It is also observed that nucleation is very slow for \(\alpha\) particles, much lower than equilibrium at early stages but after annealing at high temperature \(\alpha\) particles grow and lead to alloy partitioning and eventually lead to equilibrium. Heating the Ti-5553 also suggest that heating at high temperatures, namely at 600°Celsius or annealing at low temperature 350°Celsius but as a two stage process shows similar clustering and formation of a precipitates.

Understanding

As mentioned in the summary section of the report are mechanism of \(\alpha\) formation takes place in the titanium alloy. The motivation of studying this phenomenon is integral to understanding the strength of titanium alloys and how to make them more reliable. There are two methods by which \(\alpha\) particles strengthen the titanium alloy (citation not found: f._prima_evidence_2006) (citation not found: koul_phase_1970) (citation not found: bhadeshia_mechanism_1980):

  • One process, which is extensively described in the summary, is the formation of \(\alpha\) that is assisted by metastable phases, \(\omega\) or \(\beta\)’ in some cases. Formation of \(\omega\) suggests that the alloy less concentrated with \(\beta\) stabilizers and \(\beta\)’ suggest that the concentration of \(\beta\) stabilizers is high. The formation of these \(\omega\) and \(\beta\)’ phases are coherent and uniformly distributed with in the \(\beta\) matrix and therefore are easily deformed and form low ductility titanium. Heat treatment of these alloys to higher temperature precipitates incoherent and a homogenous distribution of \(\alpha\) particles that makes the alloy harder. This process is better known as precipitation hardening. The \(\alpha\) plates are too small to be deformed plastically and act as hard undeformable particles, they also impede the movement of dislocations which gives titanium alloys high yield strength. Since dislocation also determine the plasticity of a material, have well distributed microstructures in a matrix also suggest hardening of titanium. This type of microstructural formation is also observed in bainite structure. Bainite forms as needles or plates, depending on the temperature of the transformation. the microstructural details of bainite very fine and very similar to \(\alpha\) plates. As mentioned, heat treatment can be used to increase or decrease the density of alpha formation. Below is a graph of the affect of tempertare on titanium alloy. A higher density of alpha i.e. the second image below would be a more brittle but stiffer form of titanium alloy as it would not allow dislocation to travel through it. (citation not found: callister_materials_2002) (citation not found: lutjering_titanium_2007) (citation not found: bhadeshia_mechanism_1980)

Heating Rate Affects Aging Response
  • The other process is when \(\alpha\) forms around the boundary of \(\beta\). This process occurs when \(\beta\) is slowly cooled. However this process also contribute to yield stress of titanium , though to a smaller extent. this is because the formation of \(\alpha\) around boundary are large particles and are prone to plastic deformation and subsequent cracking. This kind of \(\alpha\) formation only provides boundary hardening but does not prevent dislocation from moving through the grains. This mechanism can be used to control the toughness after precipitation hardening described above. A two stage annealing process can be used to nucleate \(\alpha\) particle on the boundaries of \(\beta\) matrix and the second stage can be used to increase grain size until desired hardenability is obtained.

    • Process: Obtaining \(\alpha\) by Slow Cooling

      In order to obtain \(\alpha\) phase titanium is heated above the \(\beta\) transus temperature and slowly cooled to the \((\alpha+\beta)\) phase field. During this transition the \(\alpha\) phase forms around the grain boundaries of the \(\beta\) phase creating a continuous layer of \(\alpha\) around \(\beta\) grains. When cooled the \(\alpha\) either continue forming the plate or grow into \(\beta\) grain as parallel plates until meeting another \(\alpha\) colony.It is important to note that \(\alpha\) formation does not occur within the \(\beta\) phase as lenticular particles. As suggested above makes the structure of the alloy very brittle. Below is an image of continuous \(\alpha\) layer at \(\beta\) the boundaries. It is quite evident that crack propagation can be a major problem.

Continuous \(\alpha\) layer at \(\beta\) the boundaries

Both mechanisms mentioned above however increase the dislocation density in the matrix. Therefore alloys experience dislocation hardening. Other methods of increasing strength include:

  • Adding other impurities- Titanium alloys are stronger than pure metals because alloying elements not only stabilize \(\alpha\) or \(\beta\) phase but also their presence in solid solution imposes lattice strains on the surrounding host atoms. Presence of hydrogen or oxygen lattice strain field interactions between dislocations and the impurity atoms in restricted dislocation movement making the alloy stronger.

  • Heat treatment - In order to obtain a high concentration of \(\alpha\) precipitates the aging temperature are vital as they usually determine the volume fraction of \(\alpha\) platelets that form with also influence a high yield strength. There are 2 ways in which \(\alpha\) particles are annealed. (1) Annealing at high temperatures (2) two stage annealing. It is often difficult to get substantial distribution of alpha particles in titanium alloys that contain a high concentration of \(\beta\) stabilizers, in theses cases studies suggest pre-aging of the sample. This allows a more uniform distribution of \(\alpha\) particles. Another study suggests that more homogeneous distribution is achieved at dislocations by cold working prior to aging. The persistent problem is that the a phase generally appears preferentially at grain boundaries, on intra-granular defects and along dislocation lines, making it difficult to obtain a uniform and dispersed a phase distribution.

Critical Analysis

This paper proves that the formation of \(\alpha\) phase is definitely dependent upon the \(\omega\) phase but is inconclusive as to which mechanism dominates the formation of \(\alpha\) particles. There is insufficient evidence of one being superior to the other. The paper uses various modern technologies and tests both hypotheses mentioned in the introduction instead of just investigating one. The problem with this paper is that the Ti-5553 alloy that is being used as a test sample has many alloying elements and therefore makes it difficult to determine where the \(\alpha\) particles are forming at the defects at grain boundaries around the alloying elements and titanium or if they are nucleating due to the presence of \(\omega\).

Future works

In order to study the affects of \(\alpha\) formation test should be conducted on titanium alloys with varying \(\alpha\) stabilizer content and testing what the optimal level of stabilizer are suitable to nucleate desired \(\alpha\) content. The affects of the duration of aging as well as temperature of aging should also be studied and a comparison should be made between alloys. It is also necessary to use better technology to pin point the location of \(\alpha\) precipitates. It would also be useful to study titanium alloys with lower number of alloying elements so that the effects of each element on the formation of \(\alpha\) particles can be individually studied without possible interference of other elements in the composition.

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