Solidification, Grain Refinement, and Isomorphic Self Inoculation

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The Ti-Al Binary System

This section provides an overview on phase and microstructure in the Ti-Al binary alloy system. Particular focus is given to the peritectic region which is of interest for industrial applications in the system.

Ti-Al Phase Diagram

The Ti-Al binary system consists of titanium, aluminum and intermetallic phases. Over time the understanding of the phases formed between the two pure metals has evolved. The first investigation into the full Ti-Al binary not solely concerned with the Al rich region was conducted in 1951 by Ogden, et.al. [19]. They established the first phase diagram of the system as can be seen in Figure 1.4A. Their work was continued by Mcpherson and Hansen who’s phase diagram was published 1954 [20]. Murry combined experimental results with thermodynamic calculations to produce an updated phase diagram in 1988 [21], shown in Figure 1.4B.
In more recent years coupling of experimental data and CALPHAD calculations have been used to update the phase diagram such as done by Schuster and Palm [22] or Witusciewicz et.al. [23] and Wang et.al. [24], Figure 1.4C and D respectively. The most recent work attempts to couple experimental data with both CALPHAD and atomistic modelling [25]. In this work the data and phase diagram used is that of Witusciewicz in 2008 [23].
Alloys of interest normally consist of less than 50 at% Al around the peritectic plateau at 1503oC and 44-48 at% Al [26]. Alloys which are processed from the liquid state in this region can include up to four phase changes from liquid to room temperature [26]. These could include the initial solidification of β grains from the liquid, transformation of β grains to α (following Burgers orientation relationship (110)β // (0001)α [27]), α to α and γ (following Blackburn’s orientation relationship (0001)α // (111)γ [28]) and finally α and γ to γ and α2 [29]. An example progression from liquid to room temperature can be seen in Figure 1.6.

Ti-Al Phases

It can be seen from the phase diagram that there are four main phases of interest for Ti-Al alloys in the peritectic region, β, α, α2 and γ. This section introduces these phases, their structure and characteristics.

β Phase

The solidifying phase of Ti rich alloys in the system is the β phase. The β phase has a body centered cubic structure consisting of eight Ti atoms on the vertices with a single Al atom encapsulated in the centre. Such a lattice can be seen in Figure 1.7.
β solidifying TiAl alloys are ideal to reduce texture, increase hot-workability, limit macro-segregation and increase the homogeneity of the as-cast microstructure [31]. The high temperature properties and castability of TiAl was also found to increase when β solidifying alloys were used [15]. In alloys designed to be β solidifying the interlamellar spacing of the room temperature structure was decreased, also improving the mechanical properties at room temperature.

α and α2 phases

The α and α2 phases are similar hexagonal close packed systems with different lattice parameters. The α phase is the natural allotrope of pure Ti at room temperature while α2 is modified by the addition of Al to the lattice. The aspect ratio of the lattice changes drastically between α and α2, with α having half the lattice parameter (a) of α2 while maintaining a similar c value [32].The lattice of α2 is shown in Figure 1.8.

γ phase

The γ phase is a face centered cubic structure consisting of alternating planes of Ti and Al [33].
γ is the intermetallic TiAl phase which exists broadly in the middle of the phase diagram, and can exist from room temperature up until it’s melting point near 1450oC. The alternation of planes results is a slight anisotropy between the C and A values of the lattice, however, the c/a ratio remains approximately 1.01-1.02 [21]. The Lattice of γ can be seen in Figure 1.9.

Microstructures

A variety of microstructures can be found in TiAl alloys. Of particular interest to this work are the structures which form on solidification of the alloy. Once solidification is complete a number of solid state phase transformations can take place which change the microstructure. Post solidification heat treatment can also be implemented to obtain further microstructures as well.

Solidification Microstructures

The two major solidification phases which can form from a TiAl melt near the peritectic plateau are α or β. These are often formed as dendrites from the melt and can be distinguished from one another by their shape, examples of both can be seen in Figure 1.10. Dendrites which are composed of the α phase have six fold symmetry as they have a hexagonal structure [34]. They can be easily distinguished as equiaxed dendrites which are seen in perfect cross section should have six primary arms 60o apart from one another with secondary arms also growing from the primary at an angle of 60o. In contrast β dendrites have a cubic structure and as such can be distinguished by their four primary arms growing at right angles and secondary arms perpendicular to the primary. Segregation phases such as γ may also form, especially between the dendrites when the liquid is particularly rich in solutes.

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Heat Treated Microstructures

As previously explained the solidification structure of α or β dendrites no longer exists at room temperature as solid state transformations take place when the alloy cools to room temperature. Example fully lamellar and duplex microstructures produced from the same processed (forged) sample of Ti-47.5Al-2.5V-1Cr can be seen in Figure 1.11 and are typical of room temperature structures. The duplex structure was obtained by annealing at 1250oC for 18h and has lamellar colonies of 100 μm with lamellar spacings of 0.2 μm along with 50 μm equiaxed gamma grains, while the fully lamellar microstructure has colonies of 200 μm with lamellar spacings of 0.5 μm [36].
Higher resolution micrographs of four such microstructures of a Ti-48Al-2Cr alloy are shown in Figure 1.12 [37]. Heat treatment at 1200oC (75oC above the eutectic) for two hours led to a Near-γ (NG) structure constituted of γ equiaxed grains with a few α2 grains at grain boundaries and triple junctions (Figure 1.12a). Increasing the heat treating temperature into the (α +γ) field can lead to a DuPlex (DP) structure constituted of a mixture of monolithic γ grains and small lamellar colonies of (α2 + γ) (Figure 1.12b). A Nearly Lamellar (NL) structure mainly constituted of big lamellar colonies and small γ grains (Figure 1.12c) can be formed with a 30 minute heat treatment near the α-transus (1360oC). Further increasing the heat treatment temperature leads to a Fully Lamellar (FL) structure organized in big lamellar (α2 +γ) colonies (Figure 1.12d).
Figure 1.12: Micrographs of the different microstructures of Ti-48Al-2Cr: (a) near-γ; (b) duplex; (c) nearly lamellar; and (d) fully lamellar [37].

Multi-Component Systems

The TiAl system is often augmented by the addition of other elements such as Cr or Nb as used in the GE alloy. The addition of Nb to TiAl can cause precipitation of a deleterious ω phase at high temperatures, this was found to worsen with Ni additions, however it could be greatly lessened by Mn additions [38]. Additions of Y to TiAl were found to cause Y2O3 precipitation as well as Y being located in the α2 and γ solid solutions. These additions assisted grain refinement and increased the strength of the alloys, additionally increasing the oxidation resistance [39]. Oxidation resistance was found to decrease with V additions, however oxidation resistance increased with Nb addition even if V was also added, this is attributed to Nb forming a solid solution with TiO2 and restricting O movement [40]. A summary of common alloying elements and their effects is given in Table 1.1.

Table of contents :

Chapter 1 Titanium Aluminium Alloys
1.1 Introduction to Ti-Al
1.2 The Ti-Al Binary System
1.2.1 Ti-Al Phase Diagram
1.1.4 Microstructures
1.3 Casting and Processing
1.4 Mechanical Properties
1.4.1 Creep
1.4.2 Fatigue
1.4.3 Fracture
1.5 Summary
Chapter 2 Solidification, Grain Refinement, and Isomorphic Self Inoculation
2.1 Solidification of Metallic Alloys
2.1.1 Overview of the Solidification process
2.1.2 Nucleation Theories
2.1.3 Solutal Growth Restriction
2.1.4 Summary
2.2 Inoculation of Metallic Alloys
2.2.1 Inoculation in The Ti-Al System
2.3 Development of Isomorphic Self Inoculation
2.3.1 ISI Criteria I: Phase and Lattice Matching
2.3.2 ISI Criteria II: Stability in the Melt
2.3.3 ISI Criteria III: Usability Factors
2.3.4 ISI Criteria IV: No Negative Effects on Final Cast Product
2.4 Summary
Chapter 3 Inoculant Alloy Selection, Production, and Characterization
3.1 Inoculant Alloy Selection
3.1.1 Selection and Description of the Bulk Base Alloy
3.1.2 Selection and Description of the Inoculant Alloys
3.1.3 Summary of Inoculant and Bulk Alloy Properties
3.2 Powder Production
3.2.1 Bulk Alloys
3.2.2 Powder Milling
3.3 Powder Characterization
3.3.1 Size Characterization
3.3.2 Microstructure Characterization
3.3.3 Effects on Milling
3.4 Summary
Chapter 4 Isomorphic Self-Inoculation Casting Trials
4.1 Inoculation Procedure
4.1.1 Inoculant Preparation
4.1.2 Number of Inoculant Particles
4.1.3 Ingot Preparation
4.1.4 Inoculation
4.2 Ingot Analysis Procedure
4.2.1 Sample Preparation
4.2.2 Measurement of Ingot Equiaxed Area
4.2.3 Grain Size Measurement
4.3 Individual Bulk Alloy Ingot Trials
4.3.1 Ti-Al-Nb Trials
4.3.2 Ti-Al-Ta Trials
4.3.3 Summary of Individual Ingot Trials
4.4 Ti-Ta Trials
4.4.1 Ti-Ta Collective Bulk Alloy Trials
4.4.2 Individual Bulk Alloy Ingot Trial
4.4.3 Summary
4.5 Conclusions
Chapter 5 Influence of Inoculant Alloys and Their Processing Parameters
5.1 Anomalous Ti-Ta Results
5.1.1 Density Effects
5.1.2 Oxygen Content
5.1.3 Superlarge Particles
5.1.4 Individual Bulk Alloy Trial
5.1.5 Summary
5.2 Ti-Al-Nb and Ti-Al-Ta Grain Refinement
5.2.1 Distribution Effects
5.2.2 Particle Break Up
5.3 Particle-Melt Interactions
5.3.1 Effect of Particle Size and Density on Velocity in the Melt
5.3.2 Diffusion between Particles and Molten Alloy
5.4 Conclusions
Chapter 6 Inoculant-Melt Interactions
6.1 Inoculant Alloy Interaction with Melt
6.1.1 Rod Dipping Procedure
6.1.2 Dissolution of Rod in Melt
6.2 Influence of Thermal Effects on Particle Grain Size
6.2.1 Particle annealing
6.2.2 As-Milled Particle Microstructure
6.2.3 Recreating Particle Thermal Treatment
6.3 Particle Dissolution in Liquid Melt
6.3.1 Influence of Fluid Flow
6.3.2 Application of the Model to Different Particle Sizes
6.4 Repercussions on Isomorphic Inoculation Trials
6.4.1 Number of Grains Present During Solidification
6.5 Conclusions
Chapter 7 Conclusions and Recommendations
7.1 Conclusions
7.2 Recommendations
7.2.1 Ti-Al Applications
7.2.2 Mechanism of Isomorphic Inoculation

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