Project B4: Ti alloy development
•	Project Chief Investigators: Barry Muddle (Project Leader), Michael Ferry
•	Senior Researchers/Research Associates: Monash: Colleen Bettles, HP Ng, Dacian Tomus; UNSW: Sammy Chan
•	Research Assistants Visiting Scholars, Hons Students etc: Monash: Daniel Curtis, Thomas Pühringer
•	External Collaborators: Allan Morton (CSIRO), Mark Gibson (CSIRO), Hamish Fraser (OSU)
B4.1: Solute-lean Titanium Alloys and Titanium Aluminides
The two areas of activity were lean alloys with tensile properties intermediate between those of commercial purity (CP) titanium and Ti-6Al-4V (for sheet metal applications) and titanium aluminide with improved ductility and formability.
A total of twelve lean alloys based on the Ti-1.5Fe-O-N and Ti-Al-Mn-(Fe) systems were prepared and annealed. The two commercial alloys CP Ti and Ti-6Al-4V were remelted, cast, and ECAPed in the same manner as the test alloys. CP Ti and the Ti-1.5Fe alloys were annealed at three temperatures in the (α+β) phase field. For the Ti-Al-Mn-(Fe) alloys the annealing temperatures were above and below the beta transus. The optimal annealing temperature differed from alloy to alloy, depending on the microstructural features produced. An assessment of the mechanical behaviour was obtained using a Shear Punch Test.
Results are shown in Figure B5. If enhanced ductility is of primary interest, the ternary Ti-1.5Fe-xN alloys could be of considerable value. Microstructural inspections of the Ti-Fe-O-N alloys showed that the annealed structures did not retain the deformation induced by ECAP, and recrystallisation had occurred. The CP Ti sheet had a grain size in the range of tens of microns. The addition of 1.5wt.%Fe refined the grain size considerably. Oxygen additions provided a further grain refinement over the binary alloy, and the complete composition containing Fe, O and N has a submicron grain size after ECAP and annealing at 700°C. In the recrystallised microstructure the α phase is equiaxed, the β phase has been retained on quenching and athermal ω is present within the β grains. The presence of the ω phase is likely to have an embrittling effect.
A re-examination of the data in a patent by Nippon Steel on Ti-Fe-O-N alloys, using both regression and a neural network analysis, make it possible to predict the strength or ductility of any alloy composition. Very small changes in the oxygen and nitrogen contents can have an enormous effect on the strength of the material. The effect is amplified by increasing the Fe content. The ductility is affected to a lesser extent by these changes in composition, but a reduction from 25% to 15% is possible by changing the oxygen and nitrogen contents in an alloy containing 1.5wt%Fe. The (strength/ductility)/composition relationship is clearly complex and is most likely related to the β transus temperature and the annealing temperature.
The Ti-Al-Mn-(Fe) alloys also show potential for improved ductility at lower strength levels than the Ti-1.5Fe alloys. All alloys containing Mn have a two-phase microstructure - equiaxed grains of the primary α phase and intergranular β phase along boundaries. Annealing in the single phase region at 1000°C and quenching resulted in a largely martensitic microstructure. The martensitic structure has had a detrimental effect on the ductility. Results for only the 800°C anneal are included in Figure B5.
Figure B5 - Diagram showing the UTS-Strain loci (based on SPT) for all the alloys evaluated, and including results for CP Ti and Ti-6Al-4V. The areas shown for each of the Ti-Fe-O-N alloys represent the results of annealing over the temperature range 700°C to 800°C.
Rapid assessment of a range of titanium aluminide alloys containing iron was conducted by means of two compositionally-graded specimens prepared using the LENS™ (Laser Engineered Net Shaping) process (illustrated in Figure B6). Powders are used as the feedstock for this process. Specimen S1 contained a gradient in Fe content at fixed Ti:Al ratio. Specimen S2 contained a fixed Fe content and a gradient in Al:Ti ratio.
After preliminary characterisation of the specimens in both the as-deposited condition and after heat treatment at 1130°C for ten hours, six promising compositions were prepared for further examination. The ones of most interest are Ti59.8Al38.0Fe2.2 in the three-phase α2 + γ + β(Ti(FeAl)) region and Ti55.3Al43.8Fe0.9 in the conventional two-phase α2 + γ region.
Figure B6 - Preparation of titanium alloy by the LENS™ process.
B4.2: Deep-Hardenable Aerospace Alloys
The deep-hardening behaviours of various metastable β-Titanium prototype alloys have been investigated with respect to the decomposition modes of β-phase. Isothermal ω- and α-phases are among the decomposed products that impart strengthening effects on β-matrix, the occurrence of which is highly sensitive to the heat treatment temperatures. A novel low-density Ti-V-Cu ternary alloy system has been developed and been shown to exhibit (i) a prominent quench-rate insensitivity on age-hardening, a crucial factor that determines the deep-hardenability of a β-Titanium alloy, and (ii) microstructural stability within a reasonable length of ageing period. Current investigations indicate that the kinetics of α transformation in β-alloy are significantly influenced by the types of precursory nucleation sites that are available for subsequent α-precipitate development. These sites include the athermal/isothermal ω and the transient β’-phase resulted from β-phase separation reaction (β->β+β’). The observed trend is that the presence of precursory phases tends to accelerate the hardening rate of the alloys upon ageing, whereas suppressing the precursor formation would induce a lengthened incubation period prior to the arrival of peak age condition. Besides, the project team has studied the effects of alloying elements and temperature on the stability and morphology evolution of isothermal ω, the outcome of which sheds light on the factors that control the ageing responses and attainable mechanical strength in relatively solute-lean metastable β-alloys. Understanding of the above would provide the basis for the design of deep-hardenable alloys with simpler and/or lower-cost alloy additions.
a)	Study of the Hardenability of Beta-III Ti-alloy
	Beta-III (Ti-11.5Mo-6Zr-4.5Sn) alloy is one of the representative commercial alloys that possess good deep-hardenability. The study focused on the effects of individual alloying elements (Zr and Sn) on the age-hardening behaviour of the binary Ti-11.5Mo base alloy. The ageing curves for the binary and ternary alloys treated at 400°C and 500°C are shown in Figure B7. Microstructural characterisations showed that the alloying additions of Zr or Sn has considerable influence on the formation of athermal ω-phase in the alloy upon quenching from the solution-treated condition (900°C).
	Sn, in particular, was found to be a prominent suppressor for athermal ω-phase. The Ti-Mo binary and Ti-Mo-Zr ternary alloys, where ω-phase was relatively abundant, were characterized with a rapid increase in strength yet followed by a gradual softening after eight hours of ageing at 500°C. The ω-suppressed Beta-III alloy, conversely, demonstrated a steady strength on prolonged ageing attributable to its comparatively finer α-precipitate microstructure. Nevertheless, it was observed that the restriction of ω formation in Beta-III has resulted in a longer incubation time (a few hours) preceding α-nucleation, the consequence of which could potentially reduce the cost-efficiency of ageing treatment in practice.
	Figure B7 - Age-hardening curves for Beta-III titanium alloy and its binary and ternary counterparts. Sn suppresses the formation of ω phase and led to more sluggish precipitation kinetics.
b)	Effect of Alloying Element on the Morphology of ω-phase in Ti-V alloys
	Ti-V-based β-alloy is a suitable candidate for aerospace applications considering the atomic mass of V, which is the lowest amongst the common β-stabilizers like Mo, Nb, Fe and Cr, etc. This study looked into the effect of Zr addition (0-5at.%) on the morphological evolution of ω, which has been known to be preferential nucleation sites for α precipitates in Ti-V alloys. The age-hardening behaviours of the alloys with varied Zr content were compared. TEM study evidenced that the alloy addition of Zr has changed the morphology of isothermal ω-phase in Ti84V16 from quasi-cuboidal to a more ellipsoidal geometry in the Ti79V16Zr5 alloy (see Figure B8). The accompanying change in the incipient hardening behaviours at 500°C was that the alloy with ellipsoidal ω (Ti79V16Zr5) exhibited a larger hardness increment than the ones with cuboidal ω, notwithstanding the fact that the hardness profiles of all alloys eventually converged. The results suggested that isothermal ω morphology may be able to alter the way with which the α-phase was nucleated in the Ti-V alloy systems. Investigation is under way to determine the differences in morphologies and habit planes of the α-phases associated with both types of ω-phases.
	Sn, in particular, was found to be a prominent suppressor for athermal ω-phase. The Ti-Mo binary and Ti-Mo-Zr ternary alloys, where ω-phase was relatively abundant, were characterized with a rapid increase in strength yet followed by a gradual softening after eight hours of ageing at 500°C. The ω-suppressed Beta-III alloy, conversely, demonstrated a steady strength on prolonged ageing attributable to its comparatively finer α-precipitate microstructure. Nevertheless, it was observed that the restriction of ω formation in Beta-III has resulted in a longer incubation time (a few hours) preceding α-nucleation, the consequence of which could potentially reduce the cost-efficiency of ageing treatment in practice.
	Figure B8 - TEM micrographs showing the different ω-phase morphologies in (a) Ti-16%V and (b) Ti-16%V-5%Zr alloys (in atomic %). The former is characterised with a near-cuboidal shape whereas the latter is more ellipsoidal due presumably to a lowered interfacial energy.
c)	Novel Deep-Hardenable β-Ti Alloys
	Athermal ω-phase forms irrespective of quench rates; and it is reported to be a precursor for isothermal ω and/or α-phases. Based on the theoretical β-Ti alloy design approach proposed by Morinaga, two alloys namely A1(Ti-10V-2Ni) and A2(Ti-10V-6Cu) have been developed with respect to their relative tendencies of athermal ω formation (Figure B9). In order to investigate the role of athermal ω-phase, if any, on the deep-hardenability of metastable β-alloys, different quench rates have been performed on the alloys specimens in analogy to the varied quenching rates on the surface and at the core of a thick-sectioned member. Microstructural inspections suggested that A1 alloy contains a higher volume fraction of athermal ω-phase than the A2 counterpart in accordance with theoretical prediction. When aged at 350°C in the ω+β two-phase region, both A1 and A2 alloys demonstrated hardening behaviours which were essentially independent of quench rates. At 500°C in the ω+β region, on the other hand, A1 was found to deviate from the quench-rate-insensitive age-hardening behaviour in conjunction with a monotonic decline in hardness with ageing time, whereas A2 was not. This was attributed to the extensive formation of autocatalytic α in the A1 alloy during slow cooling, whereas the A2 alloy retained its ω+β structure at the mean time.
	Rapid α-transformation kinetics were recorded for A1 and A2 alloys due presumably to the energetically favourable ω->α reaction in both alloys. A fairly complex -nucleation process encompassing β’->α and the emergence of non-Burgers-related α precipitates which were reported in commercial deep-hardenable alloys like Beta-C and Beta-III, has been found to be operational in A2. The multiple modes of α-nucleation during isothermal ageing was likely to be responsible for the high and sustained hardness values demonstrated by A2 on continued ageing (>90% of the maximum hardness, 460 Hv, was maintained for 8 hours). It is also of substantial interest to note that incorporation of Cu has activated an additional precipitation system with Ti to form intermetallic Ti2Cu compounds, which possibly reinforced the mechanical strength of the Ti-V-Cu alloy and compensated the softening of α-precipitates upon prolonged ageing.
	Figure B9 - Phase stability map showing the locations of A1 and A2 alloys in the Bo-Md space. Bo= Bond order; Md=Metal d-orbital level. The “twin” region is more favourable for the formation of athermal ω than the “slip” counterpart. (After Morinaga et la., 6th World Conference on Titanium, v.6, pp.1601-1606, 1998).
Activity plan for 2008
The plan is to continue work on the aluminide alloys, and possibly one of the low-alloy-addition areas depending on consultation with the CSIRO Light Metals Flagship. An aluminide alloy composition has been identified that will lead to the three-phase microstructure initially targeted. It is recommended that the next phase of the work concentrate on this composition. Heat treatment is the critical factor in achieving the desired properties. The work would continue with ingot-style production of samples to optimise the thermomechanical treatment.
However, concurrently, the behaviour of the alloy prepared from powder feedstock must be established and it is therefore considered essential that quantities of aluminide powder be available for this phase of the work. Besides, the precipitation behaviour of Ti-Cu intermetallic phases such as Ti2Cu will be further studied in β-titanium alloys containing V and Mo. The effect of β-stabiliser concentration on the precipitation behaviour of Ti2Cu will be studied through compositionally-graded β-alloys.