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Corrosion Science 111 (2016) 531–540

Contents lists available at ScienceDirect

Corrosion Science

j ourna l h omepage: www.elsev ier .com/ locate /corsc i

xidation behavior of binary Ti-xW (0 ≤ x ≤ 30, wt%) alloys at 650 ◦Cs a function of W concentration

. Samimia,b,∗, P.C. Collinsa,b

Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, 50011, United StatesThe Center for Advanced Non-Ferrous Structural Alloys (CANFSA), a joint NSF I/UCRC between the Colorado School of Mines (Golden, CO 80401) and the

owa State University (Ames, IA 50011), United States

r t i c l e i n f o

rticle history:eceived 7 February 2016eceived in revised form 20 May 2016

a b s t r a c t

The role of composition on the oxidation behavior of the Ti-W system at 650 ◦C was investigated utilizinga compositionally graded specimen, Ti-xW (0 ≤ x ≤ 30, all compositions in wt%). Microstructural evolu-tion of the base material and thickness of the oxide layers were assessed as a function of composition.

ccepted 30 May 2016vailable online 31 May 2016

eywords:. Titanium. Oxidation

Some of the observations with regard to the evolved base metal microstructure, including: � lath and� rib/precipitates and lamellar structure, were found to be comparable to what is reported for Ti-Mosystem. It was also shown that formation of a thinner oxide does not necessarily imply on the loweroxygen ingress.

© 2016 Elsevier Ltd. All rights reserved.

. TEM

. Introduction

The relatively low oxidation performance and subsequenturface embrittlement rank high among the most importantife-limiting characteristics for structural Ti-based alloys at ele-ated temperatures. The oxidation behavior of Ti-based alloys isnfluenced by several factors involving both the material (e.g., com-osition) and the exposure conditions (e.g., time, temperature,tmosphere). To interpret the overall oxidation performance of

material system it is necessary to understand the role of indi-idual constituents, both qualitatively and quantitatively on theifferent stages of oxidation. The prospect of understanding andredicting oxidation behavior of multi-component Ti-based alloys

s rather daunting, given the complex nature of oxidation reactionsnd the numerous operating mechanisms in multi-component sys-ems. Any prediction must be made on an understanding of theperating mechanisms for, firstly, oxidation behavior in binary sys-ems and, secondly, enhancing/obstructing tendencies of the binarylemental species in the presence of a third element. Thus, an exper-mental approach has been adopted that maximizes throughput to

robe the effect of alloy composition on oxidation behavior whileimultaneously holding other variables (e.g., temperature, time,tmosphere) precisely constant [1–3].

∗ Corresponding author at: Department of Materials Science and Engineering,owa State University, Ames, Iowa, 50011, United States.

E-mail addresses: psamimi@iastate.edu, pe.samimi@gmail.com (P. Samimi).

ttp://dx.doi.org/10.1016/j.corsci.2016.05.038010-938X/© 2016 Elsevier Ltd. All rights reserved.

Although the assessment of oxidation for Ti and simple Ti-based systems have been extensively documented, the majorityof the efforts are focused on the oxidation behavior of complextechnical alloys (e.g. �-21s, Ti-6Al-4V and Ti-aluminides) ratherthan a systematic exploration of composition space, particularlyfor the elements that are not common alloying elements for tita-nium (e.g. W) [4–17]. This dearth of binary and ternary systemsmakes the interpretation of compositionally-mediated operatingoxidation mechanisms and near-surface microstructural evolutionnearly impossible.

To date, W has not been an elemental constituent to any techni-cally important commercially used Ti-based alloy. This is due to twocharacteristics, namely its very high density (particularly avoidedin aerospace applications) and the difficulties associated with seg-regation issues in casting or the partial reaction of W particles inpowder metallurgy [18,19]. Despite the traditional absence of thiselement in Ti alloys, its influence on the oxidation behavior of Tialuminide alloys have been investigated in some research efforts[20–22]. According to Shida et al. [20], for a mixed TiO2 + Al2O3oxide scale formed on a Ti-35.4 wt% Al alloy (containing 1–6 wt% W)during oxidation at 900 ◦C, W segregated into the Ti oxide and ledto a slower growth rate for this phase. It was also reported thatinternal oxidation was prevented in W containing alloys as a resultof significant reduction in the oxygen solubility in the intermetallic

TiAl systems [20]. The critical concentration of Al required for theformation of a protective Al2O3 oxide layer in Ti-Al alloys can bereduced by addition W as it increases the activity of Al and acts assintering aid for the formation of Al oxide [1,20,21].

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32 P. Samimi, P.C. Collins / Corr

The purpose of this study is to assess the influence of the con-entration of W (across a wide composition range) on the oxidationroperties of Ti-xW binary compositions at 650 ◦C using a composi-ionally graded binary Ti-xW (0 ≤ x ≤ 30 wt%) specimen. The Laserngineered Net Shaping (LENSTM) was employed to achieve thisoal. The advantage of such a systematic approach is to maintaindentical testing conditions across the composition range and tovoid potential experimental variability regarding time, tempera-ure and atmosphere. Both the oxide scale and the metal substrateere characterized using advanced characterization techniques to

ssess different aspects of oxidation and oxygen-mediated reac-ions (e.g., phase transformations) as a function of local averageomposition. Although this paper is focused on the influence ofomposition on the oxidation behavior of Ti-W system, where rele-ant the evolution of the bulk microstructure will also be discussedelative to the parent microstructures.

Such a combinatorial approach and utilization of simple binaryystems provide mechanistic insights for the subsequent under-tanding of the oxidation behavior of more complex Ti-based alloysi.e. commercially available alloys). The present work is part of

more inclusive project on the oxidation assessment of otherinary Ti-X systems (X = selected alloying element) including Ti-o, Ti-Cr and Ti-Al [23–26], though correlations with industrially

sed alloys has been drawn [27]. Briefly, these three systems wereelected because the alloying elements Mo, Cr, and Al representhe three distinctive types of elements in Ti-based alloys, namely

monotectoid �-stabilizer (Mo), a eutectoid �-stabilizer (Cr), andn �-stabilizer (Al). Similar to Mo, W is a monotectoid �-stabilizer,nd it is thus reasonable to expect some common observations witho, especially when considering the subsurface microstructural

volution.

. Experimental methods

A compositionally graded Ti-xW (0 ≤ x ≤ 30 wt%) specimen wasroduced using an Optomec LENSTM 750 at the University of Northexas from high purity elemental metal powders of Ti (99.9% pure,150 mesh from Alfa Aesar) and W (99.8% pure, plasma spray grade

rom Micron Metals). The LENSTM technology uses a computer-ided design (e.g. CAD) file from which a machine code tool pathan be generated for the subsequent laser deposition of three-imensional components. The CAD file is first translated into a *.stlle, and subsequently sliced into a sequence of layers with a nom-

nal thickness of 0.25 mm. Each layer consists of multiple parallelines with a nominal hatch width of 0.38 mm. The tool path infor-

ation that is generated with these variables is communicated to

he computer-controlled motorized stage and a deposition headconsist of focusing lens and powder nozzles). The 2D (x,y) in-lane motion of the stage accompanied by -z vertical motion ofhe deposition head produce near-net-shape metallic pieces.

Fig. 1. a) Schematic of the compositionally graded Ti-xW

cience 111 (2016) 531–540

The LENSTM is equipped with two independently controlledpowder feeders which were loaded with pure Ti powder in powderfeeder #1 and with a Ti–30 W mechanically mixed elemental pow-der blend in powder feeder #2. An inert gas (here Ar) carries thepowders from powder feeders into a controlled atmosphere box.The fluidized powders are injected (via four convergent Cu noz-zles) into a localized melt pool created by a focused high energyNd:YAG laser (350–500 W). A 6 mm thick Ti-6Al-4V substrate wasused as the base for laser deposition of the powder blend and in-situalloying.

The oxygen level was maintained at a level below 20 ppmthroughout the deposition and the specimen was deposited inthe form of a 38 mm × 25 mm × 12 mm rectilinear solid. The inde-pendent computer control of the powder flow rate allows forpre-programmed incremental changes in the relative mass flowrate from powder feeders and consequently variation in the localcomposition along the length of the sample. The final product pro-duced for this research effort was a compositionally graded Ti-xWspecimen where x ranges between 0 and 30 wt%.

The specimen was longitudinally sectioned into several pieces(thus conserving the composition range in every piece, see Fig. 1(a))and subject to a � solution heat treatment at 975 ◦C for 100 h fol-lowed by water quenching. The solutionization temperature wasselected to be well above the beta transus for every compositionalong the gradient. The time was selected to allow for sufficientdiffusion of W, a notoriously slow diffusing element in Ti, and thewater quench was conducted to allow for a more fundamentalcomparison among different binary titanium systems. To minimizeoxidation, the samples were encapsulated in an evacuated and Arback-filled quartz tubes. In addition, Ar was flowing constantlythrough the tube furnace during the solution heat-treatment (seeFig. 1(b)). Several pieces of titanium sponge were also placed in thequartz tube to getter up the residual oxygen and further protectthe specimen from oxidation during extended solution heat treat-ment. Following solutionization, the samples were polished priorto the oxidation tests to assure that the exposed metal surface wasflat and uniform. The samples preparation steps included grind-ing using 240–800 grit SiC abrasive papers followed by polishingwith 0.04 colloidal silica suspension. Cleaning of the samples fol-lowing grinding and polishing was carried out using a sequence ofsolutions starting with acetone, followed by water+surfactant, andfinally with methanol.

The polished and cleaned samples were oxidized at 650 ◦C forthree different holding times of 25, 50 and 100 h. The oxidation testswere carried out in a box furnace and the samples were placedwith the polished surface oriented upward and exposed to stilllaboratory-air. The samples were cross-sectioned after oxidation

tests and the cross section of the oxidized surface was polishedfollowing the aforementioned steps.

specimen and b) solution heat treatment setup.

P. Samimi, P.C. Collins / Corrosion Science 111 (2016) 531–540 533

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Fig. 2. a–f) Backscattered electron micrographs of six selected compo

The microstructure and composition of the oxide scale andetal substrate were characterized using a field emission gun (FEG)

EITM Nova NanoSEM 230 with an integrated energy dispersivepectrometer (EDS) as well as a Tecnai G2 F20 TEM operating at00 kV accelerating voltage. The TEM was equipped with both aigh angle annular dark field (HAADF) STEM detector and energyispersive spectroscopy (EDS) capability. Site-specific TEM sam-les were prepared using an FEI DualBeamTM (FIB/SEM) Nova 200anoLab.

. Results and discussion

.1. Microstructural evolution of the base material

The electron backscattered micrographs showing theicrostructure of six selected compositions after solutioniz-

ng and water-quenching are presented in Fig. 2(a–f). It can beeen that the microstructure consists of martensite laths uponuenching across the entire studied composition range. Depend-

ng upon composition, two different types of martensites (i.e.,exagonal �′ and orthorhombic �′′) are possible in Ti-based alloys.

n this system, it is expected that the hexagonal �′ will form forompositions below 7.5 wt% W and orthorhombic �′′ martensiteill form for compositions above 7.5 wt% [28]. The martensitic

tructures become unstable in Ti-based alloys when the martensitenish temperature (Mf ) drops below room temperature, often

ollowed by the onset of a decomposition of the parent bcc � phaseo � + �. Given the relatively little research into the Ti-W binaryystem, there is no general agreement in the literature regardinghe beginning and ending composition of � formation in the Ti-Wystem. The onset of athermal � formation upon quenching fromolutionizing temperature has been reported to be 23.7 wt% W,ccording to Collings [29] and 20 wt% W according to Bagariatskiit al. [30]. For complete � retention, W contents of 22.4, 25 and0 wt% are reported in the literature [31–33].

Fig. 3 (a–d) show the cross-sectional microstructures of theegions just beneath the oxidized surface for four selected com-ositions after 25 h oxidation at 650 ◦C. It can be seen that theicrostructure consists both � and � phases across the compo-

ition range in the graded specimen; however, the volume fractionf these phases change with addition of W due to the � stabilizingffect of this element. The � phase exists as laths and the � phasehat has precipitated takes the form of distinct and globular parti-les at the �-lath interfaces and also fine scale precipitates within

he � laths.

When compared to the microstructure of other binary systemsreviously studied by the authors, i.e. Ti-Mo [23] using identicalechniques and under the same oxidation condition, it becomes

s after solutionizing at 975 ◦C for 100 h followed by water quenching.

apparent that the overall morphology in Ti-W system (regardlessof the size and phase fraction of the � laths and � phase particles)is comparable to what is reported for Ti-Mo system (see Fig. 4(a,b)).It can be seen that the inter �-lath � phase particles/ribs in Ti-Wsystem are significantly smaller in size, likely due to the relativediffusivity of the two elemental species.

The presence of nano-scaled � phase particles within the � lathswas also previously observed in the case of Ti-Mo system [23] andwas attributed to the ingress of oxygen into the metal substrateduring oxidation exposure that drastically alters the partitioningcoefficient of the alloying between the � and the � phases (strongrejection of the alloying element from � phase) and creates a largethermodynamic driving force to favor the homogeneous nucle-ation of � precipitates in � phase. The same hypothesis (oxygenassisted precipitation of � in �) is postulated for the Ti-W system,given the similarities of the post-oxidation microstructures. How-ever, the composition ranges over which this phase transformationoccur is different. In the Ti-Mo system, the oxygen-assisted precip-itation of intra-lath � particles is observed for the compositionsbelow ∼7.5 wt% Mo while this composition limit is extended up to∼25 wt% W in the Ti-W system. This difference is directly relatedto the stability of the martensites of the two systems, confirmingthat fundamental aspects of the starting microstructure govern theinfluence of oxygen on the resulting microstructural evolution. Inaddition to the homogenous precipitation of intra-lath particles,the � phase also precipitates as discrete particles that decorate thedefect structures of the supersaturated martensite (i.e. twins anddislocations) as shown in Fig. 5(a,b).

Fig. 6(a–d) shows the SEM backscattered micrographs repre-senting the subsurface microstructure of Ti-3W, Ti-9W, Ti-25W andTi-33W compositions, oxidized for 50 h at 650 ◦C. It is apparent thatthe evolution of the base material beneath the oxide scale and theoverall morphology of the phases present are quite similar to theobservations made for the oxidation time of 25 h in Fig. 3(a–d) forthe identical compositions. It is interesting to note that althoughthe maximum concentration of W in the compositionally gradedspecimen was intended to be 30 wt%, higher contents of W wascaptured at the top portion specimen (corresponding to the soluterich region). This can be caused by the gradual segregation of themechanically mixed Ti and W powders in the powder feeder hop-pers during the deposition resulting in the local deviation fromthe targeted composition. For the Ti-33W composition region, adistinctive Widmanstätten-type microstructural morphology wasobserved. There is also an increase in the volume fraction of the� phase for longer oxidation exposure times, as evidenced when

comparing Ti-3W in Fig. 6 (50 h) and Fig. 3 (25 h). This observationis consistent with the rejection of W from the �-phase upon ingressof oxygen, consistent with previous results made on the Ti-xMo sys-

534 P. Samimi, P.C. Collins / Corrosion Science 111 (2016) 531–540

Fig. 3. a–d) Cross-sectional backscattered electron micrographs from the subsurface of four compositions after 25 h oxidation at 650 ◦C.

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Fig. 4. Backscattered electron micrographs from the subsurface regi

em. Similar trend for microstructural changes was observed as thexidation time was extended up to 100 h.

Another distinct microstructural feature observed in the gradedi-W specimen after high temperature exposure is an �/� lamel-ar structure resembling a prototypical morphology resultedrom a cooperative growth mode (e.g. eutectoid transformation).

ig. 7(a,b) show the presence of such morphology in a Ti-22Womponent after the oxidation time of 25 h. This lamellar struc-ure which was also observed in the case of Ti-Mo system [23] ands understood to be a product of discontinuous precipitation where

a) Ti-3Mo and b) Ti-3W compositions after 50 h oxidation at 650 ◦C.

the parent microstructure consisting of � + � phases is being con-sumed and the identical phases (i.e. � + �) are precipitating but witha different morphology and, importantly, a different composition(i.e., higher O in the �, higher W in the �). However, notable dif-ferences are observed when comparing the two systems regardingthe locations and the composition range over which this lamellar

morphology forms and develops. In the Ti-Mo system nucleation ofthe lamellar structure happens in the regions with high concentra-tion of oxygen (i.e. regions in a close proximity to the surface andgrain boundaries of the prior � grains) for the compositions above

P. Samimi, P.C. Collins / Corrosion Science 111 (2016) 531–540 535

Fig. 5. a–b) Bulk precipitation of � phase within the � laths and decoration of twins and dislocations with the � phase particles.

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Fig. 6. a–d) Backscattered electron micrographs from the region just below th

.5 wt% Mo. For the Ti-W however this phase transformation takeslace near to the partially reacted W particles either close to theurface or in the bulk regardless of the local average composition.

al/oxide interface of four selected compositions after 50 h oxidation at 650 ◦C.

It was previously shown that the formation of similar lamel-lar �/� structure is an oxygen-induced phase transformation (i.e.,a discontinuous precipitation reaction) [23] and presence of this

536 P. Samimi, P.C. Collins / Corrosion Science 111 (2016) 531–540

Fig. 7. a,b) The lamellar morphology develop

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ig. 8. EDS composition profile across the interface of an arbitrary selected W par-icle and Ti matrix for the solution heat treated and water quenched specimen.

emarkably similar morphology inside the bulk and away from thexidized surface is not expected. Thus there must exist anotherxygen source that triggers the formation of this lamellar structureround the partially reacted W particles, regardless of their locationithin the specimen. One hypothesis is the retention of tungsten

xide phase (WO2) on unmelted W powder particles during theeposition, considering the higher melting point of this oxide com-ared to pure Ti. Such unmelted particles are expected to exist at

ow energy densities, given the presence of relatively cold liquidue to short reaction times and complex convective flow patterns.uch an oxide layer can ultimately react with the surrounding Tiatrix during extended high temperature exposure and release the

tomic oxygen. Fig. 8 shows an EDS line scan recorded across thenterface of an unmelted W particle and Ti matrix in the solutioneat treated and water quenched specimen. Although the contrastssociated with the presence of tungsten oxide phase is not visi-le at the interface (apparently due to the small oxide uptick), It

s clear that the concentration of O is slightly higher around thenterface area which supports (but does not definitively prove) theforementioned hypothesis about the potential source of requiredxygen for discontinuous precipitation of � + �.

.2. Oxide scale

The thickness of the oxide scale was measured across the com-osition range for the three exposure times and plotted as a

unction of W content (see Fig. 9 (a)), to assess the influence of Wn the scaling behavior of Ti-W system. The reported scale thick-ess values are averages of six independent measurements taken

rom the cross-section SEM images for each composition, as they

ed from a partially reacted W particle.

are shown for the three selected components in Figs. 9(b–d). It isapparent that plots of scale thickness for all three exposure timesfollow a similar trend, ranging from ∼1.5 �m scale thickness forlow W contents to ∼50 nm for Ti heavily alloyed with W. For anidentical composition, no major variation in the scale thickness isobserved with the passage of time from 25 to 100 h which is consis-tent with the parabolic growth rates, commonly observed for thecommercial � + � Ti alloys such as Ti-6Al-4V and Ti-6242 for similartemperature ranges [34,35].

It is well established that the parabolic oxidation rate law is pre-dominantly associated with the diffusion of oxygen into the metalsubstrate and which is considered to be the rate determining fac-tor and the subsequent establishment of an oxygen concentrationgradient owing to the presence of a relatively thin oxide layer andlarger diffusivity of O in the scale rather than the metal (∼50 timeshigher) [36]. After the metal substrate is saturated and the oxy-gen concentration of the interface reaches a critical level, there willbe a transition from parabolic to linear oxidation rate law accom-panied with heavy oxide formation. The linear oxidation stage isgoverned primarily by nucleation and growth of the oxide and forwhich the diffusion of the ionic species across the scale becomesthe predominant rate determining factor [37,38]. Accordingly, anyparameter that can suppress diffusion of oxygen in the metal sub-strate and retard the onset of linear oxidation stage would decreasethe scaling rate, e.g. lowering the oxidation temperature, includingalloying elements that reduce oxygen solubility in Ti [36,38–42]. Asshown above, the addition of W leads to the reduction of the scalingrate, presumably as a consequence of lowered O solubility in Ti, andthus postpones the starting point of the linear oxidation stage andheavy oxide formation. In addition smaller number of Ti cations inthe rutile lattice and their lower mobility as a result of the presenceof W cations could further hinder the ionic transportation throughthe oxide scale [43].

Care must be taken while interpreting the results, and quanti-tative assessments of oxygen ingress into the metal substrate mustbe coupled with scale thickness measurements to reveal the actualinfluence of W on the overall oxygen uptake, including both the Oconsumed by the formation of the oxide and the O dissolved in themetal substrate.

Cross-sectional STEM micrographs of the oxide scale and metalsubstrate for a Ti-3W composition, oxidized for 100 h are presentedin Fig. 10(a–c). The oxide scale consists a compact outer layer anda porous inner layer. Considering the fact that the oxidation reac-tion progresses with the inward migration of oxygen anions and

the metal/oxide interface is the oxidation front, the compact outerregion was the oxide formed on the metal surface in the earlystages of oxidation followed by the porous layer. The noticeable

P. Samimi, P.C. Collins / Corrosion Science 111 (2016) 531–540 537

Fig. 9. a) Thickness of the oxide scale as a function of W content for three exposure times and b-d) backscattered electron micrographs of Ti-3W, Ti-10W and Ti-33Wcomponents after 50 h oxidation at 650 ◦C.

Fig. 10. a–c) The STEM micrographs of a Ti-3W component after 100 h oxidation.

538 P. Samimi, P.C. Collins / Corrosion Science 111 (2016) 531–540

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Fig. 11. The STEM micrographs recorded from the subsurface region of

hange in the compactness of the oxide is attributed to the sin-ering and recrystallization of the initial oxide crystals during therolonged high temperature exposure of the oxide [38,42]. Theresence of the nano scale inter-lath � phase particles in the sub-urface microstructure is clearly evident, and strongly resemblesbservations of the monotectoid Ti-xMo study.

Shown in Fig. 11 are STEM micrographs recorded from the cross-ection of the oxidized surface for a Ti-25W composition oxidizedor 100 h oxidation. The morphology which can be referred to asWidmanstätten” can be seen in the base material unlike Fig. 10for Ti-3W). There also exists a considerable variation regarding

he size of � phase particles, moving from surface towards the bulk.he presence of oxide crystals was observed ahead of the oxidationront (i.e., isolated crystals enveloped within in the metal substrate)

idized surface for a Ti-25 W component after 100 h oxidation at 650 ◦C.

within the first ∼500 nm distance from the metal/oxide interfacewhich indicates clearly that internal oxidation phenomenon hastaken place. The TEM specimens prepared from Ti-3W (see Fig. 10)and Ti-12W compositions (not presented) however were free ofinternal oxides. This is rather surprising since the addition of Wis reported to decrease the O solubility of Ti and thus reduces theinternal oxidation susceptibility in the Ti aluminides. Furthermore,one of the requirements for the occurrence of internal oxidationis to have a solute element with a thermodynamically more sta-ble oxide scale compared to the base metal [44,45]. Notably, thisrequirement is not satisfied in the case of Ti-W system where the

oxide stability of solute, i.e. W, is considerably higher than that ofbase metal, i.e. titanium. Thus, this result is quite surprising andcontradicts the requirement.

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According to the results of oxide thickness versus W concentra-ion (see the plot shown in Fig. 9), higher contents of this alloyinglement would reduce the scaling rate of Ti. On the other hand,he observation of internal oxidation for Ti-25W implies a higherevel of oxygen ingress compared to the alloys with lower soluteevels (e.g. Ti-3W and Ti-12W). This is an example of anomalousxidation behavior in Ti alloys where larger amount of dissolvedxygen in the metal substrate does not necessarily correspond with

higher scaling rate. If oxide is born internally, as discrete oxiderains, its morphology is preserved even after it is incorporatednto the oxide scale (external oxide). Importantly, this morphol-gy cannot be protective and act as a barrier to oxygen diffusionuring the high temperature exposure. Thus, internal oxidation sig-ificantly degrades the oxidation performance of the alloy and istrongly avoided in high temperature applications. Further studiesre required to reveal the mechanism of this unexpected internalxidation. One might attribute this to the high volume fraction of

stabilized � phase underneath the metal/oxide interface, con-idering the faster diffusion rate of oxygen through � compared to

phase [5,46,47].

. Conclusions

Rapid assessment of the influence of W content on the oxidationehavior of Ti was conducted using LENSTM technology and theollowing salient conclusions have been drawn:

. Different morphologies evolved during the high temperatureexposure across the composition range including: � lath and� rib/precipitates (inter- and intra-lath), lamellar structure thatforms around the partially melted W particles and Widmanstät-ten � and � for very high concentration of W.

. In general, and as expected, the observations made regarding themicrostructural evolution of the base metal in Ti-W system arecomparable to those of the Ti-Mo system including: precipita-tion of fine scale � particles within the � laths and formation of�/� lamellar structure as a result of discontinuous precipitationwith a cooperative growth mode. This means that it may be pos-sible to draw some generalized conclusions regarding the typeof alloying element and the resulting response to oxidation forTi-based alloys. The composition ranges associated with thesedistinctive morphologies are quite different: precipitation of �in � is not only limited to the subsurface region and the lamellarstructure predominantly form in the regions around the partiallyreacted W particles regardless of the location.

. The scale thickness follows a descending trend with increase inW content and appears to be unresponsive to the oxidation expo-sure times ranging between 25 and 100 h, indicative of parabolicoxidation rate law.

. Internal oxidation takes place for very high solute levels (e.g.Ti-25W) where the base material has a Widmanstätten mor-phology. This phenomenon indicates that the lower scaling rate(observed for high W contents) does not necessarily imply on thelower oxygen ingress into the metal and these two could followan opposite trend under certain condition.

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