Microstructure of Cast Titanium Alloys

7/29/2019 Microstructure of Cast Titanium Alloys

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MATERIALS FORUM VOLUME 31 - 2007

Edited by J.M. Cairney and S.P. Ringer Institute of Materials Engineering Australasia

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MICROSTUCTURE OF CAST TITANIUM ALLOYS

M.J . Bermingham, S.D. McDonald, M.S. Dargusch, D.H. StJ ohn

CAST Cooperative Research Centre, School of Engineering, The University of Queensland, Brisbane, Queensland,

Australia

ABSTRACT

Due to the significant raw-material costs, the near net-shape manufacture of titanium alloy components is an attractiveproposition. Processing through melting and solidification allows manipulation of the bulk composition and control ofthe microstructure. This research examines the effect of commonly used alloying additions on the as-cast microstructureof titanium alloys. In particular, the effects of incremental additions of aluminium and vanadium are used as a basis forexplaining the as-cast microstructure of the most commonly used titanium-based engineering alloy, Ti6Al4V.

1. INTRODUCTION

Titanium has emerged as a very attractive metal fornumerous applications. It has the highest strength todensity ratio in comparison to other widely used metalssuch as iron, nickel and aluminium based alloys. Thetensile strength of titanium alloys is comparable to thatof some stainless steels, as well as iron and cobaltbased superalloys1. Titanium has exceptional corrosion

resistance in many environments, often exceeding thecorrosion resistance of stainless steels.

Although titanium has properties superior to manyother materials its applications are limited primarily toits high cost. Traditionally titanium was confined to

niche markets such as military and commercialaerospace. However emerging markets for titaniuminclude chemical processing, biomedical, marine andconsumer goods such as sporting equipment. Titaniumdemand is expected to double over the next decade2.

The high cost of titanium makes net shapemanufacturing routes very attractive. Casting is a nearnet shape manufacturing route that offers significantcost advantages over forgings or fabricated structures.In addition, titanium components that are cast and hotisostatically pressed (HIP) exhibit mechanical

properties comparable to forged counterparts. Like

other cast metals, the final properties of titaniumcastings are highly dependant on the microstructureand therefore control of solidification is veryimportant.

In pure titanium, solidification occurs at 1668C withthe formation of a body centered cubic crystal structurereferred to as -titanium. An allotrophic transformationoccurs at 882C where the BCC structure transforms tohexagonal close-packed. The HCP structure is referred

to as -titanium, and the temperature above which themicrostructure is 100% -titanium is known as the -transus. The addition of alloy elements can affect the -transus temperature. Certain elements will raise the -

transus and thus are referred to as alpha stabilizers, and

other elements will lower the -transus, known as betastabilizers.

This work investigates the mechanisms ofsolidification and microstructural evolution in as-casttitanium alloys. The influences of aluminium andvanadium on the as-cast microstructure of commercialpurity (CP) titanium are investigated.

2. EXPERIMENTAL

The influence of aluminium on cast titanium wasexamined by making individual aluminium additions tocommercial purity (CP) ASTM grade 2 titanium prior

to melting, while the influence of vanadium wasexamined by remelting commercial Ti-6wt%Al-4wt%V (ASTM grade 5) samples. One cylindricalsample of diameter 22 mm and height 17 mm wasprepared for each melt. When required, high purityaluminium (99.999 wt %) was added to the sample by

drilling a hole into the side of the sample and placingthe appropriate proportion of aluminium into the holeto give the desired bulk composition. This method ofalloying is similar to that used by previousresearchers3. All samples were adjusted to equalweights of approximately 25.5 grams.

Three samples of CP titanium, Ti-6Al and Ti-6Al-4Vwere cast with the averaged chemical results shown inTable 1.

Table 1. Nominal and analysed composition of theexperimental castings (three-cast averages).

Average Analysed Composition

NominalAl V C O N Fe Ti

Composition

CP Ti 0.017


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Melting and casting of the 9 samples took place inrandom order using a tungsten arc melting furnaceunder a protective argon atmosphere (Autocast 230,

Dentaurum ) with the melt cycle being controlled toachieve identical cycles between alloys. Casting tookplace into a copper mould of dimensions shown inFigure 1 that was maintained at room temperature.

Figure 1. Schematic of the copper mould used forcasting, and the casting unit.

Temperature measurements were obtained using aType B thermocouple. The thermocouple was insertedinto a specially designed mould of the same

dimensions in Figure 1, where the tip of thethermocouple was located in the approximate center of

the casting. The thermocouple wire was shielded by aceramic sheath, and only the very tip of the wires wereexposed to the molten metal. This resulted in the fastestresponse time possible. The sampling speed was 74534

Hz.

After casting, the samples were sectionedlongitudinally down the center and polished by firstlygrinding to 1200 grit silicon carbide paper followed byfinal polishing in a solution containing 10% H2O2 and a90% colloidal silica solution. The samples were thenetched (3ml HF, 30ml HNO3, 67ml H2O2) followed by

examination using optical and scanning electronmicroscopy.

3. RESULTS

The microstructure of all samples predominatelyconsisted of equiaxed prior- grains. A macrographillustrating the equiaxed grains of a CP titanium castingcan be seen in Figure 2.

Figure 2. Cross section of CP Ti casting consistingpredominantly of equiaxed prior- grains.

A variety of internal -morphologies were presentwithin the equiaxed prior- grains. The average prior-

grain size in all samples was within the 400-800 mrange. The unalloyed samples and the samplescontaining only aluminium all exhibited similarmicrostructures, however the addition of vanadiumaltered the microstructure. The major differences canbe seen in Figure 3. All samples contained transformed present in a variety of -morphologies such as

widmansttten,

Figure 3. [A] Typical microstructue of CP Ti: grainboundary (a), fine accicular (b), widmansttten

(c), serrated (d). [B] Typical Ti6Al4V microstructure

containing very fine acicular (e), fine acicular and (f) and prior- grain boundaries (g).


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acicular, lamellar, serrated and plate-like , some ofwhich have been labeled in Figure 3 in accordance withthe literature4-6.

As seen in Figure 3 (b), the addition of vanadiumradically alters the observed microstructure. Themicrostructure now consists predominantly of very fine

acicular and some of this acicular is delineated byintergranular.

The thermal data collected during solidification

indicated the casting experienced very rapid cooling inthe copper mould. Initial cooling rates were in excessof 150C per second over the first 1-2 secondsdecreasing to approximately 30C per second duringthe transition.

4. DISCUSSION

The complicated phase transitions that occur withintitanium and its alloys make understandingsolidification and microstructural formation moredifficult than that of many other foundry alloys. Under

equilibrium conditions, solidification occurs at 1668Cwith the formation of the high temperature BCC -titanium phase. Upon further cooling, a phase changeto HCP -titanium occurs at 882C. The addition ofalloying elements complicates this further, as certainelements may lower or raise the transition

temperature. Furthermore, solidification of titaniumcastings rarely occurs under equilibrium conditions.

Examination of the as-cast CP titanium microstructureyields little information about the solidification aspectsof the high temperature phase. Only prior- grains

remain, and within these grains exists a multitude ofvarious -morphologies. The prior- grains remainvisible because various -morphologies nucleate fromthese boundaries, including grain boundary . Inaddition, the -phase does not cross prior- grainboundaries.

The addition of 6wt% aluminium to the CP titanium

did not change the as-cast microstructure. The fact thatthe addition of aluminium did not induce

microstructural change is not surprising afterconsidering the binary Ti-Al phase diagram shown in

Figure 4. The Ti-Al phase diagram shows thataluminium has a very high solid solubility in titanium,

and at the low levels used in this research, nointermetallics or second phases form. Furthermore,aluminium is an alpha stabilizer, and hence no betatitanium is expected to remain in the as-cast

microstructure. EDS analysis of the microstructureconfirmed the presence of aluminium in solid solution

in the alpha phase, and no indication of preferentialaluminium rich -morphologies could be determined.

Figure 4. Binary phase diagrams for (a) Ti-Al and (b)Ti-V7.

The addition of 4wt% vanadium to the Ti-6Al didsubstantially impact the as-cast microstructure.However, once again there was no evidence of thesolidification process involving the L transformation. Prior- grain boundaries were clearly

visible as in the case of the CP and Ti-6Almicrostructure, however the internal phases within theprior- grains were altered dramatically. Instead of thestructure described above the interior of the prior-grains almost completely comprised of very fine

acicular + . This / mixture is expected from theTi-V phase diagram (Figure 4 (b)) as vanadium is a stabilizer and allows and to coexist at room

temperature.

It remains unclear whether the -Ti in either CP Ti, Ti-6Al or Ti-6Al-4V first nucleates and grows via adendritic or cellular mechanism, as no evidence ofdendrites can be seen in the as-cast microstructure. It isstated in the literature that dendritic growth

morphologies are often obtained in Ti-6Al-4V castingshowever the + transition eliminates evidence ofprior dendrites

4. To examine this further, a

supplementary experiment was performed in an

attempt to observe the suspected dendritic morphologyof the beta phase. A range of phase diagrams were

investigated to select an insoluble element whichwould be rejected during initial solidification and be


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unlikely to go back into solid solution. As shown inFigure 5 boron satisfied this criteria. The selection ofboron was further supported by prior observations in

the literature that boron-containing intermetallics arefound scattered throughout as-cast structures whenadded to common titanium alloys

8, 9.

Figure 5. Binary Ti-B phase diagram7.

One boron containing casting was made by addingboron in the form of an Al-4B master alloy to a CP Tisample in the same manner as discussed for thealuminium-containing samples. All variables andcasting parameters were maintained the same as those

of the previous experiments. The final composition forthis alloy was Ti-3.25wt%Al-0.135wt%B.

It is evident that the addition of 0.135 wt% boronsignificantly altered the as-cast microstructure. Assuspected, the presence of an insoluble element

delineated the -dendrites making the structure clearlyvisible, as shown in Figure 6. Numerous clusters of rodshaped boron-containing particles were observed in theinterdendritic regions. These rods were approximately2-10 m long and the presence of boron was confirmedby EDS. A phase which resembled fine widmansttten

was present within the dendrites. As seen in Figure 6,

the -phase overlapped interdendritic regions (and theborides). This indicates that in three-dimensions thedendritic structure and the interdendritic phases offer

little barrier to the growth of the -phase. It is known

that the -plates grow during the transitionhowever from the observations of this paper it remains

unclear where these plates initially nucleate, although itis probable that they nucleate and grow from prior-grain boundaries, as suggested in the literature1.Unlike the other alloys investigated, the undulating

dendritic interface made it difficult to clearlydistinguish the prior- grain boundaries in the boron

containing sample.

Figure 6. SEM (Secondary electron mode) images ofTi-3.25Al-0.135B showing (a) the dendritic

microstructure delineated by intermetallics and (b)interdendritic boron-containing particles offer no

substantial barrier to the growth of-phases

Although it is clear that the influence of boron in

titanium promotes solidification via a dendriticmechanism it still remains unclear whether CPtitanium, Ti-6Al or Ti-6Al-4V also solidifydendritically or by other mechanisms. A model whichpredicts whether a liquid alloy will solidify in a planar,cellular or dendritic fashion is shown in Equation 110.

kDkmC

VG

S

)1(0 Equation 1

Where G temperature gradientVs - growth velocityM - liquidus gradientC0 - composition of the alloyk - partition coefficientD - diffusion coefficient in the liquid

This equation shows that it is a combination of thesolidification conditions (i.e. left hand side of

Equation 1) and alloy properties (i.e. right hand side of

Equation 1) that predict the growth morphology of thesolidifying phase. When the term G/Vs is minimized,the constitutional undercooling increases and there isan increased tendency for dendritic growth. C0, m, k


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and D are properties of the alloy, and hence thesevariables will vary with the addition of Al, V and B.

The fact that the boron-containing casting had a visibledendritic morphology could be related to either achange in solidification structure or simply thedelineation of dendrites via the presence of aninsoluble phase. After consulting the phase diagrams inFigure 4 and Figure 5, it becomes apparent that in the

case of the boron addition, the right side of Equation 1is infinitely larger than the equivalent calculation foraluminium and vanadium additions used in this study.This is because the partitioning coefficient, k, for boronin titanium approaches 0, whereas those of aluminiumand vanadium in titanium approach 1. This does not

however discount the possibility of a dendritic growthmorphology in the aluminium and vanadium

containing castings because the G/Vs term hasnt beenconsidered. The obtained temperature measurementdata indicate that the castings were subject to very high

initial cooling rates, in the order of several hundreddegrees celsius per second. The rapid cooling rateduring solidification may increase the thermal

undercooling and effectively reduce the G/Vs ratio,promoting a solidification instability and hence adendritic growth morphology. Furthermore, it is knownthat in pure metals it is possible to obtain dendritic

growth morphologies if the undercooling is sufficient11

and it is noted that trace impurities were present in all

alloys used in this study. In light of this, it is proposed

that all of our castings including CP Ti solidify via adendritic mechanism and that the addition of boronsimply allowed this structure to be visualised with ease.

A schematic showing the proposed solidification andmicrostructural evolution of cast titanium alloys ispresented in Figure 7.

5. CONCLUSION

A series of titanium castings were produced and theeffects of aluminium, vanadium and boron on the as-cast microstructure investigated. No difference orvariation in the microstructures could be determined

between the commercial purity (CP) titanium and theTi-6Al. However, the presence of vanadium altered the

microstructure significantly from the CP and Ti-6Alsamples. The addition of boron also changed the castmicrostructure. When boron was present in low

concentrations (0.135wt%), no prior- grain boundariescould be observed and an extensive network of boroncontaining particles delineated -Ti dendrites. Despite

being present above the -transus temperature these

intermetallics offered no barrier to the growth of the -phase. A model was presented and it was proposed

that all castings solidified via dendritic mechanisms.The absence of dendrites in the room temperaturemicrostructures of CP, Ti6Al and Ti6Al4V wasattributed to the high solubility of the alloy additionsand the lack of any segregated phases to delineate thedendritic structure.

Figure 7. Schematic of proposed microstructural evolution in titanium castings. In the presence of soluble elements a

dendritic structure is possible but no evidence of this structure exists at room temperature. Insoluble elements allowprior-dendrite arms to be visualised due to the presence of intermetallic precipitates. In both cases the morphology of

the dendrites will depend on both the thermal conditions during solidification and the alloy composition.


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References

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21(4), pp. 240-244.3. F.A. Crossley: SAMPE Journal, 1986, vol.

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4. ASM Handbook: Titanium and titaniumalloys,1990, vol. 15, pp. 824-835.

5. ASM Handbook: Titanium and titaniumalloys, 1990, vol. 9, pp. 458-475.

6. American Society for Metals: MetalsHandbook. 8th ed, 1972, vol. 7.

7. ASM Handbook : Alloy phase diagrams,1990,vol. 3.

8. J. Zhu, et al.: Materials Science &Engineering, 2003, Vol. A339, pp. 53-62.

9. S. Tamirisakandala, et al.: Scripta Materialia,2005, vol. 43, pp. 1421-1426.

10. M.C. Flemings: Solidification processing,McGraw-Hill, New York, 1974.

11. W. Kurz, and D.J. Fisher: Fundamentals ofsolidification, 3rd ed, Trans Tech PublicationsLtd, 1989.

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Microstructure of Cast Titanium Alloys