A new tungsten-free c –c ’Co–Al–Mo–Nb-based superalloy
S.K.Makineni,B.Nithin and K.Chattopadhyay
?
Department of Materials Engineering,Indian Institute of Science,Bangalore 560012,India
Received 14September 2014;revised 7November 2014;accepted 9November 2014
Available online 29November 2014
We present the ?rst report of a tungsten-free cobalt-based superalloy having a composition Co–10Al–5Mo–2Nb.The alloy is strengthened by cuboidal precipitates of metastable Co 3(Al,Mo,Nb)distributed throughout the microstructure.The precipitates are coherent with the face-centred cubic c -Co matrix and possess ordered L12structure.The microstructure is identical to the popular c –c ’type nickel-based superalloys and that of recently reported Co–Al–W-based alloys.Being tungsten free,the reported alloy has higher speci?c proof stress compared to existing cobalt-based superalloys.
ó2014Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.
Keywords:Cobalt-based superalloy;c –c ’microstructure;Solvus temperature;Density;Speci?c yield strength
Central to the development of nickel-based superal-loys for high-temperature applications is the precipitation of coherent Ni 3Al intermetallic compound (c ’)having ordered L12structure in the face-centred cubic (fcc)nickel matrix (c )[1–5].The precipitates and the low-energy c –c ’interface provide high-temperature strength and stability to these alloys.In the Co–Al system,the phase Co 3Al does not exist,while in the Co–Ti system,a compound Co 3Ti with ordered L12structure (c ’)has been reported.However,this phase is not stable at high temperature (>750°C)[6,7].Therefore the report on coherent precipitation of an ordered c ’(Co 3(Al,W))in cobalt matrix in a ternary alloy Co–9.2Al–9W that is stable at high temperature (990°C)by Sato et al.[8]opened up new possibilities for developing cobalt-based superalloys with c –c ’structure.Several subse-quent reports have dealt with the in?uence of additional alloying elements on the stability,microstructure and prop-erties of this new class of alloys [9–15].Study of the mechanical properties of these alloys,particularly at higher temperatures,indicates early promise [16–18].Further,the creep properties are found to be superior to those of exist-ing cobalt-based alloys and comparable to that reported for nickel-based superalloys [19–21].Recently,the oxidation properties of these “W ”-containing alloys have attracted increasing attention [22–26].These studies have indicated the possibility of the formation of protective oxide coatings at high temperatures.Hence Co–Al–W-based alloys are exceptional in terms of stability and high strength com-pared to other cobalt-based superalloys.
The main drawbacks of the present Co–Al–W-based alloys are their high density and relatively poorer ductility
compared to nickel-based superalloys.Since tungsten is the main alloying addition,the speci?c strengths of these alloys are low and hence not suitable for applications such as in aerospace industries where a good strength to weight ratio is crucial.The densities of tungsten-bearing cobalt-based superalloys vary from 9.3to 10.5gm cm à3whereas the nickel-based superalloys have densities in the range of 7.9–8.5gm cm à3[10,27].In addition,the presence of tung-sten makes homogenization of cast Co–Al–W alloys di?-cult.Therefore,although promising,Co–Al–W alloys have to overcome severe hurdles to replace nickel-based superalloys even in niche applications.
The present work reports a new c –c ’cobalt-based super-alloy which is free of tungsten.This alloy has much lower density and exhibits high speci?c yield strength when com-pared to existing cobalt-based superalloys including Co–Al–W alloys.After heat treatment,c ’precipitates in this alloy having ordered L12structure are uniformly distrib-uted throughout the fcc c -Co matrix.
Alloy of composition Co–10Al–5Mo–2Nb (at.%)was arc melted under argon in the form of a 30g ingot on a water-cooled copper hearth using a laboratory-scale vac-uum arc melting unit.The alloy was melted 10–12times to obtain a homogenized alloy composition throughout the ingot.Subsequently,the melt was cast into 3mm rods using a vacuum arc suction casting unit equipped with a water-cooled split copper mould.The rods were further solutionized at 1300°C for 15h under vacuum (10à5mbar)followed by quenching in cold water.Small samples from the solutionized rods were cut and sealed in vacuum quartz tubes.The sealed samples were then aged at 800°C for 2h in a box furnace and subsequently furnace cooled.For transmission electron microscopy (TEM)sample preparation,3mm disks from the aged sample were
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cut and mechanically polished to a thickness of$80l m. The?nal samples were prepared by using a twin-jet electropolishing unit with a solution of methanol and5 vol.%perchloric acid atà30°C.Di?erential scanning calorimetry(DSC)was used to determine the solvus temperature of the aged sample under argon with a heating rate of10°C minà1.The hardness of the sample was measured using a Vickers microhardness tester with a load of0.5kg.
Compression tests to evaluate the0.2%proof stress of the alloy were performed using a DARTEC hydraulic machine operated with a strain rate of10à3.The density of the alloy was measured in accordance with ASTM stan-dard B311-08[28]at room temperature while the volume fraction of c’precipitates in the aged alloy was measured using ASTM standard E562-11[29].The evolution of the microstructure of the aged samples was further investigated by TEM using a FEI F30microscope equipped with a?eld emission gun.Analysis of the composition of the precipi-tates was carried out by energy-dispersive X-ray spectrom-etry performed in the microscope operating in STEM mode (nanoprobe).
A TEM di?raction pattern taken in the[001]zone axis from the aged Co–10Al–5Mo–2Nb alloy is shown in the inset of Figure1a.The di?raction pattern shows the pres-ence of superlattice re?ections along with the main matrix re?ections,con?rming L12ordering in the fcc c-Co matrix. Figure1a shows a dark-?eld micrograph taken from the 100superlattice re?ection in the[001]zone axis,revealing cuboidal precipitates with L12ordering.The mean size of these precipitates is40±20nm and they are distributed homogeneously throughout the c-Co matrix.The precipi-tate morphology and distribution is analogous to the L12 ordered precipitates found in Co–Al–W alloys and nickel-based superalloys.Figure2shows the composition pro?le across the c–c’interface.The total content of molybdenum and niobium is around13.6at.%in the c’precipitates while aluminium is about10.4at.%,yielding a total of24.1at.% with the rest being cobalt.This composition is close to the stoichiometry Co3(Al,Mo,Nb).From the composition pro-?le we observe that the aluminium content is almost uni-form in both c-Co matrix and c’precipitate.However, molybdenum and niobium are de?cient in matrix and richer in ordered precipitates.
DSC measurements were performed for one of the aged samples weighing$85mg which was melted in an alumina crucible with a heating rate of10°C minà1under?owing argon to determine the incipient melting temperature of the alloy.Figure3shows the DSC heating curve for the aged sample.The incipient melting point of the alloy was found to be1315°C.An additional endothermic peak also appears at lower temperature,which may correspond to the solvus temperature of the alloy where dissolution of c’pre-cipitates into the c-Co matrix takes place.In order to verify this,samples with similar weight were heated at a rate of 10°C minà1in a platinum crucible to100°C below the incipient melting temperature and subsequently cooled at the same rate.The inset of Figure3shows the cooling curve.A well resolved exothermic peak appears corre-sponding to the solvus temperature of the alloy at866°C. The Vickers hardness of the aged sample was found to be 390HV at a load of0.5kg.The measured density of the alloy is8.36g cmà3.The value is comparable to those of
(a) (b)
500 nm 010
10 0 [001
]
500 nm
Figure1.TEM dark-?eld image taken from the100superlattice L1 ordered spot in the matrix[001]zone axis di?raction pattern(shown in inset)for:(a)Co–10Al–5Mo–2Nb alloy peak aged at800°C for2 and subsequently furnace cooled;and(b)Co–30Ni–10Al–5Mo–2Nb alloy aged at900°C for100h and furnace cooled.
S.K.Makineni et al./Scripta Materialia98(2015)36–3937
nickel-based superalloys and is much lower than that the alloy Co–9Al–9.8W,which has a density of9.82g cmà3. Figure4a shows a comparison of the densities of some commercially available cobalt-based superalloys and Co–9Al–9.8W alloy with the present alloy(Co–10Al–5Mo–2Nb).Figure4b shows stress–strain plots of the com-pression test for both Co–9Al–9.8W(processed through the identical route adopted in the present case and aged according to Ref.[8])and the present Co–10–Al–5Mo–2Nb alloy.The inset table reports values of0.2%proof stress and speci?c0.2%proof stress for both alloys at room temperature.For Co–9Al–9.8W alloys the0.2%proof stress is$780MPa,while for Co–10Al–5Mo–2Nb alloy the value is720MPa.Thus,the room temperature0.2% proof stress for Co–10Al–5Mo–2Nb alloy is comparable to that of Co–9Al–9.8W alloy.The speci?c0.2% proof stress for Co–10Al–5Mo–2Nb is estimated to be 86.1MPa gà1cmà3,i.e.higher than79.4MPa gà1cmà3 obtained for Co–9Al–9.8W alloy.
As mentioned earlier,in the phase diagram of Co–Al [30],no phase of the type Co3Al exists.However,Co3Al phase with metastable c’structure has been reported in Co–14%Al[31,32].Ishida has estimated the critical disso-lution temperature for c’to be870°C[34].However,uni-form distribution of L12ordered c’phase could not be observed in these alloys[33].Instead one observes discon-tinuous precipitation of CoAl phase in fcc matrix[35–37]. Therefore,the observation of Sato et al.[8]that W addition stabilizes the c’phase with L12ordered structure is of great interest.In the Co–Mo and Co–Nb phase diagrams[30],a phase with stoichiometry Co3X(X=Mo,Nb)exists.How-ever,these phases exhibit DO19structure(ordered hexago-nal).Since molybdenum belongs to the same group as tungsten in the periodic table,it is expected that replace-ment of tungsten with molybdenum should also lead to the formation of ordered L12precipitate of the type Co3(Al,Mo)in Co–Al–Mo alloys.Co–Al–Mo alloys have indeed been investigated[38]and it was found that ordering of Co3(Al,Mo)with L12structure does not occur.On aging these alloys yield CoAl phase with B2structure and Co3Mo phase with DO19structure.Therefore,addition of Nb plays a crucial role in stabilizing the L12structure in our alloy. This results in a uniform distribution of c’phase in c matrix across the sample and no other phase was visible.
For high-temperature application,the stability of the c’structure is of importance,and this also depends on solvus temperature.The Co–9Al–9.8W alloy has a solvus temperature of990°C.Di?usion couple experiments have shown that c’decomposes at900°C at longer exposure times[39,40].The solvus temperature of the current alloy is866°C.Long-term aging at800°C indicates that c’starts decomposing at35h of exposure.This shows that Co3(Al,Mo,Nb)c’precipitates are metastable in nature. However,addition of Ni which replaces Co can dramati-cally change the solvus temperature(990°C for30at.% Ni addition)and improves the stability.This has been shown previously for Co–Al–W alloys[41,42].Figure1b shows a dark-?eld micrograph taken from the100 superlattice spot in the[001]zone axis from an alloy where an equivalent amount of Co in the original alloy has been replaced by30%Ni.After100h of aging at900°C the alloy shows no sign of decomposition of the ordered c’phase. Our initial results suggest that the nickel-added alloys exhi-bit0.2%proof stress under compression in excess of >500MPa at870°C.Detailed investigation on these aspects is under progress and will be reported elsewhere [45].
In conclusion,the present paper reports the synthesis of a new cobalt-based alloy with addition of2%Nb along with5%Mo to Co–10Al.The alloy shows c–c’microstruc-ture after heat treatment and exhibits substantial precipita-tion hardening.The alloy has a lower density compared to existing cobalt-based superalloys,including Co–Al–W-based alloys,and may be a low-density alternative to Co–Al–W alloy.Further study of this alloy,including mechan-ical properties and alloying additions to increase the solvus temperature,is in progress.
The authors would like to acknowledge the microscope facility available at the Advanced Facility for Microscopy and Microanalysis(AFMM)centre,Indian Institute of Science,Ban-galore.One of the authors acknowledges the?nancial support from Department of Science and Technology in the form of a J.C.Bose national fellowship.
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