Solution Heat-treatment of Nb-Modified MAR-M247 Superalloy

M A T E R I A L S C H A R A C T E R I Z A T I O N 7 5 ( 2 0 1 3 ) 2 1 4 – 2 1 9

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Solution heat-treatment of Nb-modified MAR-M247 superalloy

Paulo Ricardo Soares Azevedo e Silvaa, Renato Baldana,⁎, Carlos Angelo Nunesa,Gilberto Carvalho Coelhoa, b, Alex Matos da Silva Costaa

aUniversidade de São Paulo (USP), Escola de Engenharia de Lorena (EEL), Departamento de Engenharia de Materiais (DEMAR),Pólo Urbo-Industrial Gleba AI-6, Caixa Postal 116, 12600-970, Lorena, São Paulo, BrazilbUniFoa, Centro Universitário de Volta Redonda, Núcleo de Pesquisa, Campus Três Poços, Avenida Paulo Erlei Alves Abrantes, 1325,Bairro Três Poços, 27240-560, Volta Redonda, Rio de Janeiro, Brazil

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +55 12 3159 991E-mail addresses: [email protected]

(C.A. Nunes), [email protected] (G.C. C

1044-5803/$ – see front matter © 2012 Elseviehttp://dx.doi.org/10.1016/j.matchar.2012.11.00

A B S T R A C T

Article history:Received 5 March 2012Received in revised form 26 August2012Accepted 15 November 2012

MAR-M247 superalloy has excellent mechanical properties and good oxidation resistance atelevated temperatures. Niobium is an element known as γ′ phase hardener in nickel-basedsuperalloys, besides promoting homogeneous distribution of MC carbides. This work isinserted in a project that aims to evaluate the total replacement of tantalum by niobiumatoms in MAR-M247 superalloy (10.2 Co, 10.2 W, 8.5Cr, 5.6 Al, 1.6 Nb, 1.4 Hf, 1.1 Ti, 0.7 Mo,0.15 C, 0.06 Zr, 0.015 B, Ni balance—wt.%). Based on microstructural characterizations (SEMand FEG-SEM, both with EDS) of the as-cast material and heat-treated materials as well asutilizing Thermocalc simulations and experiments of differential thermal analysis (DTA),heat-treatment at 1260 °C for 8 h was chosen as an ideal condition for the solution ofNb-modified MAR-M247 superalloy. The hardness of as-cast and ideally solution treatedmaterials was 390±14 HV and 415±6 HV, respectively.

© 2012 Elsevier Inc. All rights reserved.

Keywords:MAR-M247Solution heat-treatmentNiobiumNickel-based superalloyThermocalc

1. Introduction

Solution heat-treatments are common in nickel-based superal-loys and play a key role for the application of thesematerials athigh temperatures [1,2]. The objectives of this heat-treatmentare the complete solutioning of γ′ phase in the γ matrix andminimizing the segregation fromnon-equilibriumsolidificationin preparation for the aging heat-treatment [3]. Then, by agingheat-treatments, it is possible to achieve a controlled γ′ phasere-precipitation, producing a uniform and isotropicmaterial [4].

Theoretically, the solution heat-treatment should be carriedout between γ′ solvus and solidus temperatures in nickel-basedsuperalloys, as described by Semiatin et al. [5]. However,segregation from the non-equilibrium solidification limits thereal ‘window’ of treatment due to incipient melting below thesolidus temperature of the alloy. This ‘window’ is usuallybetween 100 and 200 °C [6].

2; fax: +55 12 3153 3006..br (P.R.S. Azevedo e Silvaoelho), [email protected].

r Inc. All rights reserved.6

Although the primary γ′ precipitates dissolve quickly inthe γ matrix, a substantial time is required for the dissolutionof γ/γ′ eutectic regions and to minimize the segregation of thematerial, because these materials usually have several heavyalloy elements that diffuse slowly in the matrix [7]. Typicalsolution heat-treatments for conventional MAR-M247 super-alloy are carried out at 1230 °C for 2 h and 1250 °C for 4 and5 h, as published by Kattus [8], Gasko et al. [9] and Baldan [10],respectively.

According to literature, niobium is a refractory element thatis normally present in the composition of nickel-based super-alloys, found mainly in the γ′ phase (replacing aluminum) andin the formation of different types of carbides at the grain-boundaries [11,12]. Considering solution heat-treatment, niobi-um increases γ′ solvus temperature, avoiding the dissolution ofthis phase at lower temperatures. Additionally, niobium pro-motes the hardener effect of γ′ phase at higher temperatures by

), [email protected] (R. Baldan), [email protected] (A.M.S. Costa).

215M A T E R I A L S C H A R A C T E R I Z A T I O N 7 5 ( 2 0 1 3 ) 2 1 4 – 2 1 9

increasing its volume fraction and thermal stability [13]. For thisand other reasons, about 50% of casting nickel-based superal-loys has between 1.0 and 2.5 wt.% of niobium in its composi-tion, as reported by Sims [14].

In this work, an Nb-modified MAR-M247 superalloy wasproduced by the total replacement of tantalum atoms byniobium atoms, taking into account the chemical similarity ofboth elements and the lower cost of niobium. The bestcondition for solution heat-treatment of this superalloy wasevaluated through microstructural characterizations of theas-cast and heat-treatedmaterials, correlating and comparingthe results with experiments of differential thermal analysisand simulations with Thermocalc software.

Fig. 1 – a and b — Simulation of the number of mols' phasebetween 600 and 1400 °C for Nb-modified MAR-M247superalloy.

2. Materials and Experimental Procedure

The conventional MAR-M247 superalloy was modified by thetotal replacement of tantalum by niobium atoms, the compo-sition of the new Nb-modified MAR-M247 superalloy is asfollows (wt.%): 10.2 Co; 10.2W; 8.5 Cr; 5.6 Al; 1.6 Nb; 1.4 Hf; 1.1 Ti;0.7 Mo; 0.15 C; 0.06 Zr; 0.02 B; Ni balance. The alloywas preparedin a vacuum inductionmelting furnace and cast into cylindricalrods (15 mm diameter×130 mm length) at Açotécnica Compa-ny (Jandira, São Paulo, Brazil).

After cutting the bars using electro-discharge machining(EDM), samples for solution heat-treatmentswere encapsulatedin quartz tubes under argon atmosphere and heat-treated in atube furnace at 1185, 1240, 1260, 1280 and 1300 °C for 2, 4 and8 h, followed by air cooling. A K-type thermocouple was placednear the sample to determine the real temperature of thesample. The temperature of the heat-treatments was chosenfrom simulations with Thermocalc software (TTNI8 database),based on data for the liquidus, solidus and γ′ solvus temperatures.

For microstructural characterization the samples wereprepared following conventional metallographic techniquesand etched with a glyceregia solution (1/6 HNO3, 2/6 HCl, 3/6glycerin, vol.%). The microstructural analyses were done withthe aid of a conventional SEM LEO 1450VP model and aFEG-SEM JEOL JSM-7001F model, both with EDS detectors. Thehardness values of all samples were determined using aBuehler Micromet equipment with a load of 500 gf for 30 s.

Differential thermal analysis (DTA) experimentswere carriedout in order to determine the γ′ solvus, incipient melting andliquidus temperatures of the as-cast Nb-modified MAR-M247superalloy. The experiments were performed in a LABSYS DTA/DSC Setaram Instrumentation device. Cylindrical samples with200 mg each were heated in an alumina crucible from 25 up to1500 °C with heating rate of 20 °C/min in argon flux. Thereference sample was an empty alumina crucible. These resultswere compared with those from the Thermocalc simulations.

3. Results and Discussion

Fig. 1a shows the simulation of number of mols of phasebetween 600 and 1400 °C for Nb-modified MAR-M247 superal-loy. The interpretation of these results was based on the workof Saunders et al. [15]. It can be seen that the volume fractionof γ′ phase decreases and that of γ phase increases as the

temperature increases. Additionally, the alloy has γ and MCcarbides precipitating almost simultaneously at 1360 °C.Finally, the ideal region for solution heat-treatment (betweenthe γ′ solvus and solidus temperatures) of the alloy is athree-phase region composed by γ, MC carbide and MB2

boride phases and the temperature range is between 1210and 1310 °C. Fig. 1b shows the amount of minor phases fromThermocalc simulations for the same temperature range. It canbe seen that MC carbide is stable and does not decompose inthe whole temperature range. In addition, the M6C and M23C6

carbides may form in specific temperature ranges.Fig. 2 shows theDTA thermogram from as-cast Nb-modified

MAR-M247 superalloy. The interpretation of these results wasbased on thework of Sponseller [16]. It is possible to observe thepresence of three major regions. The one in blue refers to γ′phase dissolution and does not show clear peak since the γ′phase dissolves gradually in γ over awide range of temperature.However, the γ′ dissolution final temperature is importantbecause it is associated with γ′ solvus temperature (number 1).

Fig. 2 – Differential thermal analysis (DTA) of as-castNb-modified MAR-M247 superalloy. (For interpretation of thereferences to color in this figure, the reader is referred to theweb version of this article.)

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The region represented in green should refer to the simulta-neous melting of γ and/or MC carbides. The temperature of thebeginning of this event is associated with incipient melting(number 2) and it is lower than the solidus temperature of thematerial due to the segregation. Finally, the region representedin red refers either to the melting of the remaining γ or MCcarbides. Possible thermal events associated to the formation ofthe minor phases indicated in Fig. 1b were not detected in theDTA thermogram.

Table 1 shows the values of γ′ solvus, incipient melting,solidus, MC or γ melting and liquidus temperatures obtainedwith Thermocalc simulations and our DTA results from bothas-cast Nb-modified and conventional MAR-M247 superal-loys. The liquidus and γ′ solvus temperatures calculated withThermocalc and those found in DTA experiments are in goodagreement. The incipient melting temperature of 1281 °C

Table 1 – Thermocalc simulations and temperatures ofthermal events from DTA for Nb-modified and conven-tional MAR-M247 superalloys, both in the as-castcondition.

Event Nb-modifiedMAR-M247superalloy

ConventionalMAR-M247superalloy

Thermocalc(°C)

DTA(°C)

Thermocalc(°C)

DTA(°C)

γ′ Solvus 1210 1238 1222 1225Incipient melting(γ or MC)

– 1281 a – 1340

Solidus 1310 – 1315 –MC or γ melting – 1351 – –Liquidus 1360 1370 1380 1343

a Sample heat-treated at 1280 °C showed incipient melting.

Fig. 3 – Micrographs of as-cast Nb-modified MAR-M247superalloy: (a) Dendritic microstructure from solidification;and (b) γ/γ′ eutectic in the interdendritic region as well as γ′phase precipitated in γ matrix.

found in the DTA experiments of Nb-modified MAR-M247superalloy was confirmed by microstructural characterizationof samples heat-treated at 1280 °C, as presented below. Bycomparing the results of Thermocalc simulations and DTAexperiments (Table 1), it is possible to observe that the γ′ solvus,solidus and liquidus temperatures do not change significantly,but the incipient melting temperature of 1340 °C for conven-tional as-cast MAR-M247 is higher than the temperature of1281 °C for Nb-modified MAR-M247 superalloy.

Fig. 3 shows the as-cast microstructure of Nb-modifiedMAR-M247 superalloy. There can be seen a segregated dendriticmicrostructure (Fig. 3a) with primary dendrite arm spacing ofabout 50 μm. Additionally, there are γ/γ′ eutectic regions andMC carbideswith high aspect ratio in the interdendritic regions.The borides, predicted by Thermocalc, were not observedprobably due to their very small quantity and size.

Tables 2 and 3 show, respectively, EDSmeasurements fromthe dendritic/interdendritic regions and MC carbides for bothconventional and Nb-modified MAR-M247 superalloys. Theresults have shown that Ni, Co and Cr do not have strongpartition between the dendritic and interdendritic regions.However, W is found mostly in the dendritic regions. Themain elements forming the carbides are Ti, Hf, W and Nb for

Table 2 – EDS measurements from the dendritic/interdendritic regions (wt.%).

Element Nb-modified MAR-M247 Conventional MAR-M247

Dendritic regions Interdendritic regions Dendritic regions Interdendritic regions

Ni 63.1±0.5 65.9±1.2 59.7±0.7 62.9±1.0Co 11.8±0.1 10.6±0.6 9.7±0.3 9.3±0.2W 11.7±0.2 7.0±0.6 14.3±0.3 8.6±1.8Cr 8.6±0.2 8.9±0.9 8.2±0.6 9.1±0.4

Table 3 – EDSmeasurements from the MC carbides (wt.%).

Element ConventionalMAR-M247

Nb-modifiedMAR-M247

Ta 52.5±2.7 –Nb – 45.4±4.6Hf 16.7±0.6 17.0±3.3W 14.0±4.8 18.8±3.4Ti 10.5±1.9 15.0±4.5


Nb-modified and Ti, Hf, W and Ta for conventional MAR-M247superalloy.

Heat-treatments carried out at 1185 and 1240 °C have notshown any importantmodification of the as-castmicrostructure

Fig. 4 – Micrographs of Nb-modified MAR-M247 superalloy heat-t8 h; and (d) 1280 °C for 2 h.

for treatments of reasonable duration. Fig. 4 shows the micro-structure of Nb-modified MAR-M247 superalloy heat-treated at1260 °C for 2, 4 and 8 h and 1280 °C for 2 h. It can be seen thatincreasing the time of heat-treatment reduces the segregationand the size of carbides. In the material heat-treated at 1260 °Cfor 2 h (Fig. 4a) it is possible to note the presence of dendritic andinterdendritic remaining regions, even though they are muchsmaller than those in the as-cast material (Fig. 3a). After 4 h ofheat-treatment at 1260 °C (Fig. 4b) these regions are notobserved, but the chemical gradient is not eliminated. Thematerial heat-treated at 1260 °C for 8 h (Fig. 4c) has shown a veryhomogeneous γ matrix and thus, was chosen as the idealsolution heat-treatment condition for Nb-modified MAR-M247superalloy.

reated at: (a) 1260 °C for 2 h; (b) 1260 °C for 4 h; (c) 1260 °C for

Fig. 6 – Micrographs of Nb-modified MAR-M247 superalloy

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At higher temperatures of treatment, such as 1280 °C for2 h, the material showed incipient melting evidences in theinterdendritic regions (Fig. 4d).

Fig. 5 shows in detail the region of incipient melting, whichis characterized by the presence of large cluster of carbides witha γ′ film, besides a shrinkagehole caused by re-solidification.Allsamples heat-treated at 1280 and 1300 °C have shown incipientmelting regions.

Fig. 6 shows micrographs obtained in a FEG-SEM of as-cast(interdendritic region) and heat-treated material after idealsolutioning condition (1260 °C for 8 h). It is possible to observethe presence of cubic γ′ precipitates with approximately 0.5 μmin the dendritic region of the as-cast material (Fig. 6a) and with0.1 μm in the heat-treated material (Fig. 6b). The primary γ′particles in the as-cast material are the combination of smallcubic particles. However, at the temperature of solution heat-treatment, primary γ′ phase begins to break down and dissolvesgradually into a small pieces known as secondary γ′ up to thecomplete γ′ phase dissolution, obtaining a homogeneous mate-rial composed by the γ matrix, MC carbides and MB2 borides(according to Thermocalc simulations). The γ′ precipitates presentin the solutionmaterial are formed via solid state reaction duringair cooling from the temperature of heat-treatment to roomtemperature, associated to a decrease of aluminumand titaniumsolubility in γ.

Fig. 7 shows the hardness of Nb-modified MAR-M247superalloy after the different solution heat-treatments. Thehardness of as-cast and after ideal solution heat-treatmentNb-modified MAR-M247 superalloy (1260 °C for 8 h) is 390±14 HV and 415±6 HV, respectively. It shows that the hardnessdid not vary significantly between the as-cast and heat-treated samples.

obtained in a FEG-SEM showing the γ′-phase precipitated inγ matrix: (a) dendritic region of as-cast material; and (b) aftersolution heat-treatment (1260 °C for 8 h).
4. Conclusions
The experimental results of DTA andmicrostructural character-izations of Nb-modified MAR-M247 superalloy showed a goodagreement with Thermocalc simulations. It was possible to

Fig. 5 – Micrograph of Nb-modified MAR-M247 superalloyheat-treated at 1280 °C for 2 h showing incipient meltingregion.

determine an ideal condition for solution heat-treatment ofNb-modified MAR-M247 superalloy (1260 °C for 8 h). The micro-structure of the material heat-treated in this condition hasshown fragmented carbides and a homogeneous γ matrix with

Fig. 7 – Average hardness of Nb-modified MAR-M247superalloy after solution heat-treatments.


fine γ′ precipitates (approximately 0.1 μm). It was an observedevidence of incipientmelting in samples heat-treated at 1280 °C.The hardness of as-cast material is 390±14HV and 415±6 HVafter ideal solution heat-treatment.

Acknowledgment

The authors are grateful to the ProgramPró-Engenharias—CAPES(984/2008), the FAPESP (2010/05211-1), the CNPq (476005/2010-8),the Açotécnica Company (Jandira, São Paulo, Brazil) and to theInstituto Superior Técnico, (Lisbon, Portugal) for the use of theirFEG-SEM facilities. Scholarships from FAPESP and CAPES, 2009/17337-2 and 141857/2009-9, respectively, are also appreciated.

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Solution Heat-treatment of Nb-Modified MAR-M247 Superalloy

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