Materials Science and Engineering A 550 (2012) 97 102

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A

jo ur n al hom epage: www.elsev ier .co

Effect o mmeltin

J. Li, H.MLaboratory of L iversitBeijing 100191

a r t i c l

Article history:Received 3 FebReceived in reAccepted 8 ApAvailable onlin

Keywords:Rene41Laser melting Standard heatMechanical pr

ced bd at ostrug elecweretility.gnme

of th stan

be m

1. Introduction

Rene41 is a nickel-base superalloy developed as a high-strength, high temperature material in the mid-1950s based onits good tensile strength and creep-rupture strength combinedwith an excIt has been[13]. Howwelding reticity and scasting prorapid solidwhich builddesign modings, since involved. Laimproved hrapid solidiical propertreported els

Rene41ening mechgamma primheat treatm

CorresponE-mail add

LMD Rene41 was solution treated at 1065 C for 4 h/air quenched toroom temperature and then aged at 760 C for 16 h/air cooled. Themicrostructure and mechanical properties of as-deposited, solutiontreated and solution-aged material were discussed in this paper.

0921-5093/$ http://dx.doi.oellent oxidation resistance at the range of 650900 C. widely used for hot section parts in aircraft enginesever, a high tendency of cracking during forging andstricts its use, which is due to its bad thermoplas-erious segregations during conventional slow-coolingcess. Laser melting deposition (LMD) is a layerwiseication material additive manufacturing technologys complex-shaped components from a computer-aidedel [4,5]. The technology offers considerable cost sav-extensive machining and welding processes are notser melting deposited Rene41 was preferred due to itsomogeneity and ner microstructure resulting from thecation cooling rate. The microstructure and mechan-ies of as-deposited alloy was previously studied andewhere [6].

is a precipitation-hardened superalloy, and its strength-anism depends on the size and the morphology ofe phase and carbides, both of which are affected by

ent. In order to improve the mechanical properties, the

ding author. Tel.: +86 10 8231 7102; fax: +86 10 8233 8131.ress: wanghm@buaa.edu.cn (H.M. Wang).

2. Experimental

The commercially available, vacuum non-contacting plasmamelting argon atomized Rene41 powders were used in thisstudy, and the detailed parameters of the LMD process have beenreported elsewhere [6]. The LMD alloy was given a solution treat-ment at 1065 C/4 h/air quenched to room temperature followedby an aging treatment at 760 C/16 h/air cooling. Metallographicsamples (two samples for each condition) were mechanicallypolished and etched in a chemical solution of 3 g CuSO4, 80 mlHCl, 20 ml absolute alcohol and were immersed in ammoniumperoxydisulfate:H2O solution in weight ratio of 1:20 for severalminutes to remove corrosion products. Microstructure of sampleswere characterized by JSM-5800, eld emission high resolutionscanning electron microscopy S4800 and Cambridge-S360 (SEM)equipped with League-2000 energy dispersive spectroscopy (EDS).As the quantitative metallographic investigation is basically astatistical measurement, a large number of images at differentmagnications were analyzed with only one of each is shown here.The carbon extraction replica technique was used to prepare sam-ples to identify the interdendritic phase precipitated during theaging treatment by JEM-2100F transmission electron microscopy(TEM). This technique consisted of a single-stage carbon extraction.

see front matter 2012 Elsevier B.V. All rights reserved.rg/10.1016/j.msea.2012.04.037f heat treatment on microstructure andg deposited Ni-base superalloy Rene41

. Wang , H.B. Tangaser Materials Processing and Manufacturing, Beihang University (formerly Beijing Un, China

e i n f o

ruary 2012vised form 29 March 2012ril 2012e 13 April 2012

deposition treatmentoperties

a b s t r a c t

Ni-base superalloy Rene41 was produThe LMD material was solution treatefor 16 h followed by air cooling. Micralloys were characterized by scanninthe tensile properties and hardness hardness, low strength and low ducand ductility, and the subsequent aliThe structureproperty relationshipsstudied and results indicated that thenot suitable for LMD alloy and had tom/locate /msea

echanical properties of laser

y of Aeronautics and Astronautics), 37 Xueyuan Road,

y the laser melting deposition (LMD) manufacturing process.1065 C for 4 h followed by air quenching and aged at 760 Ccture of the as-deposited, solution treated and solution-agedtron microscopy and transmission electron microscopy, and

tested. The solution-aged Rene41 superalloy showed high The extensive precipitation of resulted in high hardnessnt and coalescence of precipitates induced low strength.e alloy under three different heat treating conditions weredard heat treatment recommended for wrought Rene41 wasodied to realize optimum mechanical properties.

2012 Elsevier B.V. All rights reserved.

98 J. Li et al. / Materials Science and Engineering A 550 (2012) 97 102

Fig.

A carbon laetching proin order to measured ufor a dwell from eightperformed column tengauge lengtof three teexamined b

3. Results

3.1. Micros

The micconsisted osome ne Mareas. The MThe size ofthat in the ibeen discus

The solu and MC cchemical seto the intertreated allocontained uprecipitatedhas been reFig. 2 that 4dissolve allvisible undeof the un-dand tendedof the initiaof a similardier [8]. Acnear-equiaxtreatment. laser meltinas that of tgrains to eqabsent here

icrostructure of the as-deposited Rene41 superalloy after solution treat- 1065 C for 4 h and cooled in air: (a) un-dissolved particles and carbides,recipitated particles, (b) splitting of some coarse particles and hyper-recipitated particles.

ot easy to etch clearly, the solution and aged alloy revealedumnar grain morphology clearly.

solution treated alloy was given an ageing treatment. Fig. 3d the microstructure of the aged alloy consisting of parti-

different sizes, MC carbides and some other interdendritic which precipitated in the ageing treatment. The interden-hases were equiaxed or strip-shaped and had an average

of approximately 300600 nm (Fig. 3b and c). According composition and selected area diffraction (SAD) pattern 1. Microstructure of the as-deposited Rene41 superalloy.

yer is sputtered on the specimen surface and then ancedure (1HF:7HNO3 etching solution) was performedcollect the replicas. The hardness of the samples wassing a Vickers microhardness tester using a 1000 g loadtime of 18 s. The average hardness value was calculated

indentations. Room temperature tensile tests wereon a MTS880 universe tensile testing machine usingsile test specimens with a diameter of 5 mm and ah of 25 mm. The reported tensile data are an averagest results. The surfaces of the failed specimens werey SEM to study the fracture characteristics.

tructure

rostructure of the as-deposited alloy Rene41 (Fig. 1)f ne rapid directionally solidied columnar grains withC carbide particles distributed in the interdendriticC carbides were found to be enriched in Ti and Mo.

precipitates in the dendritic cores was larger thannterdendritic zones, and possible reasons for this havesed elsewhere [6].tion treatment has been applied to dissolve the coarsearbides formed during solidication and to reduce thegregation due to partitioning of some of the elementsdendritic regions. The microstructure of the solutiony presented in Fig. 2 revealed that the austenite matrix

Fig. 2. Mment atand re-pne re-p

were nthe col

Theshowecles inphasesdritic plengthto then-dissolved particles, MC carbides and ultrane re- particles. The solvus temperature for in Rene41ported to be 1065 C [7], however, it was clear from

h solution treatment at 1065 C was not sufcient to the coarse particles. The splitting of particles wasr higher magnications as indicated in Fig. 2b, and mostissolved particles possessed near-cubic morphology

to grow to split into smaller ones from their edgesl precipitate toward its center. A detailed description

splitting process phenomenon was given by Grosdi-cording to the micrograph and the EDS analyses, theed or script-like MC carbides decreased after solutionIn addition, the columnar grains which resulted fromg deposition process remained approximately the samehe as deposited alloy. Transformation from columnaruiaxed grains which was reported elsewhere [9] was. Although grain boundaries of the solution treated alloy

(Fig. 3d), it where M reM23C6 carbat 760 C fothere was ecarbon diffuis believed ture similarreplacing Mcontent is might be th

During asize, the (Fig. 4). Theshort chainquently (Figinterface ofcould be inferred that the particles were mainly M23C6,presented Cr and Mo. The precipitation of the discreteides was caused by carbide reactions during the agingr 16 h. During aging at 760 C for a relatively long time,nough time for the formation of M23C6 carbides throughsion and reaction with the Cr and Mo. Furthermore, itthat M6C carbides which have a complex cubic struc-

to M23C6 were inclined to form during aging partially23C6 when the Mo content is high [10]. Since the Mo910 wt% in Rene41 alloy, the interdendritic phasese mixture of M23C6 and M6C.ging at 760 C, in addition to an increase in their mean particles also underwent a change in morphology

cuboidal precipitates tended to group together ins or blocks of several particles and coalescence subse-. 4c and e). The TEM micrograph in Fig. 4d showed the

the and coarsened was covered by an interfacial

J. Li et al. / Materials Science and Engineering A 550 (2012) 97 102 99

Fig. 3. Micros umnacarbides, (c) b carbi

dislocation different co

3.2. Tensile

The roothe as-depoand a minitical materthat after sostrength (Yas-depositein both thewith the soshowed imand RA. Micin Table 2 rthe as-depohigh microhspecicatio

Table 1Size and morp

Material con

As-depositedLMD + soluti

LMD + solutitructure of LMD Rene41 superalloy after solution and ageing treatment: (a) the colright eld image showing the M23C6 carbides, (d) SAD pattern corresponding to thenetwork. Size and morphology of precipitates innditions were given in Table 1 for a clear comparison.

properties and hardness

m temperature tensile properties of alloy Rene41 insited, solution treated and solution-aged conditions

mum allowable specication for Rene41 as aeronau-ial [3] were given in Table 2. These results indicatedlution treatment, there was a drastic decrease in yieldS) and ultimate tensile strength (UTS) compared to thed material while substantial improvement took place

elongation (EL) and reduction in area (RA). In contrastlution treated alloy, alloy in solution-aged conditionprovement of YS and UTS, but considerable drop in ELrohardness of samples under three different conditionsevealed that the hardness varied obviously, and bothsited and the solution-aged alloy showed considerableardness values compared to the minimum allowable

n, with samples after solution-aged showed the highest

hardness. Instandard heto meet theand microh

3.3. Fractog

The fracthe solutiontreated matpled rupturfailure resp

In contrface of the cracks (Fig. (Fig. 6b), mo(Fig. 6c), thchains or blthe interfac

hology of precipitates in different conditions.

dition Volume fraction (vol.%) Average size or leng

2327 250 50 on treated 57 120170 (average s

ones after splitting)

on-aged (16 h) 7001700 (average chains or blocks)r grain morphology, (b) different sizes of particles, MC and M23C6des in (c). conclusion, LMD Rene41 alloy which experienced theat treatment recommended for wrought Rene41 failed

minimum allowable specication, with higher EL, RAardness but lower YS and UTS.

raphy and sub-surface of fracture

ture surfaces of the solution treated alloy as well as-aged material were examined by SEM. The solutionerial showed homogeneous deformation and fully dim-e (Fig. 5), suggestive of a transgranular ductile mode ofonsible for the high value of elongation of about 42.2%.ast to the solution treated alloy, the fracture sur-solution-aged material showed that lots of secondary6a) followed the columnar grain boundaries apparentlyreover, the surface contained considerable ne dimplese size of which is close to that of coalesced , that is,ocks of several . Note that the crack propagated alonge between and coalesced (Fig. 6d).

th (nm) Morphology

Big and small cuboidsize of small Some coarse particles are splitting

into smaller ones while some otherultrane precipitates

length of Short chains or blocks of severalparticles

100 J. Li et al. / Materials Science and Engineering A 550 (2012) 97 102

Fig. 4. Microgseveral particl

4. Discussi

The as-da Ni-base ascript MC cwhile someThe splittinticles or cuobserved inthe splittinresearcherswhile otherelastic anis[15]. Furtheduring coolmatrix. How

Table 2Mechanical pr

Material con

As-depositeLMD +1065

LMD +1065

Wrought +1

a The minimraphs of phases in alloy solution treated at 1065 C for 4 h and subsequently aged at 7es which was induced by the coalescence of adjacent precipitates, (d)16 h, and TEM m

on

eposited LMD Rene41 superalloy consisted mainly ofustenitic matrix, precipitate and near-equiaxed orarbides. After solution treatment, some dissolved

other coarse failed to dissolve and grew to split.g of coarse precipitates into a number of small par-boids during cooling or long time ageing has been

other Ni-base alloys [8,11,12] and the mechanism ofg phenomenon has not been completely clear. Some

[13,14] suggested it was an elastically driven process calculations showed the interface instability driven byotropy and diffusion eld generated particle splittingrmore, a small amount of ultrane re-precipitateding due to the reduction of solubility of Ti and Al in ever, no other types of carbides were detected after

solution treing the procthe relativeto the relat

The solument and sof growth oof adjacenttogether inlescence suraft-like mreported inthe mist bthe changeaging treatmthe release

operties and hardness of alloy Rene41 evaluated at room temperature.

dition 0.2%YS (MPa) UTS (MPa)

d 852 9.4 1117 35.1 C/4 h/AC 563 27.3 838 40.3 C/4 h/AC +760 C/16 h/AC 835 10.4 1083 30.8 065 C/4 h/AC +760 C/16 h/ACa [3] 880 1175

um allowable specication in manuscript.60 C for (a) 12 h, (b)14 h, (c) 16 h, showing short chains or blocks oficrograph showing dislocations at /coarsened interface, (e) 18 h.

atment though a portion of MC carbides dissolved dur-ess. This could be attributed to the high cooling rate andly sluggish precipitation dynamics of M23C6 comparedively fast continuous precipitation kinetics of phase.tion treated material then underwent an ageing treat-everal microstructure changes occurred. The processf precipitates seemed to occur through coalescence

particles, and the precipitates inclined to group short chains or blocks of several particles and coa-bsequently. The tendency for precipitates to formicrostructures in the absence of applied stress was

several alloys and was related to the magnitude ofetween particles and matrix [1619]. In addition tos of precipitates, M23C6 carbides precipitated duringent due to the dissolution of partial MC carbides and

of C to the matrix.

%EL %RA Hardness (HV)

21 2.5 22.7 1.5 426.58 8.542.2 6.8 38.7 6.3 261.63 25.212 4.1 20.8 8.2 448.8 10.712 12 400

J. Li et al. / Materials Science and Engineering A 550 (2012) 97 102 101

Fig. 5. Fractographs of the solution treated Rene41 superalloy tested at room tem-perature: (a) overall view and (b) high magnication view showing dimple-ductilefracture.

The tensile properties and hardness of the alloy in three differentconditions were compared to correlate the fracture characteris-tics with microstructure. The low strength values and hardnessassociated with high ductility of the solution treated alloy wereobviously due to the dissolution of during solution treatment.The matrix became much softer and the softer matrix could relaxthe stress concentration at the interface of two phases which wasbelieved to be the origin and propagation paths of cracks to frac-ture. This delayed the fracture and allowed the material to deformwell and nally led to a transgranular mode of failure. It couldbe seen that the YS and UTS of the alloy were partially increasedthrough ageing treatment but were still lower than that of the as-deposited alloy. Moreover, the ageing treatment brought a hugedrop to the EL and RA. The improvement in the YS and UTS wasattributed to the extensive precipitation of , which also resultedin the increase of hardness. However, particles aligned parallelto each other to minimize mist stress and coalesced subsequentlyto reduce interface area. Meanwhile, the alignment and coars-ening described above widened matrix corridors and facilitatedthe movement of dislocation [16]. Coherency of interfacebecame lost when the particles attained a certain size, and theenergy associated with the strained interface became too high,thus the original ordered interface was replaced by a disorderedinterface and interfacial dislocation networks formed. Generally,strength of alloy was governed by the inverse of interparticlespacing [20]. The increase of spacing between the coarsened

resulted in low strength. The deformation was localized at theinterfaces and decohesion contributing to microvoid nucleationoccurred. From fracture surface observations, it could be seen thatthe morphology consisted of ductile regions with considerable nedimples.

Fig. 6. Fractographs of the solution-aged Rene41 superalloy tested at room temperature: (a) overalsecondary cracks follow the columnar grain boundaries, (c) high magnication view showing considerpropagated along the interface between and coalesced .l view showing lots of secondary cracks and (b) revealing that theable ne dimples, (d) sub-surface of fracture revealing that the crack

102 J. Li et al. / Materials Science and Engineering A 550 (2012) 97 102

5. Conclusions

The extensive precipitation of particles during heat treat-ment resulted in high hardness and ductility, but the subsequentalignment and coalescence of precipitates induced decrease instrength. This suggests that the recommended standard heat treat-ment for wrought Rene41 alloy is not suitable for the LMD alloyfor optimum mechanical properties.

Acknowledgements

This research was supported by National Natural Science Foun-dation of China (Grant No. 50671004), National Outstanding YouthScience Foundation of China (Grant No. 50625416), Cheung KongScholars Innovative Research Team Program of Ministry of Educa-tion (Grant No. IRT0805), State Key Basic Research Program of China(Grant No. 2006CB605206).

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Effect of heat treatment on microstructure and mechanical properties of laser melting deposited Ni-base superalloy Rene411 Introduction2 Experimental3 Results3.1 Microstructure3.2 Tensile properties and hardness3.3 Fractography and sub-surface of fracture

4 Discussion5 ConclusionsAcknowledgementsReferences