Dendritic growth of primary c9 precipitates innickel based alloy
K. Zhao*1 and L. H. Lou2
Crystal growth from all three states of matter, i.e. vapour, liquid and solid, produces the dendritic
morphology. A well known vapour case is the snowflake. Most liquid metal products solidify by
forming dendrites. Crystal growth from an already crystalline matrix is also found to produce
precipitates with a dendritic morphology. However, dendritic precipitates were scarcely observed
to form from liquid state. In the present paper, a directionally solidified nickel based superalloy
was designed to show the dendritic growth of precipitates from the liquid state. The formation of
carbides led to the negative temperature gradient and constitutional supercooling in the residual
liquid melts as a result of the release of latent heat and ejecting of forming precipitate elements
respectively. This resulted in the formation of dendritic primary precipitates.
Keywords: Nickel based superalloy, Precipitate, Dendrite
According to the precipitation behaviour, c9 phase canbe classified into two types: primary c9 grows from liquidmetals, and secondary c9 precipitates grow from thesolidified c matrix.1 The growth morphology of c9 hasbeen proposed to progress from spheres to cubes, todoublet of plates, to octet of cubes and then to thinplates.2 In addition, dendritic secondary c9 precipitateshave been observed in specimens subjected to heattreatment,3–9 and their development has been thought asa very natural phenomenon.10 Dendritic growth hasbeen thought to occur only when a diffusion processdominates the rate at which the phase transformationproceeds.11
The dendritic c9 precipitate, by virtue of its extendedsurface, has a considerably increased surface free energyand is therefore thermodynamically unstable comparedwith the equilibrium shape. The origin of the dendritemust therefore result from the growth kinetics. Thedevelopment of dendritic secondary c9 precipitates hasbeen well elucidated by Grosdidier et al.7 The first stageoccurs in a highly saturated matrix, and the c/c9 interfaceis still coherent. Therefore, although the growthmechanism forces the eight branches of the octodendriteto extend in the n111m directions, the dendrite armspresent steps along the {001} planes to accommodatethe elastic strain. When the dendrite arms are sufficientlyaway from one another, secondary and even ternarybranching occur. As growth progresses, it is more andmore controlled by the diffusion of solute atomsthrough the depleted matrix, and the growth ratedecreases. At this stage, trapping of mobile dislocations
is often sufficiently advanced, and with the associatedloss of coherency, the interface becomes smoother.Further aging allows the eight different units of theoctodendrite to coarsen to reduce the surface area of thec/c9 interface. This leads to the formation of a dendriticshape.
However, the formation of dendritic c9 precipitatesfrom liquid state needs further consideration. Dendriticstructures forming from liquid metals involve twodistinctly different growth conditions, which differ inthe way in which the latent heat of fusion is carried awayfrom the interface.12 One is growth from an undercooledmelt, in which, generally, an equiaxed dendritic crystalforms. In this case, the latent heat of fusion is dissipatedthrough the cooler liquid ahead of the interface. Thenegative temperature gradient in the liquid at theinterface is gained. Another case is directional solidifica-tion or constrained growth, in which a positivetemperature gradient in the liquid is imposed so thatthe latent heat of fusion is dissipated through the solid.
In the present paper, experiments were performed onan experimental nickel based superalloy. The nominalchemical composition is Ni–0?3C–16Cr–8W–1?5Mo–5Al–NbzCo,5 (wt-%). The alloy was directionallysolidified by high rate solidification process in anindustrial vacuum induction furnace. The solidificationof the c matrix proceeds by dendritic growth. At the endof the solidification process, several types of carbides,such as MC, M6C and M23C6, form by the release of thelatent heat in the grain boundary or the interdendriticarea. As the dendritic c9 precipitates were surrounded bythe c matrix, electrochemical corrosion was applied toget rid of the c matrix using a solution of 21 mLH3PO4z17 mL H2SO4z12 mL H2O (2 V dc and0?2 A).
Figure 1 illustrates the possible site for the formationof the dendritic c9 precipitates from the liquid state. Itwas provided that the temperature gradient in the
1National Key Laboratory of High Temperature Structural Materials,Beijing Institute of Aeronautical Materials, Mail Box No. 81, Division 1,Beijing 10095, China2Institute of Metal Research, Chinese Academy of Sciences, Shenyang110016, China
*Corresponding author, email [email protected]
� 2012 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 20 September 2010; accepted 18 November 2010DOI 10.1179/1743284710Y.0000000033 Materials Science and Technology 2012 VOL 28 NO 1 83
dendritic c matrix was imposed by the high ratesolidification process without the influence of the latentheat. Consequently, the positive temperature gradientcorresponds to the dashed line.
The solidification of the c matrix proceeds bydendritic growth. At the end of the solidificationprocess, several types of carbides, such as MC, M6C,M23C6, etc., form by the release of the latent heat in thegrain boundary or the interdendritic area. The enthal-pies of formation of carbides13–16 are listed in Table 1. Itwas provided that the carbide reaction occurred at aconstant pressure. Then, Q5DfH. Consequently, it wasinevitable that the temperature gradient was changed asa result of the release of latent heat Q by the formation
of carbides. Carbon is a strong positive segregationelement. Accordingly, there exist three preferring pre-cipitate sites for carbides. The solidification interface ofcarbides is a source of latent heat, which needs to bedissipated. The superposition of the latent heat and thepositive temperature gradient resulted in the negativetemperature gradient (see B1 and C1 in Fig. 1), whichfacilitated the precipitation of dendritic c9. In the case ofA, carbides precipitated below the secondary dendriticarms, and no dendritic primary c9 precipitates will beobserved due to the lack of a negative temperaturegradient in the residual liquid melts (see A1 in Fig. 1). Band C had proper temperature gradient conditions forthe precipitation of dendritic c9 precipitates. Equiaxeddendrites can be found in the supercooled liquid, as wellas growing as a result of constitutional supercooling.MC carbides absolutely contain no aluminium, whereasM6C and M23C6 contain a few.17 Accordingly, duringthe precipitation of carbides, c9 forming elements, suchas aluminium, were ejected and concentrated in theresidual liquid melts. This may lead to constitutionalsupercooling. Hence, the combination of constitutionalsupercooling and negative temperature gradient resultedin the primary equiaxed dendritic c9 precipitates.Actually, B2 and C2 corresponded to B and C inFig. 1 respectively.
Dendritic crystal growth patterns, with the hierarch-ical structure of the primary, secondary and higherorder branches, have fascinated scientists for severalcenturies.18 At very low precipitate sizes, interfacial
1 Schematic diagram of formation of dendritic primary c9
precipitates, in which B and C are satisfied with proper
conditions for dendritic growth of precipitates, and B2
and C2, which correspond to B and C respectively,
show equiaxed dendritic primary c9 precipitates
Table 1 Enthalpies of formation of carbides
Carbides Structure DfH298/kJ mol21 DfH1753/kJ mol21
Cr23C6 Cubic 28.6 (Ref. 15) 29.4 (Ref. 15)Cr7C3 Hexagonal 210.5 (Ref. 15) 210.7 (Ref. 15)TiC Cubic 2139¡6 (Ref. 16) …NbC Cubic 266.78 (Ref. 17) …TaC Cubic 273.22 (Ref. 18) …
a branches growing from corners of cubic c9 precipitate;b branches growing from four edges of cubicprecipitates
2 Growth rhythm of dendritic primary precipitates
Zhao and Lou Growth of primary c9 precipitates in nickel based alloy
84 Materials Science and Technology 2012 VOL 28 NO 1
energy is prominent, and spherical or quasi-sphericalshapes are favoured. The anisotropy of surface tensionand instabilities results in side branching. The growthdirection is a compromise between the direction ofmaximum thermal gradient and the direction of crystal-lographic favourable growth. Although the temperaturefield of the residual liquid was complex, the growth ofdendritic primary precipitates was regular (see Fig. 1).Branches of precipitates were found to arrange along thesame direction. It was also found that both eight cornersand four edges of cubic c9 precipitates extend to formdendritic c9 precipitates, as clearly shown in Fig. 2. Thesize of the nucleation core growing from the corners was,200 nm, whereas that growing from the edges was100 nm.
Further growth of the branches occurred. Thisresulted in the hierarchical structure of the primary,secondary and higher order branches of the dendriticprimary c9 precipitates (see Fig. 3). However, in mostcases, not all branches had the opportunity to grow at
the expense of liquid melts. Figure 3a shows that threebranches were preferred to grow, leading to theformation of anisometric dendritic c9. Furthermore,the observed dendrite growth pattern appeared to betypical equiaxed dendritic c9 precipitates, with threeorder branches (see Fig. 3b). Some branches of manyprimary equiaxed precipitates were observed to separateon the surface of large carbides (see Fig. 1, C2). Thisindicates that there existed a poor combination of oneorder branches as well as that between equiaxed onesand carbides.
ConclusionIn summary, dendritic c9 precipitates have beenobserved to form from residual liquid melts in anexperimental directionally solidified nickel based super-alloy. The generation of the dendritic primary precipi-tates can be attributed to the formation of carbides thatlead to the negative temperature gradient, and constitu-tional supercooling resulted from the release of latentheat and ejecting of c9 forming elements.
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a branches growing from corners of cubic c9 precipitate;b branches growing from four edges of cubicprecipitates
3 Hierarchical structure of dendritic primary precipitates
Zhao and Lou Growth of primary c9 precipitates in nickel based alloy
Materials Science and Technology 2012 VOL 28 NO 1 85
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