Characterisation of 20Cr32Ni1Nb alloys in as-cast and Ex-Service conditions by SEM, TEM and EDX

Materials Science and Engineering A 374 (2004) 398–408

Characterisation of 20Cr32Ni1Nb alloys in as-castand Ex-Service conditions by SEM, TEM and EDX

Q.Z. Chena,∗, C.W. Thomasb, D.M. Knowlesb

a Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Rd, Hong Kong, Chinab Materials Performance Technologies, PO Box 31310, Lower Hutt, New Zealand

Received 20 October 2003; received in revised form 13 March 2004; accepted 13 March 2004

Abstract

The microstructure of a 20Cr32Ni1Nb alloy in the as-cast and Ex-Service conditions has been fully characterised using scanning electronmicroscopy (SEM)/transmission electron microscope (TEM) and EDX. Nb-rich MC and Cr-rich M23C6 carbides were identified in both theas-cast and Ex-Service samples. Minor addition element Mn is present more in M23C6 carbides than in other phases. In addition, a Si-richintermetallic compound formed on the MC carbides along the grain boundaries in the cast alloy which had been in service around 760◦Cfor about 4 years. The Si-rich phase was identified as G-phase by the experimental and simulated TEM diffraction patterns. EDX of TEMrevealed that its molecular formula is Nb6 (Ni,Fe)16 (Si,Cr)7.© 2004 Elsevier B.V. All rights reserved.

Keywords:Cast alloy; Fe–Ni-base; Carbide; Silicide; TEM

1. Introduction

The use of cast alternatives to more traditional wroughtalloys has become a common industry practice in somepetrochemical applications because of their low manufacturecost for specific more complex components. A particularexample is bull-T’s in reformer application where a numberof manufacturers have started to supply cast components toammonia plants as alternatives to the conventional wroughtproduct. Essentially these castings with base compositionsof Fe–32Ni–20Cr-based (wt.%) are used as alternatives to800H and 800HT. The cast version of alloy 800 has dif-ferent silicon, carbon and minor element additions such asniobium. The changes of alloy chemistry, combined withsegregation associated with the casting process, lead todifferent mechanical properties. It has been reported thatfatigue or creep fatigue have occurred in manifold sys-tems at different plants[1]. To understand the mechanicalperformance, however, it is important that the microstruc-ture of the newly developed cast materials in as-cast andparticularly in-service conditions is fully characterised.

∗ Corresponding author. Tel.:+86-852-28597095;fax: +86-852-28585415.

E-mail address:[email protected] (Q.Z. Chen).

There are few reported works on the microstructure ofcast 20Cr32Ni1Nb variants. Hoffman and Gapinski[2]stated that G-phase (Ni16Nb6Si7) and�′-phase (Nb3Ni2Si)were present at the grain boundaries of the cast materi-als after ageing at 1038–1204◦C for 3 h, using electronback-scattered diffraction (EBSD) of scanning electron mi-croscopy (SEM). However, such a technique cannot clearlydistinguish between these two phases. They also foundvery fine precipitates in interdendritic regions, which wasinferred to be either M23C6 or M7C3. But both EDX inSEM and conventional X-ray diffraction could not confirmthe phase due to their small size. Hence, the objective ofthis work is to exclusively identify precipitates in a cast20Cr32Ni1Nb alloy using transmission electron microscope(TEM) equipped with EDS.

2. Experiments and materials

The two materials investigated in this work are fabricatedwith the composition 20Cr32Ni1Nb. One is in the as-castcondition; the other, called Ex-Service, had been in-serviceat an outlet manifold system for about 4 years. The servicetemperatures are around 760◦C. The average alloy compo-sitions are listed inTable 1.

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2004.03.038

Q.Z. Chen et al. / Materials Science and Engineering A 374 (2004) 398–408 399

Table 1Actual composition of an as-cast and Ex-Service 20Cr32Ni1Nb alloys(wt.%)

Material As-cast Ex-Service

Ni 31.59 31.73Cr 20.75 21.65Nb 1.47 1.43Si 1.31 1.12Mn 0.92 0.90C 0.10 0.11Fe Balanced Balanced

Fig. 1. Microstructure of an as-cast 20Cr32Ni1Nb alloy. (a) A typicalcasting microstructure with eutectically formed MC carbides, (b) stereomorphology of MC carbides and (c) fine precipitates populating the matrixand grain boundaries (e.g. in the circled regions).

Fig. 2. EDX result of large, eutectically formed MC carbides in an as-cast20Cr32Ni1Nb alloy.

SEM and TEM samples of the as-cast material wereprepared through electropolishing in a 10% perchloric acidethanol solution at−10◦C and 35 V. Due to difficultywith preferential etching, SEM and TEM samples of theEx-Service alloy were prepared through mechanical pol-ishing and ion beam milling, respectively. SEM sampleswere observed with back-scattered electron image (BSEI),using Cambridge SEM440 and FEGSEM LEO1530. TEMfoils were examined in JEOL 2000FX and Philip CM 20microscopes.

EDX spectra were collected at 20 kV on FEGSEMLEO1530, and at 200 kV on the Philip CM 20. They wereprocessed using Link eXL and Oxford Link ISIS, respec-tively, using standard reference spectra. The spectrum linesused were Fe–K, Ni–K, Cr–K, Si–K, C–K, Mn–K andNb–L. Each datum presented in the paper is an average of10 readings.

3. Results and discussion

3.1. As-cast alloy

The microstructure of the as-cast alloy is shown inFig. 1.The basic structure consists of large interdendritic pre-cipitates (in bright contrast, and one of which is arrowedin Fig. 1a) which showed thin-plate shape (Fig. 1b) andwere formed during the latter stages of solidification, withfine particles populating the matrix and grain boundaries(Fig. 1b). EDX analysis of SEM (Fig. 2) shows that theeutectic precipitates contain high Nb (∼50 at.% of metalliccomponent), which suggests that they are MC type.

Fig. 3shows the TEM morphologies of large eutectic pre-cipitates and their diffraction patterns. Diffraction analysisreveals that the large eutectic precipitates have an ordinaryface-centre-cubic (fcc) structure and their lattice constant is∼4.40 Å. This result is in agreement with the structure ofMC phase (the first row ofTable 2 [3]).

The compositions of the fine particles were measured byEDX analysis of TEM. Although their compositions varyfrom one particle to another, basically the raw data canbe divided into two groups. One group contains significant

400 Q.Z. Chen et al. / Materials Science and Engineering A 374 (2004) 398–408

Fig. 3. (a) MC carbides in an eutectic regions of an as-cast 20Cr32Ni1Nb alloy and (b) a select area diffraction pattern (SADP) of the region in (a),beam direction is [1 1 2]MC//[1 1 2]M23C6.

Table 2Structures of possible carbides and the matrix in the 20Cr32Ni1Nb alloy

Carbide Structure type Strukturbericht(Pearson)

Space group and no.(Schonflies)

Lattice constant (Å) Ref. of atomposition

Matrix Simple fcc Fm3̄m, no. 225 (O5h) 3.57

MC NaCl-type ordinary fccB1 (cF8) Fm3̄m, no. 225 (O5h) 4.43 (NbC) [3] p.716

M23C6 Complex cubicD84 (cF116) Fm3̄m, no. 225 (O5h) 10.66 (Cr23C6) [4] p.104

M6C (�-carbide) Complex cubicE93 (cF112) Fd3̄m, no. 227 (Oh7) 11.06 (Cr3Nb3C) [4] p. 105

M3C2 OrthorhombicD510 (oP20) Pnma, no. 62 (D162h) a = 2.82; b = 5.53; c = 11.47 (Cr3C2) [4] p.109

M7C3 Complex hex.D101 (hP80) P63/mmc, no. 194 (D46h) a = 4.53; c = 14.01 (Cr7C3) [4] p.109

niobium, and the other group contains a remarkably highlevel of chromium. The average compositions of each groupare illustrated inFig. 4. It was confirmed that the particlesof high niobium are MC carbides, by diffraction analysis(Fig. 5).

As regards high-chromium-containing carbides, thereare four possible precipitates, i.e. M23C6, M6C, M7C6 andM3C2 [4]. Their structures are also summarised inTable 2.The atomic co-ordinates in the Bravais units of these fourprecipitates can be found in the references that are listed

Fig. 4. EDX results of fine MC and M23C6 in an as-cast 20Cr32Ni1Nballoy.

in the last column. Using the data of atom positions, thediffraction patterns of low indexation of these four phaseswere calculated (Fig. 6), with the voltage and camera lengthbeing 200 keV and 0.80 m, respectively. The calculationhas considered double diffraction, which occurs when adiffracted beam travelling through a crystal is re-diffractedby the same crystal[5]. For example, 0 0 2 is a forbiddendiffraction in M6C which is not present in [0 0 1] pattern,but in [0 1 1] pattern, double diffracted{1 1 1} spots appearat the position of 0 0 2 diffraction.

On the other hand, experimental diffraction patterns ofhigh-chromium-containing carbides were obtained usingTEM, as shown inFig. 7. The three diffraction patternsbelong to FCC structure. Hence, orthorhombic M7C3 andhexagonal M3C2 can be excluded. The two cubic phasesM23C6 and M6C have the same 110, 111 and 112 diffractionpatterns. Their 100 diffraction patterns have different sizeswhen using the same voltage and camera length, 100M6Cbeing larger than 100M23C6. It should be mentioned that thesimulated and experimental patterns were produced usingthe same voltage (200 keV) and camera length (0.8 m). Acomparison betweenFig. 6(b)and 100 patterns ofFig. 7(b)indicates that M23C6 is the correct phase. The lattice con-stant is ∼10.56 Å. The crystallographic relationship be-tween the matrix and M23C6 is [100]matrix//[1 0 0]M23C6,[0 1 0]matrix//[0 1 0]M23C6 and [0 0 1]matrix//[0 0 1]M23C6.However, most M23C6 particles do not have this simplerelationship with the matrix.


Fig. 5. (a) A fine MC carbide in the matrix of an as-cast 20Cr32NiNb alloy, (b) its [1 1 4] diffraction pattern, [1 1 4]matrix//[1 1 4]MC, (c) a fine MCcarbide on a grain boundary in the same alloy and (d) its [1 1 2] diffraction pattern, [1 1 2]matrix//[1 1 2]MC.

Fig. 6. Calculated diffraction patterns of (a) M23C6, (b) M6C, (c) M7C3 and (d) M3C2, the tension and camera length being 200 keV and 0.8 m,respectively. Double-diffraction effect is considered, and some of double-diffraction spots are circled.


Fig. 7. (a) A small M23C6 particle in the matrix of an as-cast 20Cr32NiNb alloy and its SADPs of (b) [0 0 1]M23C6//[0 0 1]matrix, (c) [0 1 1]M23C6//[0 1 1]matrix

and (d) [1 1 2]M23C6//[ 1 12]matrix. The intensive spots were mainly from the matrix, combined with the diffraction of M23C6. The fine diffraction spotswere from M23C6 completely.

In summary, two types of carbides, MC and M23C6, arewidely observed in the as-cast 20Cr32Ni1Nb.

3.2. Ex-Service material

The basic microstructure of the Ex-Service alloy is shownin Fig. 8. Large and continuous precipitates are presenton the grain boundaries. A high density of small particlesformed in the matrix and there is a precipitate-free zonealong the grain boundaries, as arrowed inFig. 8a. BSEI ob-servation reveals that there are three types of precipitateson the grain boundaries, which show bright, dark and greycontrast (Fig. 8b). It is seen from element maps (Fig. 9) thatthe dark phase contains high chromium and manganese, thatthe bright one has high niobium, and that the grey phasecontains silicon, niobium and nickel.

Table 3Ten EDX data of Si-rich phase (the errors were in the range of 0.5–1.5 at.%)

Element 1 2 3 4 5 6 7 8 9 10 Average

Si K 21.0 19.1 22.3 20.5 19.2 21.3 19.2 19.1 20.8 20.1 20.3Cr K 2.51 5.80 2.00 3.10 5.80 2.24 5.80 4.80 4.01 4.11 4.02Mn K 0.34 0.40 0.26 0.33 0.23 0.12 0.45 0.13 0.11 0.37 0.27Fe K 5.85 5.10 5.24 4.77 3.57 5.84 3.69 4.67 3.98 5.42 4.81Ni K 50.3 49.8 51.2 50.9 52.3 50.7 52.5 50.1 50.3 50.1 50.8Nb L 20.0 19.8 19.0 20.4 18.9 19.8 18.4 21.2 20.8 19.9 19.8

3.2.1. Identification of MC and M23C6It was found that the Si-rich (grey) phase was preferen-

tially corroded during electropolishing (Fig. 10a). Thus anelectropolished sample was used to analyse the composi-tions of the bright and dark phases by EDX of SEM. Theresult is given inFig. 10b. The bright and dark phases con-tain niobium and chromium of∼80 at.%, respectively. Theywere confirmed to be MC and M23C6 by TEM diffraction,as shown inFig. 11. Their analysis is similar to what hasbeen described inSection 3.1(Fig. 6 andTable 2).

3.2.2. Identification of G-phaseAccording to the element maps ofFig. 9, the major ele-

ments of the Si-rich phase are Si, Nb and Ni. The accuratecomposition of Si-rich phase was determined using EDX onthe TEM. Ten raw data are given inTable 3, and the average


Fig. 8. Microstructure of an Ex-Service 20Cr32NiNb alloy. (a) Continuous precipitates form on grain boundaries and fine particles in matrix, but there isa precipitate-free zone along the grain boundaries and (b) on the grain boundaries, the bright phase is Nb-rich MC and the dark one is Cr-rich M23C6.The grey phase at the circumference of MC is a Si-rich intermetallic compound, G-phase.


Fig. 9. Element maps of precipitates on a grain boundary in an Ex-Service 20Cr32Ni1Nb alloy.

composition of the 10 raw readings is given in the last col-umn ofTable 3. It can be seen that NbxNiySiz can approxi-mately represent the Si-rich phase.

Several NbxNiySiz phases have been reported in literature[6]. Their ideal compositions are summarised inTable 4.A comparison betweenTables 3 and 4indicates that thecomposition of the present Si-rich phase is most close tothat of Nb6Ni16Si7. Although the composition of the silicidephase varies from one particle to another (Si%+ Cr%) is

always close to 24%, Ni%+ Fe% around 55%, Nb around20%, and Mn is nearly zero. Hence, it is concluded thatthe molecular formula of the Si-rich intermetallic compoundis Nb6 (Ni,Fe)16 (Si,Cr)7 in the present cast 20Cr32Ni1Nballoy after service around 760◦C for about 4 years.

The binding between Ni and Fe has been reported[7],while binding between Si and Cr element in this silicidephase is unusual. It had been expected Cr combines with Nb,as it was reported that Cr, Mn and Nb are strong formers of


Fig. 10. (a) Microstructure of an Ex-Service 20Cr32NiNb sample that was prepared by electropolishing and (b) EDX result of Nb-rich (MC) and Cr-rich(M23C6) phases on grain boundaries and in matrix.

silicide phase A6Ni16Si7, in which Cr, Mn and Nb occupyposition “A” [8].

The identification of the Si-rich phase was also carried outthrough structural analysis. A6B16Si7 was arbitrarily calledG-phase in literature[9]. Silicon plays a fundamental partin its existence. Its structure is isotypic with Th6Mn23 andMg6Cu16Si7, which had been determined by Florio et al.[10], and Bergman and Waugh[11,12]. The crystallographicdata of Nb6Ni16Si7 is summarised inTable 5. Using theatomic position data in reference[12] and the lattice constantof reference[6], the TEM diffraction patterns of Nb6Ni16Si7

were simulated, as illustrated inFig. 12. Diffraction patternsof the Si-rich phase (Fig. 13) were, on the other hand, ex-perimentally obtained by TEM, as shown inFig. 14. Thereis agreement between the simulated and experimental pat-terns. The indexed [0 1 1] pattern (Fig. 14d) reveals thatthe crystallographic relationship is [0 1 1]G//[0 1 1]matrix and[1 1̄ 1]G//[1̄ 1̄ 1]matrix, which derive the third parallel direc-tions [2 11̄]G//[2 1̄ 1]matrix.

The diffraction patterns of G-phase (Fig. 14) are almostthe same as those of M23C6 carbides (Fig. 7). In fact,the two phases have the same type of diffraction patterns.


Fig. 11. (a) A MC precipitate and its Si-rich layer, and M23C6 particles formed nearby in an Ex-Service 20Cr32NiNb alloy. SADP of (b) MC, BD= [1 1 2]MC//[1 1 2]matrix, and (c) M23C6, BD = [1 1 2]M23C6//[1 1 2]matrix. BD stands for beam direction.

Table 4Ideal compositions of various NbxNiySiz phases

Phase Atomic%

Nb Ni Si

NbNiSi 33.3 33.3 33.3NbNiSi2 25.0 25.0 50.0Nb2Ni3Si 33.3 50.0 16.7Nb3Ni2Si 50.0 33.3 16.7Nb3Ni2Si5 30.0 20.0 50.0Nb4NiSi 66.7 16.7 16.7Nb4Ni4Si7 26.7 26.7 46.7Nb4Ni10Si 25.7 66.7 6.7Nb6Ni16Si7 20.7 55.2 24.1Nb9Ni20Si 30.0 66.7 3.3

Fig. 12. Simulated TEM diffraction patterns of G-(Nb6Ni16Si7) phase, the tension and camera length being 200 keV and 0.8 m, respectively.

Table 5Crystallographic data of Nb6Ni16Si7

Structure typeStrukturbericht(Pearson)

Space groupand no.(Schonflies)

Latticeconstant(Å)

Ref. ofatomposition

Nb6Ni16Si7 Complex cubicD8a (cF116)

Fd3̄m,no.225 (O5

h)11.2 [6] [12]

p. 215

The difference between them is the intensity of individualdiffractions, which however cannot be precisely detected byTEM. Therefore, the two phases have been distinguishedby their compositions that are remarkably different (Fig. 10andTable 3).

G-phase has a close relationship with�-carbide, whichhas the same structure as G-phase and was reported in similar


Fig. 13. G-phase is present on a grain boundary in an Ex-Service20Cr32Ni1Nb alloy.

Fig. 14. Diffraction patterns of G-phase: (a) [0 1 1], (b) [1 1 1], (c) [1 1 2] and (d) indexed [0 1 1] which shows that the crystallographic relationshipbetween G-phase and the matrix is [0 1 1]G//[0 1 1]matrix and [11̄ 1]G//[1̄ 1̄ 1]matrix.

Table 6Summary of precipitates in the Nb6Ni16Si7 after service around 760◦Cfor about 4 years

Precipitates Structure Latticeconstant (Å)

Approximate formula

MC NaCl type 4.40 (Nb,Ni,Fe,Cr)CM23C6 Complex fcc 10.56 (Cr,Ni,Fe,Nb)23C6

G-phase Complex fcc 10.5 Nb6 (Ni,Fe)16 (Si,Cr)7

alloys[13]. Eventually G-phase might almost be consideredas the silicon ‘version’ of the�-carbide type[14]. The highlevel of Si in the investigated precipitates indicated that theywere silicides rather than�-carbides. However, we do notintent to exclude the existence of�-carbides, as the transfor-mation from G-phase to�-carbide is very probable in thesealloy systems[14].

It was also noted that MC phase was always encompassedby the grey phase (Figs. 8, 9, and 13), which seemed to growfrom MC phase. Therefore, the fine MC particles inside thegrey phase were residual MC phase.


Clearly a new precipitate, G-phase, formed in the20Cr32Ni1Nb alloy after service around 760◦C for about 4years.Table 6summaries the precipitates in the Ex-Servicealloy.

4. Conclusions

In the as-cast 20Cr32Nb1Nb alloy, Nb-rich MC andCr-rich M23C6 are widely observed.

In the Ex-Service 20Cr32Nb1Nb alloy, in addition to MCand M23C6, a new phase, G precipitates, was formed afterservice around 760◦C for about 4 years. The silicide wasidentified as Nb6 (Ni,Fe)16 (Si,Cr)7.

Acknowledgements

The authors would like to thank University ResearchCommittee (URS Hong Kong) and Materials PerformanceTechnologies which supported this work.

References

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Characterisation of 20Cr32Ni1Nb alloys in as-cast and Ex-Service conditions by SEM, TEM and EDX

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