Scripta Materialia, Vol. 34, No. 5, pp. 163-169, 1996                                                                     ...
764                         SOLIDIFICATIONPHENOMENAIN NICKEL BASE                              Vol. 34, No. 5             ...
Vol. 34, No. 5                   SOLIDIFICATION          PHENOMENA         IN NICKEL BASE                                 ...
766                             SOLIDIFICATION        PHENOMENA         IN NICKEL BASE                        Vol. 34, No....
Vol. 34, No. 5                 SOLIDIFICATION PHENOMENA IN NICKEL BASE                                                   7...
768                             SOLIDIFICATION       PHENOMENA        IN NICKEL BASE            Vol. 34, No. 5are given in...
Vol. 34, No. 5                  SOLIDIFICATION PHENOMENA IN NICKEL BASE                                                  7...
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Solidification phenomena in nickel base brazes containing boron and silicon

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Solidification phenomena in nickel base brazes containing boron and silicon

  1. 1. Scripta Materialia, Vol. 34, No. 5, pp. 163-169, 1996 Elsevier Science Ltd Perganson Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/96 $12.00 + .OO 0956-716X(95)00577-3 SOlLIDIFICATION PHENOMENA IN NICKEL BASE BRAZES CONTAINING BORON AND SILICON SK. Tung, L.C. Lim and M.O. Lai Department of Mechanical and Production Engineering National University of Singapore Kent Ridge Crescent, Singapore 05 11 (Received April 19, 1995) (Revised September 14, 1995) IntroductionNickel base bra:zes containing boron and/or silicon as melting point depressants are used extensively inthe repair and joining of aero-engine hot-section components (l-8). These melting point depressants formhard and brittle intermetallic compounds with nickel which are detrimental to the mechanical propertiesof brazed joints (9). It is therefore of interest to find possible means to control and engineer the microstruc-tures of the braz:s and to develop brazing alloys which have minimal weakness arising from the formationof these intermetallic compounds. A better understanding of the solidification phenomena and formationof intermetallic compounds in these brazes is a first step to this end. Previous investigations in this area have shown that the microstructures developed in joints brazed withnickel base filler metals are rather complicated (9-12). A typical microstructure of a wide-gap brazed jointis shown in Fig. 1. Three distinct zones can be identified: the base metal (marked Zone I), the interfaciallayers (Zone II) and the braze main body (Zone III). Zone II is a y-nickel solid solution layer (marked E),which is present in all nickel base brazes. It is formed isothermally at the brazing temperature duringwhich dilution and depletion of the melting point depressants occur due to the erosion of the base metaland the diffusion of the melting point depressants into the base metal, respectively. This raises the meltingpoint of the molten filler metal next to the joint faying surfaces above the brazing temperature, leading tothe formation of the y-nickel interfacial layers from both faying surfaces into the melt (13). The present investigation studied the microstructural evolution in nickel base brazes containing boronand/or silicon as melting point depressant(s) in simple systems using nickel as the base metal. The basicmetallurgical reactions and formation of intermetallic compounds uncovered in these systems will be use-ful as a guide in predicting the evolution of microstructures in similar brazes in more complex systemsinvolving base metals of nickel base superalloys. The four filler metal systems investigated in this studyare: Ni-Cr-Si; Ni-0-B; Ni-Si-B and Ni-Cr-Fe-Si-B. ExDerimental ProceduresThe base metal used was commercially available pure nickel (grade 270). The tiller metals used, in theform of atomized powders, were BNi-5 (Ni-19Cr-10.2Si), Nicrobruz 150 (Ni-15Cr-3.5B), BNi-4 (Ni- 763
  2. 2. 764 SOLIDIFICATIONPHENOMENAIN NICKEL BASE Vol. 34, No. 5 Figure 1, Typical microstructure of a wide gap brazed joint.3.5%1.9B) and BNGla (Ni-14Cr-4.5Si-4.5Fe-3B). The above compositions are given in weight percent.Wide gaps of 0.5 mm in clearance, 2 mm in breadth and 6 mm in depth were made each by placing twobase metal pieces on top of one another with two spacers of nickel placed between them. One end of thegaps was first sealed with sufficient shrrry of filler metal in a liquid acrylic binder. The gap, oriented ina vertical position, was then filled with dry filler metal powder. After securing the powder in the gap withone or two drops of acrylic binder, the top end of the gap was sealed with an excess of filler metal slurry. After drying, the specimens were brazed at various temperatures in a vacuum of better than lo-’ Pa.They were held at the brazing temperature for 15 minutes and then cooled at 3.5”Umin in the furnace toroom conditions. A few specimens were water-quenched to room temperature after cooling in the furnaceto specific temperatures. The brazed gaps were then sectioned depth-wise across the gap, prepared metal-lographically and examined by means of optical and scanning electron microscopy. Energy dispersive X-ray microanalysis (BDS) was used to determine the chemical contents of the various elements in the phasesin the braze main body. The EDS system used was not sensitive in measuring the content of boron. How-ever, the values of the chemical contents determined, coupled with the microhardness of the phases meas-ured at a load of 0. I kgf, were sufficient for the identification of the phases involved (14). Results and Discussioni1 BNi-5 (Ni-19Cr-10.2SilFigure 2(a) shows the typical microstructure of a joint brazed with BNi-5. The only intermetallic com-pound found in this braze was nickel silicide, which appears white in the figure. It solidified as a compo-nent phase of the binary eutectic with y-nickel, the latter appears as the dark phase. Figures Z(b) shows the microstructures of a joint brazed at 115O*C,cooled in the furnace to 1095 “Cand then quenched into water. Only y-nickel nodules and dendrites were present among the quenchedstructure, of which the details were too fine to be revealed under the magnification used. It may be noted that the y-nickel nodules and dendrites in the specimens quenched at 1095°C appearwhite (Fig. 2b) , while those cooled to room temperature appear dark (Fig. 2a). This can be understoodby considering the difference in the solubility of silicon in y-nickel at different temperatures. As can beseen from the Ni-Si binary phase diagram (15), the solubility of silicon in y-nickel decreases significantlywith temperature below the eutectic temperature. Hence, for a specimen which has been cooled to a tem-
  3. 3. Vol. 34, No. 5 SOLIDIFICATION PHENOMENA IN NICKEL BASE 765Figure 2. (a) Typicallmicrostructure of a joint brazed with Bni-5. (b) Microsbucttmz of a joint brazed at 1150°C and water-quenchedafter cooling to 1095°C.perature above the eutectic temperature prior to quenching as in Fig. 2(b), minimum solid state precipi-tation of nickel silicide in the y-nickel would occur during the slow cooling stage. But for a specimencooled below the eutectic temperature prior to quenching, solid state precipitation of nickel silicide wouldoccur as cooling, proceeded to a temperature lower than the eutectic temperature. The fine precipitates ofnickel silicide had caused the y-nickel in the specimen in Fig. 2(a) to appear dark upon etching with Mar-ble’s reagent. Further expe:riments have confirmed that the y-nickel nodules observed in the joint brazed at 1150°Cwere largely the remnants of the partially molten tiller metal particles. In most cases, these partially meltedparticles were observed to settle to the bottom of the gap, as shown in Fig. 2(b). y-nickel nodules were notobserved in joints brazed at 1250°C at which all the filler metal particles were confirmed to have beenmelted completely ( 16). With the above observation, the solidification sequence of this filler metal can be deduced as follows:During brazing, isothermal solidification of the y-nickel layers occurred from the faying surfaces intothe melt (Zone II of Fig. 1). On cooling from the brazing temperature, dendrites of primary y-nickel solidi-fied from within the melt and from the inter-faciallayers. This enriched the remaining melt of silicon untilit reached eutecl:ic composition which then solidified into an eutectic of y-nickel and nickel silicide. Uponfurther cooling of the solidified braze to room temperature, solid state precipitation of nickel silicide oc-curred in y-nickel, causing it to appear dark upon being etched with Marble’s reagent.ii) Nicrobraz 150 (Ni-15Cr-3.5B)The microstructure of a joint brazed with Nicrobruz 150 is shown in Fig. 3. The phase marked Gl is nickelboride, while that marked G2 is y-nickel solid solution. In the centre-line region of the joint is a ternaryeutectic of y-nickel, nickel boride and chromium boride, Fig. 3(b). Chromium boride, marked G3, solidi-fied out as elongated crystals in the ternary eutectic. The solidification sequence of this braze can be deduced as follows: During brazing, the y-nickel solidsolution layers solidified isothermally from the faying surfaces into the melt. On cooling from the brazingtemperature, primary y-nickel solidified either by nucleation from within the melt as nodules or by grow-ing onto the partially melted filler metal particles. In so doing, boron and chromium were ejected into theremaining melt, shifting the composition of the melt towards the eutectic compositions. Subsequently,solidification of the binary eutectic of y-nickel and nickel boride resulted. It could be seen in Fig. 3(a) that
  4. 4. 766 SOLIDIFICATION PHENOMENA IN NICKEL BASE Vol. 34, No. 5Figure 3. (a) Microstructure of a joint brazed with Niwobruz 150 at 1125°C. (b) A magnified view showing the details of theternary eutectic.the solidification of this eutectic began from the faying surface of the interfacial zone and progressedtowards the centre-line region of the joint. The nickel boride component in the eutectic contained prac-tically no chromium. As a result, chromium was ejected into the remaining melt, which solidified even-tually into a ternary eutectic of y-nickel, nickel boride and chromium boride, Fig. 3(b). This region is thelast to solidify in the joint.iii) BNi-4 (Ni-3SSi-1.9B)Typical microstructures of joints brazed with BNi-4 filler metal are shown in Fig. 4. The various phasesformed in this braze include y-nickel solid solution layers (marked ‘M*‘) adjacent to the joint fayingsurfaces, y-nickel nodules (marked ‘M’), bulky nickel borides (marked ‘R’), binary eutectics of y-nickeland nickel boride of normal and divorced morphologies, and ternary eutectic of y-nickel, nickel borideand nickel silicide (marked ‘XI). The binary eutectic of normal morphology (marked ‘N’) mainly formedFigure 4. (a) The constituents and component phases found in a joint brazed with BNi-4. (b) Details of the ternary eutectic andnickel silicide precipitates.
  5. 5. Vol. 34, No. 5 SOLIDIFICATION PHENOMENA IN NICKEL BASE 767adjacent to the layers of single-phase y-nickel. It gave way to divorced eutectic as solidification proceededliu-ther towards the centre-line of the joint (14). Figure 4(b) shows the details of the ternary eutectic (‘X’).‘Xl’, ‘X2’, and ‘X3’ are nickel silicide, y-nickel and nickel boride respectively. Nickel silicide (marked‘m’) which is formed in solid state precipitation can be found in the outer regions of y-nickel nodules,especially those in the centre-line region of the braze. The general direction of solidification was from the base metal towards the centre-line region of thegap. The first stage of solidification involved the isothermal formation of y-nickel solid solution layersadjacent to the base metal pieces. Subsequently, primary y-nickel solidified within the molten filler as nod-ules, or grew onto the partially melted filler metal particles upon cooling from the brazing temperature.As the y-nickel formed contained much less melting point depressants, boron and silicon were ejected intothe adjacent melt, shifting the composition of the melt towards the eutectic compositions of the system,and binary eutectic of y-nickel and nickel boride of normal structure formed as solidification progressed,mainly at or near to the single phase y-nickel layers. In most other areas, the growth of y-nickel phase onthe existing y-nickel nodules continued, and the melt in between them eventually solidified into bulkynickel boride instead, resulting in divorced or degenerate eutectic of y-nickel and nickel boride. Sincenickel borides practically do not contain silicon, any remaining melt, mainly in the centre-line region ofthe braze, would become highly enriched in silicon and solidified as ternary eutectic of y-nickel, nickelboride and nickel silicide. It is also interesting to note that the ternary eutectic of this system is of quasi-divorced morplhology in that the nickel boride component is often relatively bulky and formed in thematrix of normal binary eutectic of y-nickel and nickel silicide.{iv) BNi-la fNi-14Cr-4.5Si-4.5Fe-3BjThe microstrucfure resulting Corn the solidification of the molten filler metal in the joint brazed with BNi-la was rather complex. However, it was found that the intermetallic compounds formed in this braze con-sisted only of those found in the three brazes described above; and these are, namely: nickel boride, nickelsilicide and chromium boride. A typical microstructure of joints brazed with this filler metal is shown inFig. 5(a). The phases marked L 1a, L 1b and L 1c are nickel borides; L2a and L2b are y-nickel; L3 is nickelsilicide and L4 is chromium boride. Three eutectic constituents were identified. These are: binary eutecticof y-nickel and nickel boride (L2a/Lla); binary eutectic of y-nickel and chromium boride (L2a/L4); andternary eutectic of y-nickel, nickel boride and nickel silicide (L2a/Llc/L3). Details of the ternary eutecticFigure 5. (a) Microstructure of a joint brazed with BNi-la at 1125%. (b) A magnified view showing the details of the eutecticphases.
  6. 6. 768 SOLIDIFICATION PHENOMENA IN NICKEL BASE Vol. 34, No. 5are given in Fig. 5(b). It is interesting to note that in the presence of silicon, the formation of ternaryeutectic of y-nickel, nickel boride and chromium boride was suppressed in that chromium boride solidifiedwith y-nickel as a binary eutectic instead. The fme precipitates in y-nickel (L2a) are nickel silicide whichwere formed in solid state upon cooling the solidified braze to room temperature. The solidification behaviour of this braze can be deduced as follows: During brazing, y-nickel firstsolidified isothermally from the faying surfaces into the melt. Upon cooling, primary y-nickel solidifiedas nodular dendrites from the interfacial layers. This enriched the remaining melt with boron, silicon andchromium. As freezing proceeded, binary eutectic of y-nickel and nickel boride solidified, further enrich-ing the melt of chromium. Subsequently, binary eutectic of y-nickel and chromium boride was found tosolidify on further cooling. The very last portion of the melt, which was further enriched in silicon, wasfound to solidify into the ternary eutectic of y-nickel, nickel boride and nickel silicide. Summary1. The intermetallic compounds formed in brazes of nickel base filler metals containing chromium, boron and silicon are nickel boride, nickel silicide and chromium boride.2. The intermetallic compounds are found to form as constituents of binary or ternary eutectics with y-nickel. The binary eutectics are y-nickel/nickel boride, y-nickel/nickel silicide and y- nickel/chromium boride. The ternary eutectics are y-nickel/nickel boride /chromium boride and y-nickel/nickel boride/nickel silicide.3. Upon cooling from the brazing temperature, the first formed solid is primary y-nickel, which either nucleates within the melt as nodules or nodular dendrites or simply grows onto the partially melted filler metal particles. On further cooling, boron and silicon in the melt tend to exclude one another, solidifying into different types of eutectics the constituents of which depend on the composition of the filler metal.4. When only silicon is present, the remaining melt simply solidifies into an eutectic of y-nickel and nickel silicide.5. For boron-containing filler metals, the first formed eutectic is usually the binary eutectic of y- nickel and nickel boride; this in turn enriches the melt with chromium. In the absence of silicon, this chromium-enriched melt will eventually solidify into a ternary eutectic of y-nickel, nickel boride and chromium boride. When silicon is present, the formation of the above ternary eutectic is suppressed and a binary eutectic of y-nickel and chromium boride is formed instead. When this happens, the remaining melt will be enriched in silicon and the last portion of the melt solidifies into a ternary eutectic of y-nickel, nickel boride and nickel silicide.6. For the filler metal containing nickel, boron and silicon but no chromium, after the formation of binary eutectic of y-nickel and nickel boride, the melt will become gradually enriched with silicon and eventually solidify into the ternary eutectic of y-nickel, nickel boride and nickel silicide. References 1. R.D. Eng, E.J. Ryan and J.R. Doyle, Welding Journal, 56, 15 (1977). 2. M.H. Haatkens, SAB Technical Paper Series, l-15, Oct. 1982. 3. T.J. Kelly, Welding Journal, 61, 317-s (1982). 4. J. W. Chasteen and G.E. Metzger, Welding Journal, 58, 11l-s (1979). 5. Hans van Esch and G. Marijnissen, Turbomachinery International, 9,29 (1986). 6. E. Lugscheider and Th. Schittny, Brazing and Soldering, 14,27 (1988). 7. K. G. Gove, Metals and Materials, 5,341 (1989).
  7. 7. Vol. 34, No. 5 SOLIDIFICATION PHENOMENA IN NICKEL BASE 769 8. P. R. Mobley and G. S. Hoppin III, Welding Journal, 40,610 (1961). 9. R. Johnson, M. Baron and N.J. Livesey, Third International Brazing and Soldering Conference (BABS), Paper 21, Oct. 1979, London.10. W. F. Gale and E. R. Wallach, Metallurgical Transactions A, 22A, 2451 (1991).11. E. Lugscheider and K.D. Paritz, Welding Journal, 71,329-s (1982).12. B. Grushko and B.Z. Weiss, Metallurgical Transaction A, 15A, 609 (1984).13. S.K. Tung and L. C. Lim, Materials Science and Technology, 10,364 (1994)14. S.K. Tung, L. C. Lim and M.O. Lai, “Microstructural Evolution and Control in BNM Brazed Joints ofNickel 270”, submitted to Scripta Metallurgica et Materialia.15. Smithells, Metals Reference Books, Butterworth and Co.Ltd., London, 735( 1976).16. J.R.C. Caisido, S.K. Tung and L.C. Lim, Microstructural Evolution of a Commercial Nickel Base Braze, Proceedings of Science Research Congress 1994: Faculty of Science, National University of Singapore and Ministry of Education of Singapore, Singapore, 390(1994).

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