Structure of Vacuum Brazed BNi-5 Joint of Inconel 718
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I.
INTRODUCTION
A property apparently unique to Inconel 718" among the *Trade name of Ni-based alloy manufactured by Intemational Nickel Company.
superalloys is its outstanding weldability in both the agehardened and the annealed condition. This superior weldability of Inconel 718 in the hardened condition is primarily the result of the precipitation of the Ni3Nb(3/') phase, which responds very sluggishly to thermal cycling, thereby reducing the precipitation hardening effects which usually occur during temperature changes in welding or brazing processes. The filler metals most commonly used in high-vacuum brazing of nickel-based superalloys are temary alloys of the type Ni-A-B (A and B being alloying elements). Equilibrium diagrams of most of these ternary systems are only partly known, so that, as a rule, neither the primary nor the secondary equilibrium phase can be predicted. Moreover, the interdiffusion of the base metal and the braze, which occurs during the brazing process, introduces a further complexity to the problem. Under these conditions the phase transformations and precipitation processes are of a very complicated and sometimes unpredictable nature. The BNi-5 filler metal used in the present investigation is basically a ternary alloy Ni-Cr-Si (70 wt pct Ni, 20 wt pct Cr, 10 wt pct Si). For that composition the liquidus temperature is approximately 1135 ~ the solidus temperature, approximately 1080 ~ in the presence of 0.1 pct carbon. Only a few workers have studied the Ni-Cr-Si phase diagram, and the available information is limited to a small number of isothermal sections of that diagram. 2'3,4 In the 70 Ni 20 Cr 10 Si alloy the equilibrium phases present at 850 ~ are an fcc solid solution of Ni-Cr-Si(y) and two compounds: NisSi2(0)** and Cr3NisSi2 (7r-phase).4 De Paoli5 **This phase has been calculated as Ni3lSi~2.8 B. GRUSHKO, Instructor, and B. Z. WEISS, Professor and Head, are with the Department of Materials Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel. Manuscript submitted June 21, 1983. METALLURGICALTRANSACTIONS A
confirmed, by means of X-ray diffraction, the presence of the above phases in the BNi-5 specimen which had been held at 1000 ~ for 100 hours. There is no information about any phase transformation or precipitation processes occurring in this alloy under equilibrium conditions at temperatures lower than 850 ~ Judging by De Paoli's work, the presence of 0.1 wt pct C in BNi-5 filler metal does not introduce any structural effects in the temperature range around -1000 ~ It must nevertheless be mentioned that Gladyshewskii 4 has reported the existence of another intermetallic compound Cr3Ni2Si(C) in Ni-Cr-Si alloys containing ~ 1 at. pct C (0.2 wt pct). It is assumed that structural and compositional reactions occurring during the brazing process are affected by certain aggressive actions of the filler metal, such as diffusion of the filler metal elements into the base material and erosion of the base metal by the liquid filler material. In actual fact,
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