Powder Metallurgy and Metal Ceramics, Vol. 48, Nos. 7-8, 2009
INTERACTION OF TUNGSTEN CARBIDE WITH ALUMINUM NICKELIDE Ni3Al V. S. Panov1 and M. A. Gol’dberg1,2 UDC 621.762 The paper examines the effect of the wetting time on the contact angle θ of WC and Ti wetting by Ni3A melt. It is shown that the equilibrium θ value varies between 38 and 0°. It is established that the miscibility of WC in Ni3Al intermetallic compound is 3 to 4% at T = 1500°C. A differential thermal analysis has revealed that the liquid phase in the WC–Ni3Al system shows up 20 to 25°C lower than the melting point of pure Ni3Al. A microstructural analysis has shown that WC–Ni3Al alloys contain two phases: WC-based phase and solid solution of WC in Ni3Al. The work of adhesion is calculated; it shows that chemical interaction prevails. The microstructure of the contact area is analyzed. Keywords: carbide, intermetallide, melt, wettability, temperature, solubility, microstructure, contact area, analysis, phase, x-ray pattern. The development of various technical areas calls for new wear-resistant composites to perform under heavy loads at high temperatures in corrosive media. For this purpose, powder metallurgy methods are especially significant since they combine in one material the most valuable properties, which separately do not meet all the requirements. Wear-resistant composites cannot be developed without examining the wettability of the carbide component by the adhesive phase melt, liquid phase temperature, miscibility of the components, and phase transformations [1–3]. The objective of this paper is to examine the interaction between WC and Ni3Al at different temperatures determining the properties of the composite. We used hot-pressed samples of tungsten carbide as the starting components and samples of titanium carbide for reference and Ni3Al billets produced by isostatic pressing of the powder. The wetting was examined with the sessile drop method (stationary option) [4]. The experimental and calculated results are in good agreement: the difference is no more than ±2°. Analysis of the time dependences of the contact angle of wetting of tungsten and titanium carbides by Ni3Al melt shows (Fig. 1) that the equilibrium value of θ settles within 3 to 6 min and does not change for 20 min. Over the region of melting temperatures, θ decreases rapidly and is about zero for tungsten carbide even in 3 min. The contact angle of wetting of titanium carbide reaches 38° in 4 min and no longer changes. The contact angle θ
1Moscow 2To
State Technology University, Moscow, Russia.
whom correspondence should be addressed; e-mail:
[email protected].
Translated from Poroshkovaya Metallurgiya, Vol. 48, No. 7–8 (468), pp. 100–104, 2009. Original article submitted July 17, 2008. 1068-1302/09/0708-0445 ©2009 Springer Science+Business Media, Inc.
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θ, deg 60 TiC
40 20 0
WC
0
1
2
3
4
5 t, min
Fig. 1. Time dependence of the contact angle of wetting
a
b
c
Fig. 2. Microstructure of the transition area in the WC–Ni3Al system after wetting for 4 h at 1400 (a), 1500 (b), and 1600°C (c); ×1400: light phase⎯tungsten carbide decreases quite rapidly in the initial period (after the liquid phase shows up) because the spreading resistance is determined by internal friction and inertial forces that act rapidly for molten metals and manifest themselves when the solid starts contacting with the melt. Tungsten carbide is wetted well by Ni3Al melt. The work of adhesion was calculated with the Dupre equation: Wa = σ m (1 + cos θ) , where σm is the surface tension of the melt. At 1450°C, the work of adhesion is 3.16 J/m2 for WC and 2.7 J/m2 for TC. The quite high Wa values testify that chemical interaction prevails at the interface between the refractory compound and melt. To explain the contact interaction at the melt–carbide interface, we may use a valence electron configuration model [5, 6], according to which good wetting is ensured by the ‘metal acceptor–carbide donor’ condition. With weakening metal–carbon interaction from Me (IV) to Me (VI) carbides, their free surface energy increases and the contact angle of wetting of carbides by the melt decreases. According to microstructural analysis of the contact area in the WC–Ni3Al system (Fig. 2), the intermetallide penetrates into the substrate along WC grain boundaries by capillary suction and migration mechanisms. With increasing temperature, the carbide phase boundary ‘rebuilds’ more intensively. To determine the miscibility of tungsten carbide in the liquid phase, the sample impregnated at 1500°C for 1 h was rapidly cooled to room temperature and then its x-ray diffraction was carried out. The data from the chemical and x-ray analyses show the miscibility of tungsten carbide in the intermetallide: 3–4% at 1500°C. Based on these data, the tungsten content of the sample quenched at 1500°C was 3.5% and, taking into account the lattice parameter, substantially decreased when it was cooled down. The lattice parameter of Ni3Al was 0.356 nm and of the WC–Ni3Al sample 0.371 nm after quenching at 1500°C and 0.368 nm after slow cooling in the furnace. Because of procedural complexity, it was difficult to assess the amount of tungsten carbide dissolved at room temperature.
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Ni3Al
1385 TiC−Ni3Al
1370
WC−Ni3Al 1368
300
500
700
900
1100
1300 Т, OC
Fig. 3. Results of differential thermal analysis The results of the x-ray analysis are confirmed by data on the microhardness of pure intermetallide and solid solution on its basis. The microhardness of the sample quenched at T < 1500°C turned out to be 1000– 2000 MPa higher than that of pure Ni3Al. It is 3000 MPa for the pure intermetallide and more than 3900 MPa for a solid solution of WC in Ni3Al. Because of a great scatter in data, the microhardness could not be accurately determined, but all values for the solid solution were 500–2500 MPa higher than for the intermetallide. After the miscibility was determined, a microstructural analysis of the WC–Ni3Al samples showed that the excess WC grains were prismatic, i.e., recrystallization through the liquid phase occurs. To find the melting temperature in the WC–Ni3Al system and examine the phase transformations, we carried out a differential thermal analysis, whose results are summarized in Fig. 3. It has been established that the quasibinary section of the W–C– Ni–Al system is a eutectic diagram with the eutectic temperature being above 1350°C. Alloys of Ni3Al and tungsten carbide melt and crystallize over this temperature range. The liquid phase range shifts toward the melting temperature of the Ni3Al (γ′) phase, toward lower temperatures: 1360–1370°C. This is probably associated with the microalloying of aluminum nickelide when the mixture is intensively ground and with the miscibility of tungsten carbide in the intermetallide. The melting temperature of pure Ni3Al is 1385°C and corresponds to the literature data. The cooling process slows down when Ni3Al-based melt crystallizes because a peritectic reaction proceeds. At 450–550°C, an exothermal effect is revealed for alloys, which is probably caused by decomposition of the plasticizer (polyethylene glycol was introduced into the WC–Ni3Al mixture in grinding). Below 1370°C, an endothermic process is observed, which is associated with melting of the intermetallide component. This indicates that the WC–Ni3Al solid solution melts at a lower temperature than pure Ni3Al does, and thus the miscibility of tungsten carbide in the intermetallide is ascertained. Two phases are revealed in the samples heated to 1500°C after cooling (x-ray diffraction and metallographic analyses): tungsten carbide and Ni3Al-based phase. This confirms the thermal analysis data on one phase transformation in the system⎯melting of the Ni3Al-based bonding phase. A eutectic was not revealed on metallographic sections.
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CONCLUSIONS The wettability of tungsten carbide by Ni3Al melt is examined. The WC–Ni3Al and TiC–Ni3Al systems become equilibrium within 3 to 6 min at 1450°C, and the equilibrium contact angle greatly depends on temperature and varies from 40 to 0°. In the WC–Ni3Al system, chemical interaction prevails and the intermetallide diffuses into tungsten carbide along grain boundaries. The miscibility of tungsten carbide in Ni3Al has been determined: it is 3– 4% at 1500°C and substantially lower at 20°C. This research has been sponsored by the Russian Fundamental Research Fund, Project 07-08-00035.
REFERENCES 1. 2. 3. 4. 5. 6.
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K. I. Portnyi and B. N. Babich, Nickel-Based Composites [in Russian], Metallurgiya, Moscow (1989). I. I. Kornilov (ed.), Intermetallic Compounds [in Russian], Metallurgiya, Moscow (1970). V. S. Panov, A. M. Chuvilina, and V. A. Fal’kovskii, Technologies and Properties of Hard Sintered Alloys and Associated Products [in Russian], Mosk. Inst. Stali Splavov, Moscow (2004), p. 468. V. P. Elyutin, V. I. Kostikov, and B. S. Lysov, High-Temperature Materials. Production and Physicochemical Properties of High-Temperature Materials [in Russian], Metallurgiya, Moscow (1973). G. V. Samsonov, G. I. Upadkaya, and V. S. Neshpor, Physical Materials Science of Carbides [in Russian], Naukova Dumka, Kiev (1974). G. V. Samsonov, Metal Refractory Compounds [in Russian], Metallurgizdat, Moscow (1963), p. 244.
