Synthesis of cast heat-resistant nickel aluminide alloys with tungsten boride

V.V. Gostishchev1, I.A. Astapov1
1Institute of Materials of Far Eastern Branch of the Russian Academy of Sciences, Tikhookeanskaya st., 153, Khabarovsk, 680042, Russia
Nickel aluminides of a composite structure reinforced with inclusions of refractory transition metal compounds possess big potential possibilities for a development of new materials with enhanced strength and thermal resistance. A variety of compositions of alloying systems allows one to obtain composites of different types with a set of improved characteristics. In the given work, experimental results on a production of materials based on intermetallic NiAl matrix with inclusions of tungsten boride are presented. Thermodynamic parameters of sequential reactions underlying the production of composites are determined and indicate on a high probability of an occurrence of a two-stage synthesis process. It is shown that the phase formation occurs as a result of thermally coupled exothermic reactions in NiO-Al and WO3-B2O3-Al systems. It is established that to provide optimal conditions for the formation of intermetallic compounds NiAl, Ni2Al3 it is necessary to have an excess of aluminum (40%) in the initial charge. It has been shown that the introduction of inert additive CaF2 (15%) in a charge results to the maximum yield (about 85%) of metal into an alloy. Increasing the concentration of B2O3 in the composition of the initial mixture from WO3:B2O3=1:0.15 to 1:0.45 increases the volume fraction of tungsten boride content in the synthesized alloy. At the same time, lowering the temperature causes the formation of two intermetallic phases NiAl and Ni2Al3. As the results of element analysis, X-ray diffraction and scanning electron microscopy show, the composite materials contain phases of NiAl and Ni2Al3 with inclusions of tungsten boride WB. The volume fraction of WB in NiAl-WB alloy amounted about 15% and in the alloy NiAl, Ni2Al3-WB about 25%. Microhardness of NiAl intermetallic matrix is 3,5-4,5 GPa. Microhardness of tungsten boride particles (6,3-9,9 GPa) is below the theoretical value, which is not less than 20 GPa. This is associated with up to 5 wt. % dissolution of nickel in this phase.
Received: 06 December 2016   Revised: 11 April 2017   Accepted: 25 April 2017
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