Mechanical properties of high-temperature composites made from mixtures of tungsten-coated and uncoated NiAl intermetallide granules

Abstract

387 An interesting possibility is the production of high� temperature composites with a honeycomb structure by sintering powder based on NiAl (or simple alloys based on NiAl) when each grain is coated with a thin layer of tungsten or successive layers of tungsten and molybdenum [1]. We may note the following research findings: ⎯nonuniform thickness of the tungsten coating on an individual grain (in the limiting case, no coating on a section of the grain’s surface); ⎯rupture of the coating in technological processes associated with deformation; ⎯breakdown of the continuous tungsten film to individual droplets, in the form of a tungsten alloy with Ni and Al, formed at temperatures near the melt� ing point of NiAl [2]. These processes may convert the composite from a material with a honeycomb structure to a dispersely hardened material. In the present work, we attempt to determine what proportion of grains with intact coat� ings is sufficient for the composite with an initial hon� eycomb structure to retain its high strength (within the spread of mechanical test data). The loss of honeycomb structure, with degradation to a dispersely hardened material, is simulated by mix� ing NiAl powders with and without tungsten coatings. The grain size of stoichiometric NiAl (obtained by crushing and sifting cast material) is 15–17 μm; the deposited (with carbonyls) tungsten layer on the grains is around 0.5 mm in thickness. In pressing the powders (pressure up to 70000 atm, without a protective atmo� sphere, for 5–10 s at 1350–1400°C), cracks may appear in the tungsten layer on the grains (according to observations on metallographic samples after etching with Marble’s reagent): over the perimeter of a single grain, 0.5–1.5 cracks may be observed. After pressing, the samples are cylindrical (height 5–6 mm; diameter 5–6 mm). The porosity of all the materials is no more than 8 vol %, with sufficiently uniform distribution over the volume. The whole range of compositions is investigated, from 100% pure NiAl to 100% composite with honeycomb structure. Powder mixtures are prepared in a UZDN�2T ultrasound system, by mixing weighed portions (10 g) in ethyl alcohol for 5 min. After complete drying, the material undergoes dry mechanical mixing for 10 min. The monolithic samples obtained are characterized by uniform distribution of the components over the pressed volume. Table 1 summarizes the alloy compo� sition. The mechanical behavior of the materials is studied by measuring the microhardness at 20°C and deter� mining the fracture stress of thin samples (disks) on