Character of rupture of rubbing surfaces of chromium-base alloys in the temperature range 20–1000°C

Abstract

1000~ Specimens of Cr + 5% TaC + 3% MgO + 0.2% Ti and Cr + 3% NbC + 3% MgO + 0.2% Ti alloys were prepared by sintering compacts at 1500~ and holding them for 2 h in a purified argon atmosphere. Friction and wear tests were carred out in a special apparatus in air by the method described in [I]. Metallographic examinations revealed that after sintering the two alloys had practically identical structures: They were both chromium-base solid solutions with evenly distributed i- to 2-~m MgO inclusions and TaC and NbC particles of size not exceeding 2-3 ~m (Fig. I). The porosity of the alloys in the sintered condition, determined by hydrostatic weighing, lay in the range 4-8%. In the temperature range 20-600~ both alloys experienced severe wear (Fig. 2) independent of microstructure and composition. At higher temperature, 600-I000~ the wear of the TaC-strengthened chromium alloy was only between a quarter and a half that of the NbC-strengthened alloy. The higher wear resistance of the chromium alloy strengthened with the carbide TaC compared with the alloy strengthened with NbC was linked with a number of specific wear and oxidation characteristics of this material. Fracture of sintered alloy specimens and surfaces of specimens subjected to wear tests were investigated with a Kwikscan-100 scanning electron microscope. This instrument has a large depth of focus and high resolution, and is also suitable for electron probe microanalyses [3]. The dimensions of the microscope chamber and the versatility of the instrument, enabling specimens to be moved in two directions at right angles to each other and tilted in wide ranges, made it possible to study not only surfaces disintegrated by wear but also individual particles (wear detritus). To attain optimum resolution conditions and obtain the best-quality photographs, the maximum possible accelerating voltage -- 16 kV -- was used. Objects were examined at magnifications of 200-10,000 diameters. Photographs of surfaces taken after rubbing tests at temperatures st 20-I000~ are shown in Figs. 3-7. On the surfaces of specimens tested at 20-400~ (in the temperature range of intense wear) there were clearly visible signs of layer-by-layer brittle rupture (Fig. 3). The surfaces of specimens subjected to rubbing tests at 20~ had deep scores and consisted of individual fragments of material with a strained structure. The microhardness of the strained regions was one and a half times that measured in the unstrained conditions. The rubbing of the alloys in the temperature range 20-400~ was accompanied by abrasive wear, involving the plucking out of individual particles (wear debris). On rubbing surfaces there was wear debris formed as a result of brittle rupture (Fig. 4). Examinations were made of fractures as well as of rubbing surfaces of the alloys (Fig. 5). The character of rupture in fracture was found to be identical with that found in rubbing. Disintegration during fracture was accompanied by lamination over planes parallel to the direction of deformation, with the formation of so-called "river patterns." According to [4, 5], such lamination is linked with a laminar arrangement of insoluble particles of a second phase (a carbide or an oxide) in an alloy.