EFFECT OF PURITY ON THE S TRUCTURE PROPERTIES OF MOLYBDENUM

An investigation of the effect of the purity on the structure and properties of molybdenum was conducted on vacuum melted material obtained in arc or zone-melting furnaces. The purity was varied by changing the rate of melting and the number of passes in zone refining. The ingots from the arc furnace heat were subjected to pressing and warm rolling in sheaths to rods 10 mm in diameter. The single-crystal rod obtained by zone refining was rolled in a steel sheath with total deformation over 60~c and subjected to recrystallizatio n annealing. Tensile tests were made in the IM-4R machine on samples 3 mm in diameter. Chemical analysis of the molybdenum ingots obtained by arc melting and zone-refined rods (Table 1) indicate that vacuum melting increases the purity of molybdenum as compared with the original material. Reducing the rate of melting in the arc furnace leads to removal of oxygen and to a lesser extent of silicon, nitrogen, and hydrogen. The amount of metallic impurities of copper, chromium, and iron remained unchanged within the limits of precision of the spectral analysis, while the copper concentration was within the limits of precision of the chemical analysis: (17-22) 10 -3 wt.%. Zone melting leads to elimination of metallic and gaseous impurities, while the amount of carbon decreases only after zone refining five times. The microstrueture hardly differed in samples zone refined twice and five times. In microsections f rom ingots obtained by arc melting there were numerous precipitates and gas pores in the grain boundaries and within the grains; the number of inclusions decreases with increasing purity of molybdenum (Fig. 1). On the facets (surfaces) of intergranular fractures of ingots from heats 1-3, containing -> 17 �9 10 -3 wt. ~c 02, there were numerous precipitates of a pale yellow-pink color differing in size on the shiny light background of the grain boundaries. With increasing purity of molybdenum (heats 4-6) the density of the precipitates on the facets decreases (Fig. 2). Within small areas of the facets the precipitates of the same shape are aligned in the same direction. Analysis of a large numberofintergra nularfractures showed that the directional effect is due to the crystallographic orientation of the matrix. The precipitates have the shape of diamonds and prisms, and a number of precipitates have the shape of "keys" with an angle of 120 ~ Dense films of precipitates (or "shells" [1]) covering the entire grain were not observed even in highly contaminated metal (heat 1). The connection between the number of precipitates and the amount of oxygen in