Institute of Metals Division - Titanium-Rich Regions of the Ti-C-N, Ti-C-O, and Ti-N-O Phase Diagrams

The Ti-C-N and Ti-C-O systems were investigated in the temperature range from 500° to 1400°C and in the composition range up to 2 pct C and 5 pct N or 0. Characteristic isothermal sections at 800°, 900°, 1000°, and 1300°C are presented. The Ti-N-0 system was studied in the temperature range from 900' to 1400°C with alloys containing up to 6 pct total alloying content. Characteristic isothermal sections at 1000° and 140O°C are presented. Melting-point data for all three systems are also included. THIS paper reports on one of a series of investi-gations which have been conducted on the phase diagrams resulting from interstitial alloying with iodide titanium. The other investigations involved delineation of the binary systems with carbon,' nitrogen and boron,h and oxygen. The Ti-0 binary system has also been investigated by Bumps et al.' In varying degrees, each of these interstitial elements has been shown to stabilize the low temperature a modification of titanium1-5 and each forms a face-centered cubic TiX compound (henceforth designated 6). In addition, the Ti-N and Ti-0 systems reveal a low temperature tetragonal phase (6) formed by a peritectoid reaction between a and TiX Experimental Procedure The development of experimental techniques for the study of titanium alloy systems has, to a large extent, become standardized. In this investigation, the equipment and procedures described in detail by Cadoff and Nielsenl have been used. Arc Melting: In general, binary alloys with carbon, nitrogen, and oxygen, prepared in the composition range of interest in this investigation, show negligible composition changes during arc melting. However, the possibility of the formation of some gaseous combination of alloying elements such as CO, CN, or NO during the preparation of these ternary alloys was considered. Calculations showed that the evolution of only 0.05 gram of such a gas would be detectable as a pressure change in the closed system used during preparation of these alloys. Such pressure changes were not observed. Consequently, nominal compositions have been used in plotting the data. The compositions of the materials used in the preparation of the alloys are shown in Table I. After melting for 3 to 5 min at 275 to 350 amp, the alloys were checked for homogeneity by microstruc-tural examination. Alloys containing up to 1 pct C were homogeneous in the presence of less than 3 pct N or 0. At higher alloying contents, some inhomo-geneities in the carbon distribution became evident. Alteration of the melting procedure toward longer times and higher currents did not improve the homogeneity of these alloys. Ti-N-0 alloys were homogeneous in the range to about 3 or 4 pct total alloying addition. Beyond this, almost all of the specimens showed as-cast microstructures consisting only of the phase. Consequently, inhomogene-ities could not be detected by examination of micro-structures. Ten alloys from each of the systems were analyzed for two of the elements present (oxygen being omitted in all cases and titanium being omitted in the Ti-C-N alloys). In all cases the analyses were found not to be sufficiently precise to serve as criteria for the total composition of these alloys. On the basis of phase distribution in heat-treated alloys, however, it appears that carbon is distributed throughout the alloys most uniformly, with oxygen and nitrogen following in that order. Heat Treatment: Specimens for heat treatment were wrapped in titanium sheet before sealing in the argon-filled quartz capsules. Heat-treatment times varied from 100 hr at 800°C to 0.5 hr at 1400 °C. After heat treatment the specimens were quenched by breaking the capsule in water. With the exception of alloys in the low composition region, heat treatment did not have an appreciable effect on the as-cast microstructures. Metallography: Following heat treatment, the specimens were prepared for metallographic examination by grinding on emery paper and electrolytic polishing. For the majority of the specimens a 10-sec etch with Remington "A" agent (25 pct HNO3, 25 pct Hf, and 50 pct glycerin) adequately