Technical Notes - Allotropic Transformations at High Temperature

ALLOTROPIC transformations between 1000° and 1900°C have recently been reported for several metallic elements. Since the proposed transformations are based on limited evidence, it was decided to examine the elements in question as well as some refractory metals by thermal analysis with a photoelectric technique similar to that described by Hug-gins, Roll, and Udin.1 In this method, a lead sulphide cell was used to detect thermal radiation from an electric-resistance heated strip specimen. A thermocouple spot welded to the specimen was used to calibrate the temperature of the system under steady temperature conditions, and recording was accomplished with a cathode ray oscilloscope. In the method used by the present authors, the temperature calibration was omitted and the cathode ray oscilloscope was replaced by a recording galvanometer. This technique, in which electric current passing through the specimen heats it, will be referred to henceforth as Method 1. Another arrangement was required for those cases where the specimen could not be formed into a wire or strip suitable for resistance heating. In such cases the specimen was a sintered compact heated by thermal radiation from a hot tungsten strip, Fig. 1. The specimen was insulated from the heating strip with a thin layer of thoria powder or suspended on 0.004 in. tungsten wires. The take-up spring had just sufficient tension to prevent buckling of the heating strip due to thermal expansion. This technique will be called Method 2. Both methods were tested with specimens of iron and titanium and the well known allotropic transformations, a toy and ? to 8 in iron and a to in titanium, were very clearly detected. Fig. 2 shows one of the records obtained for titanium. For this particular experiment, the iodide titanium specimen was a square rod 0.080 in. on an edge and 3 in. long.