A high-resolution transmission electron microscopy study of the precipitation process in a dilute Ti-N alloy
- PDF / 4,234,326 Bytes
- 12 Pages / 597.28 x 777.28 pts Page_size
- 22 Downloads / 176 Views
I.
INTRODUCTION
SINCE the solubility of nitrogen in titanium is very tow at room temperature, excess nitrogen atoms precipitate out of the matrix by different metastable configurations. It is seen from the recent version of the modified and updated Ti-N phase diagram, only two nitride phases, namely, the e phase of Ti/N stoichiometry (antirutile structure) and 6TiN phase (B1-NaC1 type structure), are reported, m Yet another intermediate phase, a', which is isostructural with the Ti2N phase but with a different composition, has also been reported to form preceding the formation of equilibrium Ti2N phase on isothermal aging at a temperature less than 473 K.[2,3] The a' phase has been further shown, by resistivity studies, to form as an intermediate precursor to Ti2N phase formationJ41 Details of the process and the microstructural features associated with the reaction have been looked into by conventional transmission electron microscopy (CTEM) methods.t2] High-resolution electron microscopy (HREM) examination has now been carried out to understand the intermediate stages of precipitation. It is the aim of this article to present new HREM results and examine the results and conclusions of the earlier work to arrive at a comprehensive picture of the precipitation process and transformation sequence preceding the formation of equilibrium Ti2N phase.
II.
EXPERIMENTAL
PROCEDURES
Rods, 25 mm in diameter, of pure titanium procured from Materials Research Corporation (New York, NY) have been
D. SUNDARARAMAN, Leader, Special Materials Group, and V.S. RAGHUNATHAN, Head, Physical Metallurgy Section, are with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamilnadu, India. S. RANGANATHAN, Professor, is with the Centre for Advanced Study, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted August 7, 1995. 2966--VOLUME 27A, OCTOBER 1996
used in this study. The chemical analysis of the as-received material is shown in Table I with particular reference to interstitial solutes, namely, carbon, nitrogen, oxygen, and hydrogen. Thin slices of 1 to 2 mm in thickness were cut and rolled down to 50-/xm-thick foils. The foils were then given an annealing treatment at 1123 K for 15 to 20 minutes in sealed quartz ampoules. The quartz ampoules were previously evacuated to a vacuum better than 10 -5 torr and backfilled with pure argon atmosphere. In addition, as a precautionary measure, pieces of titanium sponge were also kept by the side of the sample to take care of any traces of oxygen present in the environment. This procedure was followed in all the subsequent treatments wherever necessary. The 50-/xm-thick annealed foils were then nitrided as per the following procedure: Edwards High Vacuum International, West Sussex, England, E-306 vacuum equipment was used to nitride the foils. The foils were clamped between the current leads and the chamber was pre-evacuated to a pressure better than 2 • 10 -5 torr. Ultrahigh purity nitrogen gas (oxygen co
Data Loading...
