Effect of Processing on Oxidation of Ti 5 Si 3
- PDF / 1,847,062 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 9 Downloads / 242 Views
EFFECT OF PROCESSING ON OXIDATION OF Ti 5 Si 3 ANDREW J. THOM, YOUNGMAN KIM, AND MUFIT AKINC Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011 ABSTRACT The mechanical properties and oxidation resistance of HIPed Ti5 Si3 have been measured. HIPing submicron size powder compacts produces a crack-free, fine-grained microstructure with significantly higher hardness and toughness than a coarse-grained microstructure which contains microcracks within larger grains. Oxidation resistance is influenced by the grain size. Coarse-grained material has much lower mass gain than finegrained material in an oxidizing atmosphere and exhibits parabolic oxidation kinetics. The oxidation resistance of fine-grained material was measured between 700'C and 1000°C in air. Mass gain at 120 hours was measured to be 0.07 mg/cm2 at 700 0 C. At 900 0 C cracking of the scale leads to linear oxidation kinetics and significantly higher mass gain. INTRODUCTION Silicides have received considerable attention as potential high temperature structural materials. Of the less commonly investigated silicides[l], Mo 5 Si3 , Nb5 Si3, Cr 5 Si3 , ZrsSi 3 , Ti5Si3 , V5 Si3 , and Ta5 Si3 are particularly interesting as potential high temperature materials. Among this group of materials, Ti5Si3 is attractive due to its low density, high melting point, potential oxidation resistance, and interesting interstitial chemistry. The previous work indicates that the monolithic material suffers from microcracking [2-7] which degrades the mechanical properties. Additionally, the oxidative behavior of TisSi 3 has received only limited attention[2,7,8]. In this paper, the effect of processing upon the mechanical and oxidative behavior of Ti5 Si3 is studied. In particular, the influence of grain size on the mechanical and oxidative behavior is discussed. EXPERIMENTAL Ti 5Si3 was synthesized in 125g batches. A stoichiometric mixture of titanium and silicon was arc-melted under an argon atmosphere using a non-consumable electrode. The buttons were milled to -325 mesh (40 hours) may be due to formation of stress cracks in the scale which exposed large amounts of new surface. The linear rate at (t> 40 hours) is slightly less than the initial rate for (t=0-3 hours), indicating this to be a plausible event. For both 900 and 1000°C, the linear rates are associated with the cracking of the scales, causing short circuit paths to the base material and allowing for an increased rate of mass gain. No scale formation was observed for samples exposed to air at 700'C. The external surface was slightly tarnished, indicating the presense of a thin film. The scale formation from 800 to 1000*C is shown in Figure 3. A dense outer 15 jim thick layer forms at 800°C. Dendrites about 12 14m wide grow from this outer layer into the base material. The total thickness of the layer and dendrites is about 150 jim. At 9000 C the dense outer layer has grown to 35 jim. The dendrites have coarsened to about 50 jim in width, and the total thickness is
Data Loading...
