Surface hardening of Ti alloys by gas-phase nitridation: Kinetic control of the nitrogen surface activity
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I. INTRODUCTION
LIGHTWEIGHT structural alloys based on Ti have important aerospace, biomedical, and other applications in demanding environments. For many of these uses, however, the alloys lack sufficient surface hardness and wear resistance. A procedure analogous to surface alloying (carburization or nitridation) of steels to overcome these deficiencies would be most desirable, and a number of treatments have been proposed over the years for such “case” hardening (surface hardening) of Ti-base alloys. These include nitridation, carburization, and oxidation by gas-discharge plasmas,[1–8] high-energy ion implantation,[9,10] exposure to molten salts,[11,12] electrochemical hydrogenation,[13,14] and laserinduced surface melting in nitrogen or carbon-containing atmospheres.[1,15–18] However, these approaches often cause precipitation of carbides or nitrides, predominantly at surfaces and grain boundaries, which compromise the toughness and fatigue resistance. In the present work, we have developed a novel procedure to nitride Ti-base alloys by “kinetically controlled” nitridation, which hardens the surface with only minimal degradation of the otherwise desirable mechanical properties of these advanced structural alloys. The novelty of our process derives from the recognition that it is possible to nitride these alloys under conditions far from thermodynamic equilibrium, such that nitride second phases cannot form, and that conformal and smoothly graded nitridation can be imparted to finished components. II. THE NEW APPROACH The Ti-N phase diagram in Figure 1 indicates considerable solubility of interstitial nitrogen in titanium.[19] At 1050 °C, for example, the solubility reaches its maximum of 23 at. pct in hcp -Ti, and similar solubilities exist for -Ti–based alloys. Previous attempts to nitride various titaL. LIU, and F. ERNST, G.M. MICHAL, and A.H. HEUER, Professors, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204. Contact e-mail: [email protected] Manuscript submitted January 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
nium alloys with nitrogen-containing mixtures at atmospheric pressure invariably led to the formation of Ti nitrides, because the affinity of Ti for nitrogen is so high and Ti nitrides are so stable that their formation could not be suppressed. It is a straightforward task to calculate the nitrogen activity, or the equivalent nitrogen partial pressure, at which nitrides can form in pure Ti. At 860 °C (below the -–phase transus in Figure 1), for example, the nitrogen partial pressure needs to be below 1020 Pa to prevent nitride formation (Figure 2). Accordingly, the nitrogen partial pressures that can be realized in a nitrogen-containing gas under atmospheric pressure for nitridation absent nitride formation are many orders of magnitude too high. (Even nominally “pure” Ar, often used to generate an inert atmosphere, contains too high a nitrogen activity to avoid nitride formation.) In reality, somewhat higher
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