An investigation of reaction forming by controlled passivation of titanium-zirconium alloys with niobium.

摘要

In previous studies by the author{dollar}sp1,{dollar} it was demonstrated that rapid oxidation rates of Ti-Zr alloys containing 15-35 wt.% Zr may be significantly reduced by additions of Nb in the range 3.5-14 wt.%. The present studies were conducted to further investigate the mechanisms responsible for this behavior and the nature of the oxidation products.; Oxidation rates of Ti-Zr, reduced by an order of magnitude through small additions of Nb, were, nevertheless, three-to-five times those of pure Ti or Zr. For this reason, relatively thick (e.g., 0.050-0.250-inch) oxide ceramic layers were obtained at moderate temperatures and relatively short times. This behavior makes it economically feasible to hard-surface appropriate metallic objects while retaining strength and toughness in the interiors of the objects. Surface oxide thicknesses of 2.5-25 {dollar}mu{dollar}m with hardness of R{dollar}sb{lcub}rm c{rcub}{dollar} 70 were obtained on Ti-35Zr-10Nb (wt.%) alloy by heating in air at 500-800{dollar}spcirc{dollar}C for 10-30 minutes.; It was found that oxide layers formed in air on certain Ti-Zr-Nb alloys at temperatures between 600 and 1500{dollar}spcirc{dollar}C are monolithic structures containing uniformly distributed microporosity. These ceramic structures are durable and adherent, as opposed to the friable oxidation products obtained from conventional Ti and Zr alloys under similar conditions. By properly controlling oxidation time and temperature, it was demonstrated that metallic objects of intricate shape may be partially or totally converted to oversized ceramic reproductions.; Ceramic/metal composites, as well as monolithic ceramic structures, were obtained by partially or totally oxidizing Ti-35Zr-10Nb alloys in air at temperatures in excess of 1000{dollar}spcirc{dollar}C. Such materials may be of interest to the medical/dental community for use in hard tissue repair or replacement. In certain applications, the uniform microporosity present in the ceramic may encourage biological tissue ingrowth and improve adhesion of other materials. Ta, when added to Ti-Zr alloys, affected oxidation in an analogous manner to Nb.; Mechanical properties of Ti-Zr-Nb alloys were shown to be altered significantly by solution-treating-and-quenching, followed by precipitation of the alpha phase. Yield strength of 130-140 ksi was obtained with elastic modulus 10-12 Msi, depending upon heat treatment. In spite of predominately BCC structure, the alloys displayed ductile fracture in Charpy V-notch testing conducted at temperatures as low as {dollar}-{dollar}196{dollar}spcirc{dollar}C. Excellent fabricability of Ti-Zr-Nb alloys was demonstrated by press forging, hot rolling, cold rolling, close-die forging, and investment casting. Wrought and cast articles were subsequently converted to oversized ceramic/cermet reproductions by controlled oxidation. During hot working operations, premature oxidation of the alloys was prevented by the use of glass barrier coatings.