Niobium-silicide alloys having a surface region of enhanced environmental-resistance, and related articles and processes

Abstract

Niobium silicide articles are described. They include a surface region enriched with at least about 25 atom % germanium, which can enhance the properties of the article. Methods for preparing these articles are described as well. According to one method, an article is formed from a niobium silicide composite material which contains a selected amount of germanium. The article is then heat-treated under conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region. In another embodiment, a germanium-containing material is applied over a niobium-silicide article, and then diffused into the surface region of the article by way of a heat treatment.

1 . A niobium silicide article which includes a surface region comprising at least about 25 atom % germanium, based on the composition of the surface region. 
     
     
         2 . The article of  claim 1 , wherein the surface region comprises at least about 40 atom % germanium. 
     
     
         3 . The article of  claim 1 , wherein the amount of germanium in the surface region is in the range of about 25 atom % to about 67 atom %. 
     
     
         4 . The article of  claim 1 , wherein at least a portion of the germanium in the surface region is in the form of a niobium germanide phase. 
     
     
         5 . The article of  claim 4 , wherein the niobium germanide phase is niobium digermanide (NbGe 2 ). 
     
     
         6 . The article of  claim 5 , wherein at least about 40% of the germanium in the surface region is in the form of the niobium digermanide phase. 
     
     
         7 . The article of  claim 6 , wherein at least about 50% of the germanium in the surface region is in the form of the niobium digermanide phase. 
     
     
         8 . The article of  claim 1 , wherein the surface region extends to a depth of about 30% of the cross-sectional thickness of the article. 
     
     
         9 . The article of  claim 1 , wherein the surface region extends to a depth of about 50 microns. 
     
     
         10 . The article of  claim 1 , comprising a bulk alloy region below the surface region, wherein the bulk alloy comprises a metallic niobium-base phase and at least one metal silicide phase. 
     
     
         11 . The article of  claim 10 , wherein the bulk alloy region further comprises titanium and at least one element selected from the group consisting of rhenium and ruthenium. 
     
     
         12 . The article of  claim 10 , wherein the bulk alloy further comprises titanium and at least one element selected from the group consisting of hafnium, chromium, and aluminum. 
     
     
         13 . The article of  claim 11 , wherein the bulk alloy further comprises at least one element selected from the group consisting of silicon, zirconium, tin, tungsten, and carbon. 
     
     
         14 . The article of  claim 1 , wherein at least one oxide layer is disposed over the surface region. 
     
     
         15 . The article of  claim 14 , wherein the oxide layer is formed by a heat treatment which is carried out to form the germanium-containing surface region. 
     
     
         16 . The article of  1 , wherein the germanium in the surface region is compositionally graded. 
     
     
         17 . The article of  claim 1 , wherein at least one protective coating is disposed over the surface region. 
     
     
         18 . The article of  claim 17 , wherein the protective coating is an oxidation-resistant coating. 
     
     
         19 . The article of  claim 18 , further comprising a thermal barrier coating disposed over the oxidation-resistant coating. 
     
     
         20 . The article of  claim 1 , in the form of a turbine engine component. 
     
     
         21 . The article of  claim 20 , wherein the turbine engine component is selected from the group consisting of turbine buckets, nozzles, blades, rotors, vanes, stators, shrouds, combustors, and blisks. 
     
     
         22 . A turbine component, formed at least in part from a niobium-silicide alloy which comprises niobium (Nb), silicon (Si); and at least one element selected from the group consisting of titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al);
 wherein a surface region of the niobium-silicide alloy comprises at least about 25 atom % germanium, and at least a portion of the germanium in the surface region is in the form of the niobium digermanide phase.   
     
     
         23 . The turbine component of  claim 22 , further comprising at least one protective coating over the surface region of the alloy. 
     
     
         24 . The turbine component of  claim 22 , further comprising an oxidation-resistant coating over the surface region of the alloy; and a yttria-stabilized zirconia thermal barrier coating disposed over the oxidation-resistant coating. 
     
     
         25 . The turbine component of  claim 22 , comprising at least one hole or passageway in a niobium-silicide portion of the component, wherein the interior alloy surfaces of the hole or passageway also comprise at least about 25 atom % germanium. 
     
     
         26 . A method for preparing a niobium silicide article which includes a surface region enriched in germanium, comprising the following steps:
 (a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase, at least one metal silicide phase, and at least about 10 atom % germanium, based on total atom percent of the composite material; and then   (b) heat-treating the article formed in step (a), under heating conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region.   
     
     
         27 . The method of  claim 26 , wherein at least a portion of the increase in the level of germanium in the surface region is caused by the migration of germanium from the article formed in step (a), up to the surface region. 
     
     
         28 . The method of  claim 26 , wherein the surface region extends to a depth of about 50 microns. 
     
     
         29 . The method of  claim 26 , wherein the heat treatment is carried out in an oxidizing atmosphere. 
     
     
         30 . The method of  claim 29 , wherein the heat treatment is carried out at a temperature in the range of about 600° C. to about 1400° C. 
     
     
         31 . The method of  claim 26 , wherein the heat treatment causes the formation of the niobium digermanide (NbGe 2 ) phase in the surface region. 
     
     
         32 . The method of  claim 31 , wherein at least about 40% of the germanium in the surface region is in the form of the niobium digermanide phase, after the heat treatment. 
     
     
         33 . A method for preparing a niobium silicide article which includes a surface region enriched in germanium, comprising the following steps:
 (a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase and at least one metal silicide phase;   (b) applying a germanium-containing material to a surface of the article formed in step (a); and then   (c) heat-treating the germanium-containing material and article, under conditions sufficient to cause at least a portion of the germanium to diffuse into the surface region of the article.   
     
     
         34 . The method of  claim 33 , wherein the surface region extends to a depth of about 30% of the cross-sectional thickness of the article. 
     
     
         35 . The method of  claim 33 , wherein the germanium-containing material comprises elemental germanium, or a germanium-containing compound or mixture. 
     
     
         36 . The method of  claim 33 , wherein the germanium-containing material is applied in the form of a slurry. 
     
     
         37 . The method of  claim 36 , wherein the slurry is applied by a technique selected from the group consisting of slip-casting, brushing, dipping, spraying, pouring, roll-coating, and spin-coating. 
     
     
         38 . The method of  claim 33 , wherein the germanium-containing material is applied to the surface of the article by a technique selected from the group consisting of plasma deposition; physical vapor deposition (PVD); chemical vapor deposition (CVD); sputtering; and pack processes. 
     
     
         39 . The method of  claim 33 , wherein the heat treatment of step (c) is carried out in a vacuum or an inert atmosphere. 
     
     
         40 . The method of  claim 39 , wherein the heat treatment is carried out at a temperature in the range of about (0.8)T m  to about (1.5)T m  of the germanium-containing material, where “T m ” represents the melting temperature.