Alloy and method of treating titanium aluminide

Abstract

A method of treating a titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure by subjecting the alloy to a temperature cycle that produces the massively transformed microstructure having a refined microstructure; the method being characterised in that oxygen securing means are provided within the alloy to prevent diffusion of oxygen to the grain boundary.

1 . A method of enhancing massive transformation of a titanium aluminide alloy having a single alpha phase field by incorporating up to 0.5 at % of oxygen scouring means into the alloy which inhibits diffusion of oxygen within the alpha phase field to the grain boundary as the alloy is subjected to a temperature cycle which produces a massively transformed gamma microstructure.  
   
   
       2 . A method according to  claim 1 , wherein the oxygen scouring means is incorporated up to 0.2 at %.  
   
   
       3 . A method according to  claim 1 , wherein the temperature cycle comprises the steps a) heating the titanium aluminide alloy to a temperature above the alpha transus temperature, b) maintaining the titanium aluminide alloy at a temperature above the alpha transus temperature for a predetermined time period, c) cooling the titanium aluminide alloy from the single alpha phase field to produce a massively transformed gamma microstructure.  
   
   
       4 . A method according to  claim 3 , wherein the heat treatment further comprises the step of d) heating the titanium aluminide to a temperature below the alpha transus temperature in the alpha and gamma phase field, e) maintaining the titanium aluminide at the temperature below the alpha transus temperature for a predetermined time period to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced, and f) cooling the titanium aluminide to ambient temperature.  
   
   
       5 . A method according to  claim 1 , wherein the oxygen securing means is selected from the group comprising yttrium and hafnium.  
   
   
       6 . A method according to  claim 1 , wherein the alloy consists at least 43 at % aluminium, 0 to 9 at % niobium, 0 to 10 at % tantalum, 0.01 to 0.15 at % yttrium, the balance being titanium plus incidental impurities.  
   
   
       7 . A method according to  claim 1 , wherein step c) comprises cooling the titanium aluminide to ambient temperature.  
   
   
       8 . A method according to  claim 1 , wherein the titanium aluminide is cooled by gas cooling, oil cooling, fluidised bed cooling or salt cooling.  
   
   
       9 . A method according to  claim 1 , wherein step c) comprises cooling the titanium aluminide at a cooling rate of 4° C.S −1  to 150° C.S −1 .  
   
   
       10 . A method as claimed in  claim 1 , wherein the titanium aluminide alloy is a cast titanium aluminide component.  
   
   
       11 . A method as claimed in  claim 10 , wherein the method further comprises the step of hot isostatic pressing the cast titanium aluminide component.  
   
   
       12 . A method as claimed in  claim 1 , wherein the titanium aluminide alloy provides a compressor blade or a compressor vane.  
   
   
       13 . An alloy consisting of 43 to 50 at % aluminium, 0 to 9 at % niobium, 0 to 10 at % tantalum, 0.01 to 0.2 at % of yttrium and/or hafnium, the balance being titanium plus incidental impurities.  
   
   
       14 . An alloy according to  claim 13 , consisting of 45 to 46 at % aluminium, 7 to 9 at % niobium, 0.02 to 0.15 at % yttrium, the balance being titanium plus incidental impurities.  
   
   
       15 . An alloy according to  claim 13 , wherein the niobium plus tantalum is less than or equal to 10 at %.  
   
   
       16 . An alloy claimed in  claim 13 , wherein the titanium aluminide alloy is a cast titanium aluminide component.