Method of heat treating titanium aluminide

Abstract

A gamma titanium aluminide alloy consisting of 48 at % aluminium, 2 at % chromium, 2 at % niobium and the balance titanium plus incidental impurities was heat treated according to the present invention. The gamma titanium aluminide alloy has an alpha transus temperature T α =1360° C. The gamma titanium aluminide alloy was heated to a temperature T 1 =1380° C. and was held at T 2 =1380° C. for 1 hour. The gamma titanium aluminide alloy was oil cooled. The gamma titanium aluminide alloy was heated to a temperature T 2 =1320° C. and was held at T 2 =1320° C. for 2 hours. The gamma titanium aluminide alloy was air cooled to ambient temperature. The gamma titanium aluminide alloy has a fine duplex microstructure comprising differently orientated alpha plates in a massively transformed gamma matrix.

I claim:  
     
         1 . A method of heat-treating a titanium aluminide alloy, the titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure, the method comprising the steps of 
 (a) heating a titanium aluminide alloy to a temperature above the alpha transus temperature,    (b) maintaining the titanium aluminide alloy at the temperature above the alpha transus temperature in the single alpha phase field for a predetermined time period,    (c) cooling the titanium aluminide alloy from the single alpha phase field to produce a massively transformed gamma microstructure,    (d) heating the titanium aluminide alloy to a temperature below the alpha transus temperature in the alpha and gamma phase field,    (e) maintaining the titanium aluminide alloy 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 in the titanium aluminide alloy,    (f) cooling the titanium aluminide alloy to ambient temperature.    
     
     
         2 . A method as claimed in  claim 1  wherein in step (b) the predetermined time period is up to 2 hours.  
     
     
         3 . A method as claimed in  claim 1  wherein in step (e) the predetermined time period is up to 4 hours.  
     
     
         4 . A method as claimed in  claim 1 , wherein step (d) comprises heating the titanium aluminide alloy to a temperature about 30° C. to 60° C. below the alpha transus temperature.  
     
     
         5 . A method as claimed in  claim 1  wherein step (a) comprises heating the titanium aluminide alloy to a temperature of about 20° C. to 30° C. above the alpha transus temperature.  
     
     
         6 . A method as claimed in  claim 1  wherein step (f) comprises air-cooling or furnace cooling.  
     
     
         7 . A method as claimed in  claim 1  wherein step (c) comprises air-cooling or oil cooling.  
     
     
         8 . A method as claimed in  claim 1  wherein the titanium aluminide alloy consists of at least 46 at % aluminium.  
     
     
         9 . A method as claimed in  claim 8  wherein the titanium aluminide alloy comprises 48 at % aluminium, 2 at % chromium, 2 at % niobium and the balance titanium and incidental impurities.  
     
     
         10 . A method as claimed in  claim 9  wherein the alpha transus temperature is about 1360° C., step (a) comprises heating to a temperature of 1380° C., step (b) comprises maintaining the titanium aluminide alloy at a temperature of about 1380° C. for about 1 hour, step (c) comprises oil cooling the titanium aluminide alloy from a temperature of 1380° C. to produce a massively transformed gamma microstructure, steps (d) and (e) comprise heating the titanium aluminide alloy to a temperature of about 1320° C. for about 2 hours to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy, and step (f) comprises air cooling the titanium aluminide alloy to ambient temperature.  
     
     
         11 . A method as claimed in  claim 8  wherein the titanium aluminide alloy comprises 46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium and incidental impurities.  
     
     
         12 . A method as claimed in  claim 11  wherein the alpha transus temperature is about 1335° C., step (a) comprises heating to a temperature of 1360° C., step (b) comprises maintaining the titanium aluminide alloy at a temperature of about 1360° C. for about 1 hour, step (c) comprises oil cooling, or air cooling, the titanium aluminide alloy from a temperature of 1360° C. to produce a massively transformed gamma microstructure, steps (d) and (e) comprise heating the titanium aluminide alloy to a temperature of about 1300° C. for about 4 hours to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy, and step (f) comprises air cooling the titanium aluminide alloy to ambient temperature.  
     
     
         13 . A method as claimed in  claim 1  wherein the titanium aluminide alloy consists of 45-46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium and incidental impurities.  
     
     
         14 . A method as claimed in  claim 1  wherein the titanium aluminide alloy consists of 45-46 at % aluminium, 2-6 at % niobium, 2-6 at % hafnium and the balance titanium plus incidental impurities.  
     
     
         15 . A method as claimed in  claim 14  wherein the titanium aluminide alloy consists of 46 at % aluminium, 4 at % niobium, 4 at % hafnium and the balance titanium plus incidental impurities.  
     
     
         16 . A method as claimed in  claim 1  wherein the titanium aluminide alloy is a cast titanium aluminide alloy component.  
     
     
         17 . A method as claimed in  claim 16  comprising hot isostatic pressing of the cast titanium aluminide alloy component.  
     
     
         18 . A method as claimed in  claim 17  wherein the hot isostatic pressing of the cast titanium aluminide alloy component is concurrent with step (e).  
     
     
         19 . A method as claimed in  claim 18  wherein the hot isostatic pressing comprises applying a pressure of about 150 MPa for about 4 hours.  
     
     
         20 . A method as claimed in  claim 1  wherein the titanium aluminide alloy is a compressor blade or a compressor vane.