Method of heat treating titanium aluminide

Abstract

A gamma titanium aluminide alloy consisting of 46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium plus incidental impurities has an alpha transus temperature T α =1335° C. The gamma titanium aluminide alloy was heated to a temperature T 1 =1360° C. and was held at T 1 =1360° C. for 1 hour or longer. The gamma titanium aluminide alloy was fluidised bed, or salt bath, quenched to a temperature T 2 , where 900° C.<T 2 <1200° C., and was held at temperature T 2 for a sufficient time to allow the massive transformation to go to completion. The gamma titanium aluminide alloy was heated to a temperature T 3 =1300° C. or 1320° C. and was held at T 2 for 4 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. The heat treatment reduces quenching stresses, allows larger castings and a broader range of titanium aluminide alloys to be grain refined.

1 . A method of heat-treating 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 a 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 a temperature in the range of 900° C. to 1200° C.,    (d) maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure,    (e) heating the titanium aluminide alloy to a temperature below the alpha transus temperature in the alpha and gamma phase field,    (f) 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,    (g) 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 (f) the predetermined time period is up to 4 hours.  
     
     
         4 . A method as claimed in  claim 1  wherein step (e) 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 (g) comprises air-cooling or furnace cooling.  
     
     
         7 . A method as claimed in  claim 1  wherein step (c) comprises fluidised bed cooling or salt bath cooling.  
     
     
         8 . A method as claimed in  claim 1  comprising cooling the titanium aluminide to ambient temperature after step (d) and before step (e).  
     
     
         9 . A method as claimed in  claim 8  wherein the titanium aluminide is cooled to ambient temperature by air-cooling or oil cooling.  
     
     
         10 . A method as claimed in  claim 1  wherein the titanium aluminide alloy comprises 48 at % aluminium, 2 at % chromium, 2 at% niobium and the balance titanium and incidental impurities.  
     
     
         11 . A method as claimed in  claim 10  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) and (d) comprise salt bath, or fluidised bed, cooling the titanium aluminide alloy from a temperature of 1380° C. to a temperature between 900° C. and 1200° C. and maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure, steps (e) and (f) 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 (g) comprises air cooling the titanium aluminide alloy to ambient temperature.  
     
     
         12 . A method as claimed in  claim 1  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.  
     
     
         13 . A method as claimed in  claim 12  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, steps (c) and (d) comprise salt bath cooling, or fluidised bed cooling, the titanium aluminide alloy from a temperature of 1360° C. to a temperature between 900° C. and 1200° C. and maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure, steps (e) and (f) comprise heating the titanium aluminide alloy to a temperature of about 1300° C. to about 1320° 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.  
     
     
         14 . 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 is titanium and incidental impurities.  
     
     
         15 . 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 is titanium plus incidental impurities.  
     
     
         16 . A method as claimed in  claim 1  wherein the titanium aluminide alloy is a cast titanium aluminide component.  
     
     
         17 . A method as claimed in  claim 1  wherein 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 (f).  
     
     
         19 . A method as claimed in  claim 17  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.