Forged titanium alloy material and method for manufacturing same

Abstract

Provided is a titanium-alloy forging material in which fatigue-strength characteristics are improved without worsening ultrasonic flaw detection. A β-forged titanium-alloy forging material (1) is characterized in that the area ratio of non-flat grains, which are prior β-grains (2) having an aspect ratio of 3 or less and a diameter in the forging direction of at least 20 μm, and an α-phase ratio at the crystal grain boundary (3) of at least 80%, is less than 10%, and the area ratio of flat grains, which are prior β-grains having an aspect ratio greater than 3 and a diameter in the forging direction of 20-700 μm, and an α-phase ratio at the crystal grain boundary (3) of at least 80%, is 85% or greater, and the average orientation difference of the α-phase crystal orientation deposited at the crystal grain boundary (3) of the flat grains is at least 6°.

1 . A β forged titanium alloy materials,
 wherein an area fraction of non-flattened grains in the β forged titanium alloy material is less than 10%, where the non-flattened grains are prior β grains having an aspect ratio of 3 or less, a diameter in a forging direction of 20 μm or more, and a proportion of an α-phase in a crystal grain boundary of 80% or more; 
 an area fraction of flattened grains in the β forged titanium alloy material is 85% or more, where the flattened grains are prior β grains having an aspect ratio of greater than 3, a diameter in the forging direction of from 20 μm to 700 μm, and a proportion of an a-phase in a crystal grain boundary of 80% or more; and 
 an average misorientation of crystal orientation of an α-phase precipitated along the crystal grain boundary of the flattened grains is 6° or more. 
 
     
     
         2 . The β forged titanium alloy materials of  claim 1 , which is produced by β-forging a titanium alloy having a Mo equivalent [Mo]eq of formula (1) of more than 2.7 and less than 15:
   [Mo] eq =[Mo]+[Ta]/5+[Nb]/3.6+[W]/2.5+[V]/1.5+1.25[Cr]+1.25[Ni]+1.7[Mn]+1.7[Co]+2.5[Fe]  (1)
 
 (wherein each element symbol in brackets on the right side of the formula (1) represents a mass % content of each element in the titanium alloy. 
 
     
     
         3 . The β forged titanium alloy materials of  claim 1 , having a thickness of 30 mm or more at the thinnest portion thereof and 70 mm on average. 
     
     
         4 . A method of manufacturing the β forged titanium alloy materials of  claim 1 , comprising:
 β-forging a titanium alloy material, 
 wherein the β forging comprises:
 heating a titanium alloy material to (T β +10)° C. or higher, wherein T β  represents a β-transus temperature of the titanium alloy material, until a β crystal grain diameter of the titanium alloy material falls within a range of from 300 βm to 1000 μm, 
 
 forging the heated titanium alloy material at a forging temperature T F  [° C.], which satisfies formula (2) under conditions in which the forging temperature T F [° C.] satisfies formulas (3) and (4), to produce a forged titanium alloy material; and 
 cooling the forged titanium alloy material to a temperature lower than (T β −150)° C.,
     T   β −150≤ T   F   ≤T   β +100  (2),
 
     Ln ( S   R )+22800/( T   F +273)−18.6≤0  (3),
 
     Ln ( S   R )+22800/( T   F +273)−13.20≥0  (4),
 
 
 (wherein in the formulas (2) to (4), T β  represents the β-transus temperature[° C.] of the titanium alloy material, T F  represents the forging temperature [° C.], and S R  represents a strain rate [s −1 ] upon forging. 
 
     
     
         5 . The method of  claim 4 , further comprising:
 billet forging an ingot comprising a titanium alloy to obtain a billet; and   α+β forging the billet in an α+β two phase region prior to the heating of the titanium alloy material.   
     
     
         6 . The method of  claim 5 ,
 wherein the billet has an acicular microstructure.   
     
     
         7 . The method of  claim 4 , further comprising:
 after the cooing of the forged titanium alloy material, irradiating the forged titanium alloy material with ultrasonic waves in a direction parallel to a direction in which an amount forged by the β forging is the largest to inspect a flaw of the forged titanium alloy material.   
     
     
         8 . The method of  claim 5 , further comprising:
 after the cooing of the forged titanium alloy material, irradiating the forged titanium alloy material with ultrasonic waves in a direction parallel to a direction in which an amount forged by the β forging is the largest to inspect a flaw of the forged titanium alloy material.   
     
     
         9 . The method of  claim 6 , further comprising:
 after the cooing of the forged titanium alloy material, irradiating the forged titanium alloy material with ultrasonic waves in a direction parallel to a direction in which an amount forged by the β forging is the largest to inspect a flaw of the forged titanium alloy material.   
     
     
         10 . The method of  claim 4 ,
 wherein the forged titanium alloy material is suitable for manufacturing an aircraft engine component.