Abstract
The potential for β grain boundaries (GBs) to give rise to large Widmanstätten colonies was determined through the observation of a large amount of precipitates at β GBs in a β-metastable titanium alloy using electron backscatter imaging and diffraction. The most critical boundaries are those which transform early and where a type of Variant Selection (VS) called double Burgers VS occurs. This mechanisms take place at ‘special’ β boundaries misoriented so that an α precipitate can be related to both grains through the Burgers relation. It was shown that the most critical GBs have a disorientation at an angle of less than 10° from 49.5°/<110> or 60°/<110>. A simulation study allowed those boundaries to be quantifed in crystallographic textures typical of industrial products. Those texture have then been discussed as a function of their potential to form large Widmanstätten colonies.
Highlights
In-service fatigue performance of titanium alloys decreases in the presence of large α regions with a single orientation which may act as fatal crack initiation sites [1,2,3,4]
We focus on the β grain boundaries (GBs) first transformed. αGB precipitation was assessed as a function of the crystallographic characteristics of GBs
It reaches about 70% when θ2-BOR is below 4° and steadily decreases until it reaches the average value of 20% at θ2-BOR < 14°
Summary
In-service fatigue performance of titanium alloys decreases in the presence of large α regions with a single orientation which may act as fatal crack initiation sites [1,2,3,4] Some of these large regions correspond to Widmanstätten colonies that are nucleated on both side of a grain boundary (GB). Two colonies with the same orientation are emitted in the two adjacent β grains, and so form anormally large crystallographic domains [2,5] This mechanism only applies when neighboring β grains have one of the following four ‘special’ disorientations: Type I - 10.5°/, Type II - 49.5°/, Type III - 60°/, Type IV - 60°/ [6]. The statistical distribution of the ‘special’ boundaries are quantified in different β textures of industrial products to evaluate if some textures are preferable to avoid large colonies
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