. . . (T)itanium surfaces in contact with each other or with other metals readily gall under conditions of sliding contact or fretting. Even with light loading and little relative movement, complete seizure of surfaces can occur. This situation is caused by adhesive wear in which microscopic asperities on the metal surfaces come into contact as a result of relative sliding and they tend to weld together forming a bond at the junction which can have a rupture strength greater than the strength of the underlying metal. Fracture then takes place at one of the asperities causing metal to be transferred from one surface to the other. The debris so formed gives rise to the accelerated wear that occurs with titanium."
https://www.azom.com/article.aspx?ArticleID=1219
The bolt and bolt carrier in an AR see not only high loads, but large relative movement. This makes them very poor candidates for titanium.
Anodized titanium has a surface as hard or harder than anodized aluminum, it can be as high as RHc 70 equivalent. Core hardness of a titanium bolt would be in the RHc 35-40 range which is similar to the core hardness of Carpenter 158 or 9310. It's not the hardness, but the galling.
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