The Alpha Titanium is known to be non heat treatable but it is strengthened by Ti3Al precipitate for increased aluminum content. Doesn't this constitute a contradiction?
Seems like a tough question. All Al-Ti phase diagrams cannot agree of whether the alpha-Ti field is continous with a narrow channel between beta-Ti and Ti3Al fields, or does the Ti3Al field touches and cuts the beta-Ti field to make a peritectoid reaction and splits the alpha-Ti field into two.
In both ways, if you heat 15-20% Al-Ti , you can dissolve Al-Ti intermetallics, quench and slowly heat-treat to precipitate Ti3Al. But note carefully, The solvus line of ALpha-Ti taking Ti3Al is very much sharp, meaning quite significant undercooling would not provide too much supersaturation but kinetics would be slowed down to percipitate out Ti3Al. And also, the alpha-to-beta phase transition line is also quite steep and dangerously close to Ti solvus line. At 20% Al, only 80 C superheat to dissolve all preexisting Ti3Al for precipitation hardening would bring you into alpha-beta 2-phase zone. For 25% AL, this gap is less than 30 degrees! So you see, controlling Precipitation hardening and polymorphic transition simultaneously would be a severe hassle. But still, precipitation hardening would be theoretically possible
For the other part of alpha-Ti, stable only above 1120 C or so, the strengthening phase would be TiAl. As Ti would be quite softened at that temperature, higher %Al would provide more precipitates and better strength. But I am not sure whether quenching this Ti structure would retain metallic phase, or a double-intermetallic of TiAl-Ti3Al would form.
In short, theoretically, if Solid solution stenthening, or even composite formation of Ti-Ti3Al is done, higher Al means higher strength. But considering the kinetic condition, making this structure by heat treatment is extremely difficult.
The phase diagrams are from https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.researchgate.net%2Ffigure%2FBinary-Ti-Al-phase-diagram-1_fig1_340513723&psig=AOvVaw1xskoycgOy-u4mhcuMkEoX&ust=1639559847205000&source=images&cd=vfe&ved=0CAkQjhxqGAoTCLD2qsT64vQCFQAAAAAdAAAAABC0AQ and https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.researchgate.net%2Ffigure%2FTi-Al-binary-phase-diagram-1993Oka_fig2_238894797&psig=AOvVaw1xskoycgOy-u4mhcuMkEoX&ust=1639559847205000&source=images&cd=vfe&ved=0CAkQjhxqGAoTCLD2qsT64vQCFQAAAAAdAAAAABDDAQ
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