Aluminum would not be a substitute for yttrium. Aluminum can be used to deoxidize less reactive metals, like iron. For a metal like titanium, you need a metal that is much more reactive than it. Yttrium is more reactive than magnesium, though less reactive than calcium, which is why it has been chosen.
Moreover, aluminum is undesirable in titanium implants, even if many surgeons without scruples have used cheaper Ti-Al-V alloys taken from aviation suppliers, instead of more expensive alloys designed specifically for compatibility with living tissues, despite the fact that it was always pretty clear that such Ti-Al-V alloys are not suitable for long-term implants.
Yttrium is also not desirable for implants, so the titanium produced by this method is not good for implants, but it is good for most other applications of titanium, where yttrium is not harmful.
Delving into the paper: Al has defo been used for deoxidizing Ti but they claim it's "inadequate"
The stability of al oxyhalide with respect to al oxide and al halide is the key here? Not sure if that has been "adequately" explored either, especially in experiment
(For the sake of more collaborative conversations on HN, not just dissfests :)
It is likely that most of the titanium deoxidized with yttrium would not be used as such, but it would be used for producing titanium alloys.
For each kind of titanium alloy, depending on its chemical composition and on its intended crystal structure, yttrium may happen to be harmful or beneficial. Yttrium atoms are significantly bigger than titanium atoms. This can influence the crystal structure and the mechanical properties of the alloys, even with only a small percentage of residual yttrium.
Almost pure non-alloyed titanium (which normally contains residual quantities of oxygen and iron) is used in applications where chemical resistance is more important than mechanical resistance, e.g. for medical implants, vessels and pipes exposed to various chemicals, spoons, metal parts that will be in contact with a human body, e.g. rings or bracelets etc.
Yttrium may diminish somewhat the chemical resistance of titanium for such applications, but the resistance might still be adequate for many of these applications.
Moreover, aluminum is undesirable in titanium implants, even if many surgeons without scruples have used cheaper Ti-Al-V alloys taken from aviation suppliers, instead of more expensive alloys designed specifically for compatibility with living tissues, despite the fact that it was always pretty clear that such Ti-Al-V alloys are not suitable for long-term implants.
Yttrium is also not desirable for implants, so the titanium produced by this method is not good for implants, but it is good for most other applications of titanium, where yttrium is not harmful.