Yeah Josh Leavitt ground out a couple of Ti hatchets in a similar size/weight, that Zoe Crist and I carbidized and reground the edges on.
They were fun toys, but essentially useless. Edge holding is a problem with Ti, but Axe/Hatchets generally utilize rather blunt edge geometry. The main issue with the concept of a UL hatchet, is that you're trying to eliminate the exact variable which causes the tool to be efficient at it's task. Axes are separated (in a specific style) by head weight for a reason.
It like trying to hit someone with a paper towel tube, no matter how hard you swing it, it's just not going to do any significant damage. Hardness isn't the issue, a carbon fiber tube would be rigid but similarly useless.
A Ti hatchet will work for batoning, but the problem you're going to run into is vibration and impact forces doing more damage to your hand than the work piece. The obtuse cross-sectional edge geometry also impairs the ability to move the tool through tight wood. Ultimately, a fixed blade of a thin cross-section, high toughness alloy will be *MUCH* more efficient, hold an edge, and with proper edge geometry will out-chop the axe by about 10 to 1 even if the weight is exactly the same, since it'll hold a thin sharp edge and allow it to bite the wood.
We've considered some other options, ideally, you want to concentrate the maximum percentage of total weight into the head of the hatchet, but I don't think you can make one that's really effective for less than 12-16oz or so total weight, but that's just my opinion, and in doing that, ideally, you want to make it out of a material that has high toughness, but can take and hold a keen edge to leverage the aforementioned "bite".
Carbide impregnated Ti excels at cutting tasks (since it creates a microserrated edge), but performs poorly in impact tasks like chopping, due to the brittleness of tungsten and titanium carbide. It's also important to use an industrial quality spark depositor to create a deep/thick enough carbide layer to hold up to long term abuse.
Anyway, fun project no doubt, and man I wish I had access to a waterjet.