I don't get what you mean with 'expose the user credentials in dev tools'.
The only person with access to those dev tools is you, seeing as you are looking at them. So the only person actually seeing those credentials in dev tools, is the person looking at the browser. This is the same person who actually typed those credentials in the browser. Which is still: you.
So surely, there isn't an issue here? Yes, you can see your username and password in your browser, but isn't that information you already know?
And like Jared said, it's not because of AJAX that you can see them in your dev tools. Even if you'd send them by regular POST request, you can still see them in your dev tools if you capture the request. Only because there usually follows a redirect to a regular GET route, you won't usually see them. But, that's just because you're not looking hard enough.
If you're worried about hackers intercepting your web traffic and seeing your plain text password being sent across the internet, than get your login form to work via HTTPS as was already stated. This way, even if your request gets hacked, they still can't see what you sent.