TL;DR: No
Well… Some tried to guess, but it’s hard to tell…
Here an example, where the answer was:
Part of the key is what you put in for encryption, parts are taken from the phone (motherboard serial number, etc.) And they’re built on the fly, not stored. That’s why you can’t put an encrypted card into another phone, then decrypt it.
Now… people on the forum tried before you.
You will find documentation here:
https://source.android.com/security/encryption/metadata#configuration-on-adoptable-storage
Unfortunately, this is just way over my knowledge.
One last useful link I just found: Adoptable storage | Android Open Source Project
which states:
Because the contents of an adopted storage device are strongly tied to the Android device that adopted it, the encryption keys should not be extractable from the parent device, and therefore the storage device can’t be mounted elsewhere.
So the answer is: Google made it as hard as possible not to find out the key. You can try to find out, but I doubt you will manage.
The one thing I see you might want to try is the following workaround on another topic about this issue (which you can read for tips if you wish) the time for you to backup your data:
It might not work. I just think it’s worth a try 
Edit:
You can’t run photorec on the SD-card as it can’t be mounted elsewhere, and I doubt you can have it run on the storage of the phone… Experimenting shouldn’t hurt as it works in read-only. Good luck!