Is it permitted in Islam for a women to study abroad without any mehram

Yes it is permissible if there is no threat of her falling into sin or her life being in danger or the like. There are several hadith on the importance of studying and there is no difference between men and women in this regard.

There are several points to consider in regards to your question

– There is a gerenal rule that something is allowed unless if that particular act were to cause harm (be it studying abroad) then a person should not participate in such an act.

– As for permission from the wali, if she is married, she cannot leave the house without permission from her husband therefore if he does not allow her to travel abroad, she cannot do so.
If her parents refuse her to study abroad and her travelling would cause them distress be it out of concern for her then she cannot travel in this instance either without the permission of her parents.