Is the statement: "If this thing doesn't happen, I will memorize Surat Al-Kahf," considered a vow, and is expiation obligatory if one is unable to fulfill it?

A vow (nadhr (definition)) is not established by inner speech or by words that do not indicate commitment. Rather, the formulation must be indicative of an obligation. The thought, "If this thing does not happen, I will do such and such," held within oneself, is not considered a vow. Scholars have disliked taking vows because it increases obligations, and the Prophet, peace be upon him, said: "It (a vow) is only extracted from the miser." It is advisable to strive to memorize the Noble Quran, especially Surat Al-Kahf, due to the virtues mentioned concerning it. If the one who made the vow is unable to fulfill it due to an inability that is not expected to cease, then he must offer an expiation for breaking an oath.