You're allowed Cv comes out here for this adiabatic expansion, which is not a constant volume only because this is always true for an ideal gas.

绝热过程写下,这个式子是因为它对理想气体都成立，并没有用到等容过程的条件,只用了理想气体的条件。

麻省理工公开课 - 热力学与动力学课程节选

But it's allowed to say the internal energy is a function of temperature and volume.

但是我们也可以说内能,是温度和体积的函数。

麻省理工公开课 - 热力学与动力学课程节选