Christ voluntarily offered Himself.
Yet in doing so he was being obedient to the Father's will:
Yet it was the will of the Lord to crush him;
he has put him to grief;
when his soul makes an offering for guilt [Isaiah 53:10]
Ephesians also says that Christ was "obedient to the point of death, even the death of the cross." It was a willing obedience, but an obedience nonetheless.