Youth international Locadia, whose treble gave PSV a 3-0 win over Zwolle, got his chance after Jeremain Lens was suspended for three matches after a clash on and off the pitch with Feyenoord's Joris Mathijsen in a league match on Sunday.
The 19-year-old opened his account with a simple finish on 16 minutes after being set up by Georginio Wijnaldum.
Wijnaldum connected with Locadia again 11 minutes before the interval to double the lead and he completed his hat-trick five minutes from time.
"It looked easy today because we kept focused but because we did not finish it earlier Zwolle kept believing in it," said PSV coach Dick Advocaat.
"Reaching the final is a good finish after some turbulent days and I have to say that my players reacted well today," he added referring to the Feyenoord defeat and the altercation between Lens and Mathijsen.
Alkmaar, struggling in the league, reached the final with a surprising 3-0 victory over Ajax Amsterdam who rested four starters.
All the goals came in the final 16 minutes, sparked by Jozy Altidore's goal from close range.
The U.S international set up Johann Gudmundsson two minutes from time and added a third in stoppage time with a superb lob from 30 metres.
"This is great feeling and hopefully this match will give us confidence for the remaining 10 league matches," said Altidore.
The final will be played in Rotterdam on May 9.
