Is it possible to screencast from an iOS device to a Chromecast?
Casting (mirroring) the entire screen of your iOS device to Chromecast is not possible. Apple TV uses an Apple proprietary technology called AirPlay for screen mirroring.

Casting is only available for compatible apps which have added support for Chromecast. In compatible apps, you will be shown a rectangular button with little wireless icon in the bottom left corner, as shown in the image below: