use this attribute in XML
card_view:cardElevation="0dp"
and remember add xmlns:card_view="http://schemas.android.com/tools" in your root layout.
OR you can call cardView.setCardElevation(0) to disable shadow programmatically.
cardView.setElevation() method and CardView attribute android:elevation will throw java.lang.NoSuchMethodError in platform before Android 5.0