Ion-radio with checked="true" does not work?

orderBy.html

<ion-content>
  <ion-list radio-group>
      <ion-radio value="orderBy1" (click)="selected('orderBy1')" [checked]="orderBy1">
          Order By 1
      </ion-radio>

     <ion-radio value="orderBy2" (click)="selected('orderBy2')">
         Order By 2
     </ion-radio>

     <ion-radio value="orderBy3" (click)="selected('orderBy3')">
        Order By 3
     </ion-radio>

  </ion-list>

  <ion-item>Order by: {{orderBy}}</ion-item>

</ion-content>

orderBy.ts

  selected(value: string) {
    this.orderBy = value;
  }