Is it possible to make half of an object semi transparent and the other half non-transparent? For example I have an object like a cube. I know that the transparency of my object can be set by the alpha. But is it also possible to just make half of the cube semi transparent and the other half non-transparent?

Thanks for your help!
Yes, this can be done by assigning different materials to the same object as shown below.


I added a cube to the scene at origin and scaled it to be bigger
Then I added a sphere to the origin which is now located inside the cube
I added a material to the cube as shown below:





I then checked and turned on the transparency and decreased the the Alpha value as shown below:





Then I added a new material and set its color to red as shown below:





Then I switched to edit mode, switched to face mode and selected couple of faces and clicked assigned to the red material as shown below:





Then render and as shown below the grey material is transparent showing the sphere inside while the red material is not: