Thank you for answering.
I have tried copying and pasting the uasset to another project(inside its Folder/Content/Material)I created earlier when UE4 editor was closed. But when I tried to use it in the engine, it turned transparent and did nothing. I doubled-clicked it to open the material editor to closely check it and it was still the same. Did I forget to do something?
As to that crashing issue, yes the computer freezed up, even the clock crashed a little bit (freezed->resume->freezed again). No error message. I am using a desktop computer with AMD Radeon HD 7400 Series. The power setting is already at high performance. I have checked this before and it still crashed even so.
I have downloaded all the free content made by EP. Most of them runs fine at around 7 fps (not a powerful computer 
) when the engine scalability is set to High (Project like: Elemental Demo, Soul City & Cave, Epic Zen Garden, Composure Framework, Particle Effect, Matinee etc). I can even open the paragon map without major crashes(the clock works fine) in the loading process, though it took me around 30 minutes. So I am guessing projects like the one I mentioned above just have too much quality detail for my computer to even open it. Maybe If I let it run for a whole day, those projects can finally be opened. But is there a faster way, other than upgrading the hardware, to do so? and, could you thoroughly explain how to transfer and use the content in another project without having to open the one the content is in?