Gamma prime phase

The gamma-prime phase is what gives nickel-based superalloys their strength at high temperatures. It forms through a heat treatment that creates fine particles in the alloy. When welding, that's the tricky part: if you heat the alloy too much and cool it too fast, you won't get this phase right or you'll dissolve it again. Then the material becomes soft and loses its high-temperature strength. So you have to control the heat input and cooling rate, otherwise you'll end up with a weld that doesn't hold up at high temperatures.