The most common metal thermal interface materials are made from indium alloys which melt between 100C to 200C.
Some power devices I have come across desperately want to make the step toward metal interface materials, but cannot use them in the traditional sense because the operating temperatures of their devices is hotter than 200C. This would melt the interface material as the device ran which could lead to reliability issues.
As this image depicts, we can't cool down an electrical device by serving cold drinks. We can keep a device cool when the heat is just too much however by inserting an appropriate material at the thermal interface.
There are multiple alternative alloys for these hot applications and fortunately, these materials are well understood since they have been used in electronics soldering applications for years. Today, they have been adopted by many companies for their TIM properties, which are similar to indium. High temperature solder alloys such as 80Au/20Sn are very common, but it is also possible to use high temperature tin alloys (99Sn1Cu, 99Sn1Sb or 95Sn5Ag) or for the most extreme cases, pure gold at the interface. These materials all melt above 200C and also have high conductivities, which will minimize the resistance at the interface when the solder is compressed or reflowed.
Image courtesy of distractible.org