One of the fundamental advantages of using metal powders to make sintered parts is down to the way metals solidify as they cool - if you tried to make cast alloys with some mixtures of metals, you cannot do it. The laws governing the solidification process dictate the temperatures at which particular compositions solidify and prohibit certain compositions being stable solids of uniform composition.
Using P/M techniques, these restrictions do not apply, since during the sintering process the temperature remains below the melting point of the metals involved.
That's not to say that the P/M chemist has carte blanche to mix any metal with any other - trying to sinter a compact with 50% lead and 50% iron would be a disaster, because if you heated it to just below the melting temperature of the lead, the iron would not be affected at all, whereas if you heated it nearer to the melting temperature of the iron, the lead would have long ago left the party! But it does enable, for example, copper-steel mixtures to be sintered which are not true alloys of copper, iron and carbon.
Here is the phase diagram for the Iron-Copper system - You will see that there is pretty much zero mutual solubility of iron and copper at room temperature, but a solid phase exists with around 10% of copper dissolved in iron at 1400 degrees. So there is no room temperature homogeneous alloy of copper and iron, but by sintering a powder mix, we can produce a component which has the optimised properties we are looking for.
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