I have a special fondness for Al-Si alloys as I did some amateur foundry work years ago, along with studying some metallurgy, which I still remember a fair bit of.
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Al-Si alloys have high strength and low ductility as-cast, and flow into molds better than any other alloy. (Ductility doesn't help castings, as you can't get any strength into them by mechanical working! Compare with wrought alloys like 6061, which achieve maximum strength with rolling/drawing/etc. so are only available as plate, bar, extrusion, etc.)
Typical compositions are 8-10% or 22-28% Si, a few % of other elements to improve strength (Mg, Cu, etc.) and balance Al.
The eutectic is around 13%, which freezes suddenly at 576°C so isn't good for casting: metal shrinks on freezing, causing it to suck in from anywhere hotter, leaving voids there; it's better that this happens more gradually, with a "mushy" off-eutectic alloy.
The higher concentration is hypereutectic (literally: past eutectic), so instead of growing primary aluminum dendrites on cooling, it's full of plates of Si crystals floating in a eutectic (fine grained) matrix. The larger Si crystals make it abrasion resistant, and it's good for hot-forging as well, so is commonly used for engine parts such as pistons and connecting rods.
Needless to say, such crystals are very heavily P-doped.
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I don't know that I've ever seen the resistance probed, that'd be an interesting experiment I suppose... it should be measurably different, but still quite a low resistance over either grain (Si or Al).
Al-Si forms no intermetallic compound, it's a simple binary eutectic system. Much like Pb-Sn, for a familiar example.
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Which, for soldering, we prefer around the eutectic, because shrinkage is avoided by the low molten mass required, and because we want the joint to freeze suddenly. (SAC305 is a bit past eutectic, so tends to leave a rough surface as crystals have enough time to grow to macroscopic size -- hence its tendency to look "cold". Both silver and copper, by the way, form intermetallics -- brittle compounds that make for a stronger alloy.)
So, at the surface of that poor ex-sistor, it'll be some kind of hyperhypereutectic Al-Si alloy, I should guess. The melting point will be close to that of pure Si, ~1400°C.
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Tim