Or (more specifically) percentage compositions of alloys.

I love it when everyone is so sure!! Consider this:
"alloying contributions" yields exactly zero (0) hits on Google. This must be a well-kept secret.
"alloying elements" is enumerative in nature rather than quantitative, and so misses the point.
"share of alloy" produces all of 11 hits on Google, "shares of alloys" only 10, "shares of alloys" exactly 0, and "shares of alloy" a grand 50.
By comparison, "alloy composition" produces 15,300 hits and, as you can see by one example below, they are on track.
I don't mean to be hypercritical (really!), but let's be more careful here.

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Note added at 3 hrs 11 mins (2004-08-16 20:46:40 GMT)
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Or was that \"annoying contributions\"? :-)

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Note added at 3 hrs 44 mins (2004-08-16 21:20:04 GMT)
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Also, \"shares of alloy\" were in the field of stocks & bonds.

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Note added at 3 hrs 51 mins (2004-08-16 21:26:56 GMT) Post-grading
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Since we just had another agreement, let me add the following explanation:
Consider pewter. Its ALLOYING ELEMENTS (enumerative in nature) are tin, antimony and copper. Its ALLOY COMPOSITION is in the following range (quantitative): at least 90% tin, 2-8% antimony, and 3% copper.

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Note added at 4 hrs 39 mins (2004-08-16 22:14:26 GMT) Post-grading
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I had pressed Return before I was sure I wanted to enter the humorous comment below.
Seriously: dictionaries contain errors and so does Google. People put them there. Google results must be scrutinized, yet they provide a good test bed for dictionary results.
Consider: I have been in Europe too long and start thinking that a cell phone is called a \"handy\". (How did the Germans come up with that before we did?) So I search Google and fine 2.9 million hits for \"+handy +phone\". Now that many hits cannot be wrong, right? Then you look at their URLs and find Japan, Germany, Austria.
A bit of scepticism helps in using both a dictionary and the Internet. However, the Internet allows in-depth background reading on terms that the dictionary just lists, it provides context and usage information, and it allows us to check the source. This exercise was a good case in point.