MadSci Network: Physics

Re: What % magnetic metal will make a coin attracted to a magnet?

Date: Thu Mar 18 20:38:41 1999
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 920817853.Ph

Hi Mike,

I checked the US Mint web site at for the composition 
of the nickel.  You are correct that the Jefferson nickel contains 25% 
nickel.  The remainder of the alloy (mixture of two or more metals) is 
copper.  A metallurgist uses a phase diagram to predict the crystal 
structure and magnetic properties of alloys.  The phase diagram for copper-
nickel alloys has a curie temperature of about 300 degrees centigrade when 
the weight percent of nickel exceeds 50%.  A curie temperature is the 
temperature at which a magnetic material becomes non-magnetic. A nickel 
copper alloy with more nickel than copper will be magnetic.  The phase 
diagram in the reference book does not show temperatures less than 200 
degrees C.  I cannot tell from the phase diagram the exact percentage of 
nickel that will be magnetic at room temperature.  The room temperature 
percentage is somewhere between 25 and 50%.

The percentage of magnetic elements in an alloy does not always determine 
if the alloy is magnetic.  Many alloys can be either magnetic or non-
magnetic depending upon the crystal structure.  These alloys, when heated 
above a transition temperature, change crystal structure and become 
magnetic or non-magnetic.   A metallurgist uses phase diagrams to predict 
crystal structures and magnetic properties of alloys.

Bob Novak
Specialist, Process R&D
Carpenter Technology

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