  Both ``phason'' elastic constants have been measured from Monte Carlo
simulations of a random-tiling icosahedral quasicrystal model with a
Hamiltonian. The low-temperature limit approximates the ``canonical-cell''
tiling used to describe several real quasicrystals. The elastic constant K2
changes sign from positive to negative with decreasing temperature; in the
``canonical-cell'' limit, K2/K1 appears to approach -0.7, about the critical
value for a phason-mode modulation instability. We compare to the experiments
on i-AlPdMn and i-AlCuFe.
