If you consider what Poissons ratio is determined by, then would the thermal expansion constant not be the same for the material in both directions?, If so, this would suggest that poissons ratio wouldn't change.
Using the following data (taken from Kaye and Laby) it is possible to calculate the tempco for different materials. Aluminium is chosen here as an example:
At 20 deg C,
E=70.3*10^9 (Pa)
G=26.1*10^9 (Pa)
Tempco for E is -480 ppm/degC
Tempco for G is -520 ppm/degC
For a homogeneous isotropic solid,
Poissons Ratio =(E/2G)-1
By plotting Poisson's Ratio against temperature the tempco for Poisson's Ratio for Aluminium at 20C turns out to be +55.6 ppm/degC
I know that for Inconel 600, E as well as G decreases with increasing temperature, approximately parabolically.. Depending on these relations, Poisson's ratio changes with temperature accordingly..
For Inconel 600, Poisson's ratio first drops, and later rises again with temperature..