Bi-axial loading in LAMMPS

Dear all,
I want to study deformation of BCC material by applying bi-axial loading. I tried by applying load along x and y axis by using fix deform command as “fix 4 all deform 1 x erate {srate1} y erate {srate1} units box remap x”. The plot of sigma_xx vs sigma_yy is coming straight line instead of ellipse. Is the method I am using correct ? If not could you please suggest how to apply bi-axial loading.

Please find the attached script.

in.trial1 (5.75 KB)