The following figure illustrates (not to scale) some relevant features of the cases you simulated.
The yellow line is the linear material stress-strain curve and the elastic portion of the nonlinear material stress-strain curve. With linear materials, as the strain increases, the stress just keeps going up and up the yellow line without limit.
The red, green and blue lines are the plastic potions of the nonlinear stress-strain curves. They each start at the Yield point.
With nonlinear materials, as the strain increases, the stress goes up the elastic portion until the yield point, then follows the flatter plastic portion.
For the load you applied, when the material was Titanium, the stress did not reach the yield point, so the stress for that and the linear materials were in the 620-630 MPa range.
When the material was nonlinear steel or aluminum, the stress is slightly above the yield strength of each material.