Two SiC-particle reinforced composites were produced by powder metallurgy using a 2124 Al-alloy matrix and two powder blending techniques: ball milling and wet blending. The effect of the blending route on the stress relaxation during thermal cycling is studied by in situ neutron diffraction based on the determination of the average stresses in the matrix and the particles. The thermal stresses in both composites partially relax by creep at T ≥ 90 °C. The higher creep resistance of the composite produced by ball milling reduces relaxation in comparison with the wet blended composite. This results in average axial compressive thermal stresses of ∼-50 MPa and ∼-10 MPa after heating to 230-300 °C in the matrices of the ball milling and wet blending composites, respectively, which relax at rates ≤5 × 10^-9-3 × 10^-8 s^-1.