In this work, grain refinement and deformation mechanisms of ultrafine-grained AA 6061 alloy have been investigated using uniaxial tensile tests with stress relaxation. Initially, solutionized AA 6061 alloy samples with 3 mm thickness are subjected to constrained grove pressing (CGP) with an effective plastic strain of 1.16. The CGP samples are eventually rolled at room temperature (cold rolled) to produce ultrafine-grained thin sheets of 1 mm thickness. Due to the intense plastic strain applied during CGP and cold rolling (CR), it is practically challenging to quantify the grain size refinement by optical methods. Therefore, stress relaxation, which is an alternate transient mechanical test, is used to estimate the average degree of grain refinement for a larger length scale. In this method, the transient data from uniaxial tensile tests with stress relaxation is used to calculate the activation volume, which helps to understand the deformation mechanisms of UFG structured materials. In the present work, uniaxial tensile tests with controlled single and repeated stress relaxation are performed to determine apparent and actual activation volume in different materials conditions (solutionized, CGPed, and CGP + CR). Stress–strain curves obtained from the stress relaxation tests are compared with the monotonic tensile stress–strain curves. The results showed that the activation volume determined from single and repeated relaxation tests substantially decreased after the CGP and CGP + CR processes. The results also indicated that the grain boundary sliding is the possible deformation mechanism in CGP + CR samples and dislocation-dislocation interactions in CGP and SL samples. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.