Solid phase processing methods such as friction stir processing (FSP) offer pathways to refine the microstructure of metallic alloys through the combined action of deformation and deformation-induced heating. However, this thermomechanical coupling during FSP also leads to the occurrence of multiple competing microstructural evolution mechanisms which in turn can lead to locally varying mechanical properties, often distributed heterogeneously in the microstructure. This inherent microstructural and mechanical property heterogeneity in alloys subjected to FSP makes it rather challenging to reveal the microstructure-mechanical property relationships systematically. Therefore in this work, we systematically analyze the relationship between microstructural evolution and local microhardness in a model binary Al-4 at.% Si alloy subjected to three-pass friction stir processing. Spatially resolved high-energy synchrotron X-ray diffraction, electron backscattered diffraction, and scanning transmission electron microscopy were used to understand the heterogeneous microstructural evolution due to the FSP. Our results provide insight into how particle-stimulated grain nucleation, recovery, and recrystallization occur heterogeneously in the Al-Si alloy as a function of the distance from the stir zone, directly influencing the degree of softening. The systematic understanding developed by this work can guide future studies on the influence of FSP process parameters on the microstructural evolution mechanisms and its influence on local mechanical properties