AbstractThe formation of precipitate depleted zones (PDZ) near grain boundaries in Mg-based alloys strengthened by precipitation hardening is deemed as detrimental to the material since wide depleted zones may affect the mechanical and corrosion properties of the alloy. Experimental investigation of PDZ evolution in Mg-Zn-Sn-alloys aged at different temperatures for different times was conducted by SEM and TEM and by measuring the microhardness of near grain boundary zones at low loads. It was found that at early stages of aging (175°C, ≤1 day) the hardening is caused by formation of MgZn2 needle- and T-like MgZn2/Mg2Sn particles. The neargrain boundary zone is harder than the grain matrix, due to large round MgZn2 and Mg2Sn particles formed at grain boundaries. Increasing the aging was found to decrease the hardening in the matrix as well as in the near-grain boundary zones due to the dissolution of MgZn2 needles and the coarsening of T-like particles. Substantial microhardness decrease in the near-grain boundary zone (from 70 to ~30 HV) found at low loads (10 gr) was connected with the formation of PDZ. This was confirmed by TEM and SEM studies. Further aging, at 225°C for 1-8 days, leads to the formation of "crusts" of enlarged T-like particles around depletion zones. As a result, the microhardness of PDZ's measured at higher loads (25 and 50 gr) increases up to ~ 60÷70 HV close to the ones measured in the grain matrix.
Bar buckling in ductile RC walls with different boundary zone detailing: Experimental investigation