Microstructure, Thermal and Mechanical Properties of Non-Isothermally Annealed Al-Sc-Zr and Al-Mn-Sc-Zr Alloys Prepared by Powder Metallurgy
Abstrakt
This paper reports results of a study aimed at understanding the precipitation processes occurring during the annealing of two Al-Sc-Zr-based alloys with and without Mn prepared by powder metallurgy with subsequent hot extrusion at 350 degrees C. Samples were isochronally annealed up to approximate to 570 degrees C.
Precipitation behaviour was studied by electrical resistometry and differential scanning calorimetry. Mechanical properties were monitored by microhardness HV1 measurements.
Transmission electron microscopy examinations and X-ray diffraction of specimens quenched from temperatures of significant resistivity changes helped to identify the microstructural processes responsible for these changes. Fine (sub)grain structure develops and fine coherent Al3Sc and/or Al-3(Sc,Zr) particles precipitate during extrusion in both alloys.
The distinct changes in resistivity (at temperatures above approximate to 330 degrees C) of the Al-Mn-Sc-Zr alloy are mainly caused by precipitation of Mn-containing particles. The easier diffusion of Mn atoms along the (sub)grain boundaries is responsible for the precipitation of the Al6Mn and/or Al-6(Mn,Fe) particles at relatively lower temperatures compared to the temperature range of precipitation of these particles in the classical mould-cast Al-Mn-Sc-Zr alloys The apparent activation energy for precipitation of the Al3Sc and Al6Mn particles in the Al-Mn-Sc-Zr alloy was determined as (106 +/- 10) kJ mol(-1) and (152 +/- 33) kJ mol(-1), respectively.