Microstructural evolution and mechanical properties of cryomilled nanograined near Al-5083 alloy following deformation processing

Nanocrystalline Al-Mg alloys are being considered for light weight transportation applications because they possess significantly higher strength than the conventional coarse grained alloys. Failure strengths higher than 1000 MPa have been reported for Al-5083 alloy at New Mexico Tech, which are almost double the strength of commercial precipitation strengthened Al-alloys. Unfortunately, the ductility tends to exhibit inverse relationship to strength and therefore there is a need to find ways to increase the ductility while maintaining high strength.;In this work, we utilize a near Al-5083 alloy that was cryomilled for 24 hours in liquid nitrogen environment and consolidated by vacuum hot-pressing. The as-atomized Al-Mg powder was especially fabricated to minimize undesired impurity content to prevent premature fracture from intermetallic particles. It turned out that the final composition was slightly lower in Mn and Mg content and so the alloy is better designated as a near Al-5083 alloy. The as-vacuum hot pressed material had poor ductility because of inadequate prior-particle bonding, and therefore was subjected to deformation processing using low strain-rate extrusion at elevated temperatures. Both the strain-rate and temperature of extrusion were varied in an effort to obtain a good combination of tensile strength and ductility. In addition, the samples were annealed following extrusion in order to reduce residual stresses.;The microstructure of extruded samples were characterized using a combination of electron microscope and X-ray diffraction techniques, and revealed a multi-scale morphology that could be binned into three different sizes of grains: i) those less than 100 nm that were analyzed using the X-ray based Williamson-Hall technique and transmission electron microscopy (TEM), ii) grain sizes in the 100-300 nm regime that were best revealed using TEM and scanning electron microscope (SEM) based electron-backscatter diffraction (EBSD) techniques, and, iii) elongated larger grains with lengths in the range 3 to 7 &mgr;m that were observed using EBSD. Room temperature tensile tests with small tensile samples indicated that ultimate strengths in the range 740-760 MPa and elongation to failure better than 2.5%. These data could be produced reproducibly following extrusion at 400 C at an average strain rate of 0.05/sec, and fractography revealed a rough topography with large pull out regions that consisted of typical ductile dimple void growth at the 200-500 nm scale, while in other regions the dimples were very shallow that would suggest failure with little energy loss likely in regions of nanograins less than 100 nm. The combination of strength and ductility in the material are some of the best that have been reported for Al-5083 alloys, and likely a result of the multi-scale microstructure resulting from processing through the cryomilling and extrusion plus annealing procedures.