Microstructure of ball milled and compacted Co-Ni-Al alloys from the beta range.

Two powder alloys from the beta phase region of compositions Co(28.5)Ni(36.5)Al(35) and Co(35)Ni(30)Al(35) were ball milled for 80 h in a high energy ball mill. The formation of amorphous structure was observed after 40 h of milling and further milling did not change their structure. The analytical and high-resolution transmission electron microscopy (TEM, HREM) examination of powder structure showed that nanoparticles of L1(0) phase of size of about 5 nm were present within the amorphous matrix. The vacuum hot pressing of the milled powders under pressure of 400 MPa at 700 degrees C for 12 min resulted in the formation of compacts with density of about 70% of the theoretical one. The additional heat treatment at 1300 degrees C for 6 h followed by water quenching, led to significant improvement of density and induced the martensitic transformation manifested by a broad heat effect. The characteristic temperatures of the transformation were determined using DSC measurements, which revealed only small differences within the examined alloys compositions. TEM structure studies of heat-treated alloys allowed to identify the structure of an ordered beta (B2) phase and L1(0) martrensite.