Role of Heat Transfer Mechanisms on the Microstructure Development: Drop Tube vis-a-vis Atomization

摘要

Cooling rate and undercooling have very important bearing on the microstructure development in an alloy. Both of these parameters, directly or indirectly, are influenced by the heat-transfer mechanism during the solidification. In the present work, an insight is made on the role of heat transfer mechanisms in the microstructure development under two material processing techniques viz., drop-tube and atomization. Drop tube processing experiments were carried out on Ni_3Al, an ordered intermetallic compound, and Al-3.6wt.%Fe alloy to entrap disorder in the former and to determine the presence of metastable phases in the latter. Partial disorder could be entrapped in Ni_3Al, droplets as large as 400 μm diameter. Al-3.6wt. %Fe droplets of about 500 μm diameter revealed two-zone fine cellular-coarse cellular α-Al metastable structure. Other researchers who processed Ni_3Al compounds and Al-Fe alloys containing more than 3 wt. pct. of Fe by atomization method, have reported the entrapment of partial disorder and presence of the metastable structure in only few μm (≤) diameter particles of these two materials. As spraying was the common droplet generation technique in our drop-tube experiments and atomization experiments of others, it was interesting to observe the development of these microstructural features with respect to the varying sizes of the droplet samples. This aspect could be explained on the basis of operative heat transfer mechanisms during solidification in the drop-tube and atomization methods.