Container-less Solidification of Advance Engineering and High Entropy Alloys

Abstract

Container-less solidification helps achieving large values of undercooling by totally avoiding heterogeneous nucleation at the container walls. Undercooling is nearly always required to drive the solidification front during solidification process which is the most important processing route for metals and alloys. However, heterogeneous nucleation sites tend to limit the undercooling of a melt during the solidification process by reducing the energy required to form the critical nucleus. The most dominant factor facilitating the heterogeneous nucleation and thus lowering the undercool-ability of a melt is container wall. Electromagnetic levitation (EML) is a useful technique to avoid heterogeneous nucleation due to container walls and consequently, large undercooling values can be achieved during the solidification process. Different undercooling values result in different solidification speeds which in turn determine the microstructure and mechanical properties of materials. A high-speed video camera with frame rate of 7000pictures/ second has been utilized to observe the recalescence behaviour and to calculate the solidification velocity. This thesis includes results for the container-less solidification of hyper-eutectic NiSn and NiCoCrFe-Cu based high entropy alloys (HEAs); and its effect on microstructure and mechanical properties. Both these alloy systems are extensively studied because of their promising properties. Eutectic alloys show lower melting points than the pure elements and zero temperature range which makes them ideal for casting processes. Similarly, HEAs allow very good control over the properties by minor adjustments in composition. It has been observed that the solidification velocity for the NiSn alloy increases with increasing undercooling. This increase is slow till a certain undercooling value is achieved where a rapid increase in the solidification velocity is observed. This is mainly due to the transition from coupled towards uncoupled growth of eutectic structure. The coupled growth results in lamellar structure which is the equilibrium structure for eutectic alloys. However, uncoupled growth results in anomalous or non-lamellar structure where one phase appears to be present in the matrix of other. In case of HEA, the microstructure consists of two-phase region. A dendritic region which is depleted in Cu forms first while the Cu-rich inter-dendritic region follows afterwards. As the undercooling increases, the Cu-rich phase tends to be segregated as spherical particles and not as continuous inter-dendritic phase. Effect of undercooling on mechanical properties has been investigated by performing Vickers microhardness and nano-indentation tests. In case of NiSn alloy, hardness decreases with increasing undercooling. It is because inter-phase and grain boundaries, which improve the hardness, are more in lamellar than the non-lamellar microstructure. In case of HEA the undercooling increase leads to lower hardness of the alloy, the undercooled structure has less lamellar structure which results in lower hardness of the alloys because of less grain boundaries in it. For the HEAs, undercooling has been found to improve the mechanical properties. It can be attributed to the precipitate hardening due to Cu-rich precipitates which form at larger undercoolings. Nano-indentation for the HEA should be performed to understand the hardness contribution due to individual phases.