Dr Hon Tong Pang

Casting & Solidification

Despite the widespread use of single-crystal nickel-base superalloys, there were fundamental questions related to the solidification of these alloys which remain unanswered.

In the directional solidification casting process to produce single-crystal components, primary solidification commences with the formation of γ dendrites. This is followed by the formation of non-equilibrium inter-dendritic constituent up to the terminal stages of solidification. Recent work has shown that the inter-dendritic constituents consists of two distinct morphologies (i) fine γ / γ' morphology, and (ii) coarse cellular γ' separated by narrow γ channels.

Solution Heat Treatment

To meet increasingly demanding service conditions, higher contents of dense refractory elements were added to single-crystal nickel-base superalloys. Associated with these additions however were the formation of severely segregated as-cast microstructures which requires homogenisation via solution heat treatment prior to service.

Solution heat treatment processes need to be optimised to produce homogenised materials without incipient melting and within acceptable cost and time constraints in industry.

Phase Stability of Superalloys

The higher contents of dense refractory elements in newer generations of alloys also have the tendency to increase the susceptibility of these alloys to the formation of topologically-closed-packed (TCP) phases after prolonged exposure at elevated temperatures. These TCP phases are detrimental to the alloys as they are brittle and may therefore act as crack initiation sites. In addition, the formation of these TCP phases depletes the microstructure of refractory elements critical for high temperature performance.

Care is required during alloy design to balance the alloy compositions in order to achieve the required properties against the phase stability of these alloys.

Other interests:

Materials Testing – Differential Scanning Calorimetry, Thermogravimetric Analysis, etc.
Small-scale Mechanical Testing – Electro-Thermo Mechanical Testing
Trace element analyses - Laser Ablation-Inductive Coupled Plasma-Mass Spectrometry
Environmental Protection Performance
Microstructure-Property Relationship