Investigation into microstructure and mechanical properties of NiAl-Mo composites produced by directional solidification

Abstract Mo fiber reinforced NiAl in situ composites with a nominal composition Ni-43.8Al-9.5Mo (at.%) were produced by specially controlled directional solidification using a laboratory-scale Bridgman furnace equipped with a liquid metal cooling (LMC) device. The microstructure of as-produced composites was characterized by electron microscopy (SEM, TEM, HRTEM) and microanalysis (EDX, SAED). The strength and plasticity at elevated temperatures (700 °C, 1100 °C) were examined by tensile tests. The fracture toughness at room temperature (RT) was measured by four-point bending tests. For a constant temperature gradient (8.9 K/mm) it was found that the alignment and size/spacing of single crystalline Mo fibers were mainly controlled by the solidification rate. When the solidification rate decreased from 1.33 mm/min to 0.33 mm/min the Mo fibers arranged themselves gradually from an initially irregular distribution to an alignment parallel to the solidification direction. The strength at elevated temperatures and the fracture toughness at RT were dominated by the Mo fiber alignment: the yield strength of the composites increased from 273 MPa at 700 °C and 68 MPa at 1100 °C for a random Mo fiber arrangement to 811 MPa at 700 °C and 344 MPa at 1100 °C for well aligned Mo fibers. The fracture toughness at RT was also apparently improved from 8.7 MPa m 0.5 to 14.5 MPa m 0.5 upon Mo fiber alignment. The influence of Mo fiber alignment on the mechanical properties of as-produced NiAl-Mo composites will be discussed in terms of the active strengthening and toughening mechanisms.