Showing papers by "Marc A. Meyers published in 1996"

Effect of Mo on microstructure and mechanical properties of TiC—Ni-based cermets produced by combustion synthesis—impact forging technique

Abstract: The effect of Mo additions on the microstructure and mechanical properties of TiC-30 wt.% Ni cermets produced by the combustion synthesis-impact forging technique was investigated. The Mo content was varied between 0 and 10 wt.%, in 2 wt.% increments. Cylindrical tiles 6.35 cm in diameter and 1.27 cm thick were produced with apparent densities above 99%. Microscopically, the addition of Mo resulted in a decrease in the number of microstructural defects such as interphase debonding and binder microcracking. The microstructure consisted of a spheroidal carbide phase with a high degree of contiguity (decreasing with increasing Mo content). The Mo additions did not have a profound effect on the carbide phase morphologies (faceted vs. spheroidal), mean apparent particle diameters (3.5 μm–4.5 μm), or particle size distribution. Energy-dispersive X-ray analysis revealed Mo preferentially in the carbide phase, with trace amounts in the Ni alloy binder. A significant amount of Ti was found in solution with Ni. Vickers' microhardness did not vary significantly with Mo content and was approximately 13 GPa. Compressive strength, transverse rupture strength, fracture toughness, and Young's moduli increased with increasing Mo content; the mean values for the 8 wt.% Mo material were approximately 3400 MPa, 1300 MPa, 22 MPa m 1/2 , and 340 GPa respectively. The beneficial effect of Mo is due to the improved wettability of the Ni alloy binder on the carbide phase. Improved wettability results in a decrease in detrimental microstructural defects and an increase in the interphase bond strength and phase uniformity.

Microstructural Aspects of Dynamic Failure

Abstract: The term dynamic failure is used to distinguish special characteristics of failure of materials subjected to dynamic loading. The major feature that differentiates dynamic failure from quasi-static behavior is the presence of stress waves. These waves arise due to the applied load or due to the stresses released from a crack tip at fracture. Several phenomena are particularly relevant to dynamic failure, including:

Dynamic consolidation/hot isostatic pressing of SiC

Abstract: Shock consolidation is a method that presents a bright potential but has been limited by inevitable cracking of compacts, especially for ceramics. In an effort to eliminate cracking while retaining the unique features of shock consolidation, three novel approaches have been implemented: (1) the use of local shock-induced reactions to increase the temperature of particle surfaces and to provide a bonding phase (reaction products); (2) shock densification at a low pressure (just above the threshold for pore collapse) followed by hot isostatic pressing; (3) shock consolidation of pre-heated specimens. These techniques were applied to silicon carbide. Reduction of cracking was observed with interparticle melting and reactions. Microstructural results, mechanical properties and advantages and limitations of these approaches are discussed. It is shown that shock consolidation of ceramics is inherently limited because shock-induced cracks are introduced into the process, damaging the particles. A criterion for the plastic deformation versus fracture of ceramic powders under shock consolidation is proposed.

An equation describing the consolidation of combustion synthesized titanium diboride

Abstract: A phenomenological equation, incorporating the temperature dependence of flow stress, is described for the in-situ consolidation of combustion synthesized materials. An activation energy for the temperature dependent flow stress is calculated and indicates that, in addition to plastic deformation, other processes such as fracture and liquid-phase-assisted particle rearrangement occur.