S.K. Bhaumik

Synthesis and sintering of TiB2 and TiB2–TiC composite under high pressure

Abstract TiB 2 and TiB 2 –TiC composite compacts with 98–99% density are prepared by high-pressure sintering (HPS) of premixed powders and by high-pressure self-combustion synthesis (HPCS) from the elemental constituents. The sintering and synthesis experiments are carried out at 3 GPa in the temperature and time ranges 2250–2750 K and 5–300 s, respectively. A high sintering temperature (2750 K) is required to obtain dense monolithic TiB 2 compacts (98% density) by HPS. Compacts with a similar density are obtained at lower sintering temperature (2250 K) when 15 mol% TiC is added to TiB 2 . The composite compacts have marginally better fracture toughness than that of monolithic compacts. TiB 2 and TiB 2 –TiC compacts (99% density) are also prepared by HPCS from elemental constituents. A minimum ignition temperature of 2250 K is required to make the reaction self-sustaining. The compacts prepared by HPCS have superior fracture toughness to those prepared by HPS. The microstructures and the properties of the compacts prepared by HPS and HPCS are compared. A possible sequence of reaction during the HPCS of TiB 2 –TiC is proposed.

Machining Ti6Al4V alloy with a wBN-cBN composite tool

Abstract Titanium-based alloys are difficult-to-cut materials. The conventional tool materials (HSS and cemented carbides) fail due to the high chemical affinity of titanium and high temperatures generated while machining. An attempt has been made in this work to machine Ti6Al4V alloy with wurtzite boron nitride (wBN) based cutting tools. The mechanisms controlling the wear of the cutting tool have been found to be similar to those observed in polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) tools. The results indicate that the wBN-cBN composite tools can be used economically to machine titanium alloys.

Reaction sintering of NiAl and TiB2-NiAl composites under pressure

Intermetallic matrix composites are a new class of engineering materials for high temperature structural applications in oxidizing and aggressive environments. Attempts have been made in the present investigation to synthesize TiB2–NiAl composites, in which the matrix phase NiAl was produced in situ by reaction synthesis. The composites with 10, 15 and 30 vol.%NiAl were fabricated from the mixtures of elemental Ni and Al and TiB2 powders by high pressure reaction sintering (HPRS) and reactive hot pressing (RHP). The HPRS and RHP were carried out at 3 GPa and 900°C, and 20 MPa and 1650°C, respectively. The different phases in the sintered compacts were identified by X-ray diffraction and microstructural studies. In HPRS, the reaction was incomplete which gave rise to various intermediate phases (Ni2Al3, Ni3Al). It was necessary to anneal these compacts at 1100°C to obtain TiB2–NiAl composites with single phase NiAl matrix. The densities of the HPRS compacts were ∼99%. The hardness and fracture toughness values were in the range 10 to 20 GPa and 3.9 to 5.7 MPa√m, respectively. The RHP compacts contained AlB2, Ni2B and NiTi2 phases in addition to those present in the HPRS compacts. The RHP composites were fully dense. These had superior hardness (15–22 GPa) but inferior fracture toughness (2.9–3.8 MPa√m) compared to those obtained by HPRS.

Nickel–Titanium Shape Memory Alloy Wires for Thermal Actuators

Shape memory alloys (SMAs) are a class of functional materials which exhibit unique properties of shape recovery under external stimuli such as thermal, mechanical, and magnetic fields. NiTi SMAs find wide applications as solid-state thermal actuators in a variety of aerospace, automobile, and engineering systems. Compared to conventional actuators, SMA actuators have the advantage of a high force to mass ratio, simplicity and miniaturization of design, fewer moving parts, and silent operation. This article deals with the processing and properties of NiTi SMA wires for thermal actuator applications. The effect of thermo-mechanical processing history and the employed stress–strain–temperature conditions on the functional fatigue behavior of the NiTi SMA thermal actuators has been discussed.