C. Divakar

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.

Pressure‐induced alpha‐omega transformation in titanium: Features of the kinetics data

The increase in the electrical resistance of the specimen, associated with the α→ω transformation in Ti, has been used to obtain the fraction ζ of the ω phase as a function of time under isobaric–isothermal conditions in the pressure range 4–9 GPa and at 300±3 K. In the entire pressure range ζ–t data fit an equation of the form: ζ=1−exp−(t/τ)n, where τ and n are constants at a given pressure. The results indicate that τ decreases rapidly and n slightly with increase in pressure. An analysis of τ–p data shows that (i) the activation free energy for the total process is very high at low pressures and decreases rapidly with increasing pressure, and (ii) the activation volume in the 6–9 GPa range is −4.3 cm3 mol−1. The activation enthalpy has been obtained by measuring τ at different temperatures, and the average value in the pressure range 5–8 GPa is 12±0.5 kCal mol−1.

The kinetics of pressure‐induced fcc‐bcc transformation in ytterbium

The kinetics data for the pressure induced fcc→bcc transformation in ytterbium under isobaric‐isothermal conditions in the pressure range 3.3–4.6 GPa have been reported. The kinetics data satisfy Avrami equation: ζ=1−exp−(t/τ)n, where τ and n are constants at a given pressure. Both τ and n decrease with increase in pressure, as given by the following relations: ln τ=(36.2±1)−(6.8±0.3)p, and n=1.36+0.55p−0.168p2, where p is pressure (GPa). The activation free energy for the combined process of nucleation and growth has been estimated from the τ −p data and is found to decrease with increase in pressure. The activation free energy varies from 16 kcal mol−1 at 3.3 GPa to 11 kcal mol−1 at 4.6 GPa. The activation enthalpies at 3.3, 3.6, and 3.8 GPa have been determined by measuring τ at different temperatures. The activation volume is −17 cm3 mol−1 at 300 K. It is estimated from the τ −p data that fcc‐bcc transformation under shock loading will occur at about 7 GPa.

Solidification of selenium melt under pressure up to 6.5 GPa

Abstract High purity selenium samples were melted under high pressure (≤6.4 GPa) and quenched at various rates ranging from 2 K s−1 to 500 K s−1 and the recovered material was examined by X-ray diffraction and electron microscopy. In the entire range of pressure and cooling rate, the melt was found to solidify into a polycrystalline aggregate of the trigonal phase of selenium. The samples obtained by slow cooling of the melt at 6.4 GPa contain, in addition to crystalline phase, regions which appear to be amorphous.