S.V. Raj

Microstructural characterization of a directionally-solidified Ni–33 (at.%) Al–31Cr–3Mo eutectic alloy as a function of withdrawal rate

Abstract The Ni–33 (at.%)Al–31Cr–3Mo eutectic alloy was directionally-solidified (DS) at different rates, VI, varying between 2.5 to 508 mm h−1. Detailed qualitative and quantitative metallographic and chemical analyses were conducted on the directionally-solidified rods. The microstructures consisted of eutectic colonies with parallel lamellar NiAl/(Cr,Mo) plates for solidification rates at and below 12.7 mm h−1. Cellular eutectic microstructures were observed at higher solidification rates, where the plates exhibited a radial pattern. The microstructures were demonstrated to be fairly uniform throughout a 100 mm length of the DS zone by quantitative metallography. The average cell size, d , decreased with increasing growth rate to a value of 125 μm at 508 mm h−1 according to the relation d (μm) ≈ 465 VI−0.22, where VI is in mm h−1. Both the average NiAl plate thickness, Δ NiAl , and the interlamellar spacing, λ , were observed to be constant for VI ⩽ 50.8 mm h−1 but decreased with increasing growth rate above this value as Δ NiAl (m)=61.2 VI−0.93 and λ (μm)=47.7 VI−0.64, respectively. The present results are detailed on a microstructural map.

Effect of growth rate on elevated temperature plastic flow androom temperature fracture toughness of directionally solidified NiAl-31Cr-3Mo

Abstract The eutectic system Ni-33A1-31Cr-3Mo was directionally solidified at rates ranging from 7.6 to 508 mm/h. Samples were examined for microstructure and alloy chemistry, compression tested at 1200 and 1300 K, and subjected to room temperature fracture toughness measurements. Lamellar eutectic grains were formed at 12.7 mm/h; however, cellular structures with a radial eutectic pattern developed at faster growth rates. Elevated temperature compression testing between 10−4 to 10−7 s−1 did not reveal an optimum growth condition, nor did any single growth condition result in a significant fracture toughness advantage. The mechanical behavior, taken together, suggests that Ni-33A1-31Cr-3Mo grown at rates from 25.4 to 254 mm/h will have nominally equivalent properties

Elevated temperature deformation of Cr3Si alloyed with Mo

Abstract Four-point bend, constant load compressive creep and constant engineering strain rate tests were conducted on arc-melted and powder-metallurgy (PM) processed Cr 40 Mo 30 Si 30 specimens in the temperature range 1400–1700 K. This is a two-phase alloy consisting of (Cr,Mo) 3 Si and (Cr,Mo) 5 Si 3 phases. The PM specimens, which were substantially weaker than the arc-melted materials, exhibited a stress exponent, n , of about 2 and an apparent activation energy for creep, Q a , of 485 kJ/mol. The mechanism in these specimens appeared to be controlled by creep of a glassy phase. In the case of arc-melted specimens for which n ~ 3 and Q a ~ 430 kJ/mol, the rate-controlling creep mechanism appeared to be that dominant in the (Cr,Mo) 5 Si 3 phase. In this case, it is suggested that the Nabarro creep mechanism, where dislocation climb is controlled by Bardeen–Herring vacancy sources, is the dominant creep mechanism. Finally, an analysis of the present and literature data on Cr 3 Si alloyed with Mo appeared to suggest that the creep rate decreases sharply with an increase in the Mo/Si ratio.

Influence of Processing on the Microstructure and Mechanical Properties of a NbAl3-Base Alloy

Induction melting and rapid solidification processing, followed by grinding to 75-micron powder and P/M consolidation, have been used to produce a multiphase, NbAl3-based, oxidation-resistant alloy of Nb-67Al-7Cr-0.5Y-0.25W composition whose strength and ductility are significantly higher than those of the induction-melted alloy at test temperatures of up to 1200 K. Attention is given to the beneficial role of microstructural refinement; the major second phase, AlNbCr, improves both oxidation resistance and mechanical properties.

Microstructure, Creep and Fracture Toughness of Directionally Solidified NiAl/(Cr,Mo) Alloys Modified with Hf, Si, Ta, Ti Additions

A statistical design of experiments (DOE) strategy was implemented to optimize alloys based on the Ni-33Al-31Cr-3Mo eutectic system using small amounts of potential strengthening elements (Hf, Si, Ta, Ti). Following the analysis of the DOE results, several alloys were selected for directionally solidification (DS) utilizing a modified Bridgeman technique. The as-grown alloys were microstructurally examined by optical and scanning electron microscopy. They were also evaluated for fracture toughness at room temperature and compressive properties at 1,300K. The microstructures and mechanical properties of these DS DOE alloys are discussed and compared to the directionally solidified Ni-33Al-31Cr-3Mo base composition.