S.K. Makineni

Phase evolution and crystallography of precipitates during decomposition of new “tungsten-free” Co(Ni)–Mo–Al–Nb γ–γ′ superalloys at elevated temperatures

This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered tungsten-free γ–γ′ alloys of base composition Co–10Al–5Mo–2Nb with or without the addition of Ni and Ti. On prolonged aging at 800xa0°C of the Co–10Al–5Mo–2Nb alloy, needle-shaped DO19-ordered precipitates with stoichiometry of Co3(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc γ-Co with composition different from the original matrix, CoAl with B2 ordering and Co3(Mo, Nb) with DO19 ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the γ′ phase. The Ni-containing alloy exhibits no phase decomposition until 100xa0h of aging at 800xa0°C without any significant effect on γ′ volume fraction (76xa0%). However, at 950xa0°C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr3Si-type cubic phase, a hexagonal Laves phase, rhombohedral μ phase, and solid solution of Co phase. The γ–γ′ microstructure in the Co–10Al–5Mo–2Nb and Co–30Ni–10Al–5Mo–2Ta alloys is not stable at 800 and 950xa0°C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990xa0°C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of γ′ precipitates, even after aging at 950xa0°C for 100xa0h.

Effect of Cr addition on γ–γ′ cobalt-based Co–Mo–Al–Ta class of superalloys: a combined experimental and computational study

The present article deals with effect of Cr addition (10xa0at.%) on the partitioning behavior and the consequent effect on mechanical properties for tungsten-free γ–γ′ cobalt-based superalloys with base alloy compositions of Co–30Ni–10Al–5Mo–2Ta (2Ta) and Co–30Ni–10Al–5Mo–2Ta–2Ti (2Ta2Ti). Cr addition leads to a change in the morphology of the strengthening cuboidal-shaped γ′ precipitates to a spherical shape. The site preference of Cr atoms in two alloy systems (with and without Ti) has been experimentally investigated using atom probe tomography with the supportive prediction from first principles DFT-based computations. Cr partitions more to the γ matrix relative to γ′. However, Cr also has a strong effect on the Ta and Mo partitioning coefficient across γ/γ′ interfaces. The value of partition coefficient for Mo (KMo) becomes <1 with Cr addition to the alloys. Results from ab initio calculations show that the Cr atoms prefer to replace Mo atoms in the sublattice sites of the L12 unit cell. The solvus temperature of about 1038 and 1078xa0°C was measured for 10Cr2Ta and 10Cr2Ta2Ti alloy, respectively, and these Cr-containing alloys have very low densities in the range of ~8.4–8.5 gm/cm−3. The 0.2% compressive proof strength of 10Cr2Ta2Ti alloy yields a value of 720xa0MPa at 870xa0°C, substantially better than most Co–Al–W-based alloys and many of the nickel-based superalloys (e.g., MAR-M-247).

Enhancing elevated temperature strength of copper containing aluminium alloys by forming L1(2) Al3Zr precipitates and nucleating theta `' precipitates on them

Strengthening by precipitation of second phase is the guiding principle for the development of a host of high strength structural alloys, in particular, aluminium alloys for transportation sector. Higher efficiency and lower emission demands use of alloys at higher operating temperatures (200u2009°C–250u2009°C) and stresses, especially in applications for engine parts. Unfortunately, most of the precipitation hardened aluminium alloys that are currently available can withstand maximum temperatures ranging from 150–200u2009°C. This limit is set by the onset of the rapid coarsening of the precipitates and consequent loss of mechanical properties. In this communication, we present a new approach in designing an Al-based alloy through solid state precipitation route that provides a synergistic coupling of two different types of precipitates that has enabled us to develop coarsening resistant high-temperature alloys that are stable in the temperature range of 250–300u2009°C with strength in excess of 260u2009MPa at 250u2009°C.

Enhancement of High Temperature Strength of 2219 Alloys Through Small Additions of Nb and Zr and a Novel Heat Treatment

This paper presents a detailed investigation on the effect of small amount of Nb and Zr additions to 2219 Al alloy coupled with a novel three-stage heat treatment process. The main aim of the work is to increase the high temperature strength of 2219 alloy by introducing thermally stable L12 type ordered precipitates in the matrix as well as by reducing the coarsening of metastable strengthening θ″ and θ′ precipitates. To achieve this, small amounts of Nb and Zr are added to 2219 alloy melt and retained in solid solution by suction casting in a water-cooled copper mould having a cooling rate of 102 to 103 K/s. The suction cast alloy is directly aged at 673xa0K (400xa0°C) to form L12 type ordered coherent Al3Zr precipitates. Subsequently, the alloy is solution treated at 808xa0K (535xa0°C) for 30xa0minutes to get supersaturation of Cu in the matrix without significantly affecting the Al3Zr precipitates. Finally, the alloy is aged at 473xa0K (200xa0°C), which results in the precipitation of θ″ and θ′. Microstructural characterization reveals that θ″ and θ′ are heterogeneously precipitated on pre-existing uniformly distributed Al3Zr precipitates, which leads to a higher number density of these precipitates. This results in a significant increase in strength at room temperature as well as at 473xa0K (200xa0°C) as compared to the 2219 alloy. Furthermore, the alloy remains thermally stable after prolonged exposure at 473xa0K (200xa0°C), which is attributed to the elastic strain energy minimization by the conjoint Al3Zr/θ′ or Al3Zr/θ″ precipitates, and the high Zr and Nb solute-vacancy binding energy, retarding the growth and coarsening of θ″ and θ′ precipitates.