Microstructural stability of Co-Re-Cr-Ta-C alloy strengthened by TaC precipitates

It is becoming increasingly clear that new materials that can operate at substantially higher temperatures than Ni-base superalloys are needed for future gas turbines. High melting Co-Re-Cr based alloy strengthened by carbides, particularly the MC type carbide, shows promise [1]. A fine dispersion of globular TaC precipitate is exploited for this purpose. Additionally Cr, which is mainly added to improve oxidation resistance, also stabilizes lamellar M23C6 type Cr carbide. The microstructure of a Co-Re-Cr-Ta-C alloy with the two types of carbides is seen in Fig. 1. Please click Additional Files below to see the full abstract

High temperature oxidation behaviour of Mo-Si-B-based and Co-Re-Cr-based alloys

Enhancing the efficiency of aerospace gas turbine engines requires materials that can be used at very high temperatures. This study summarises the current stage of alloy development of Mo-Si-B-based and Co-Re-Cr-based alloys regarding the high-temperature oxidation resistance. Since refractory metals, such as Mo and Re, suffer from catastrophic oxidation, the main task of research is to find alloying elements that improve the oxidation behaviour of these alloys. It is well-known that Mo-Si-B-based alloys are prone to catastrophic oxidation at intermediate temperature below 900°C as non-protective scales form on the metallic surface. It was found that the additions of Ti between 27 at.% and 29 at.% lead to a substitution of the Mo3Si phase exhibiting poor oxidation resistance by Mo5Si3 phase showing excellent oxidation behaviour in the wide temperature range from 750°C to 1300°C. The improved oxidation resistance can be attributed to the formation of a protective duplex layer consisting of a silica matrix with embedded TiO2 particles. Further, Ti additions cause an increased creep resistance due to the formation of Ti-rich silicides and a notable reduction of the alloy density. In Co-Re-Cr-based alloys, Cr should fulfill a twofold task: (i) to achieve favorable mechanical properties and (ii) to form a protective chromia layer during oxidation. Unfortunately, high Cr concentrations required to assure the reliable oxidation resistance induce formation of coarse particles of the Cr2Re3-type σ-phase that possesses inherently poor oxidation resistance. Additionally, these coarse particles deteriorate the mechanical properties of the material. Recent efforts to adjust the microstructure of the Co-Re-Cr-based alloys reveal that Ni additions significantly refine the particle size of the σ-phase. Moreover, Ni-containing alloys show improved oxidation resistance due the formation of a protective Cr2O3 scale. Obviously, Ni supports the lateral growth of Cr2O3 nuclei and enhances the Cr diffusivity in the metallic matrix

Gauge and General Coordinate Invariance in Non-Polynomial Closed String Field Theory

An appropriate field configuration in non-polynomial closed string field theory is shown to correspond to a general off-shell field configuration in low energy effective field theory. A set of string field theoretic symmetries that act on the fields in low energy effective field theory as general coordinate transformation and antisymmetric tensor gauge transformation is identified. The analysis is carried out to first order in the fields; thus the symmetry transformations in string field theory reproduce the linear and the first non-linear terms in the gauge transformations in the low energy effective field theory.Comment: 29 page

Topological Landau-Ginzburg Model of Two-Dimensional String Theory

We study a topological Landau-Ginzburg model with superpotential W(X)=X^{-1}. This is argued to be equivalent to c=1 string theory compactified at the self-dual radius. We compute the tree-level correlation function of N tachyons in this theory and show their agreement with matrix-model results. We also discuss the nature of contact terms, the perturbed superpotential and the flow of operators in the small phase space. The role of gravitational descendants in this theory is examined, and the tachyon two-point function in genus 1 is obtained using a conjectured modification of the gravitational recursion relations.Comment: 22 pages, harvmac, Mehta Research Institute and Tata Institute Preprint MRI-PHY/13/93, TIFR/TH/93-6

Kleinian Singularities and the Ground Ring of C=1 String Theory

We investigate the nature of the ground ring of c=1 string theory at the special A-D-E points in the c=1 moduli space associated to discrete subgroups of SU(2). The chiral ground rings at these points are shown to define the A-D-E series of singular varieties introduced by Klein. The non-chiral ground rings relevant to closed-string theory are 3 real dimensional singular varieties obtained as U(1) quotients of the Kleinian varieties. The unbroken symmetries of the theory at these points are the volume-preserving diffeomorphisms of these varieties. The theory of Kleinian singularities has a close relation to that of complex hyperKahler surfaces, or gravitational instantons. We speculate on the relevance of these instantons and of self-dual gravity in c=1 string theory.Comment: 26 pages, Phyzzx macro, TIFR/TH/92-3

Shear melting and high temperature embrittlement:theory and application to machining titanium

We describe a dynamical phase transition occurring within a shear band at high temperature and under extremely high shear rates. With increasing temperature, dislocation deformation and grain boundary sliding is supplanted by amorphization in a highly localized nanoscale band, which allows massive strain and fracture. The mechanism is similar to shear melting and leads to liquid metal embrittlement at high temperature. From simulation, we find that the necessary conditions are, lack of dislocation slip systems, low thermal conduction and temperature near the melting point. The first two are exhibited by bcc titanium alloys, and we show that the final one can be achieved experimentally by adding low-melting point elements: specifically we use insoluble rare earth metals (REMs). Under high shear, the REM becomes mixed with the titanium, lowering the melting point within the shear band and triggering the shear-melting transition. This in turn generates heat which remains localized in the shear band due to poor heat conduction. The material fractures along the shear band. We show how to utilize this transition in the creation of new titanium-based alloys with improved machinability.Comment: Accepted for PR

Coexistence of Two Cubic-Lattice Co Matrices at High Temperatures in Co-Re-Cr-Ni Alloy Studied by Neutron Diffraction

In situ neutron diffraction measurements were performed during heating to high temperature and cooling for a Co-17Re-23Cr-25Ni alloy. The allotropic transformation of the Co matrix and the evolution of the low-temperature hexagonal and high-temperature cubic Co phases were studied. A surprising observation was the splitting of the face-centred cubic (fcc) Co phase peaks at high temperature during heating as well as cooling. The phase evolution was monitored, and an appearance of the secondary fcc phase could be linked to the formation of σ phase (Cr2Re3 type) associated with a compositional change in the matrix due to diffusion processes at high temperature

Influence of σ Phase on the Allotropic Transformation of the Matrix in Co-Re-Cr-Based Alloys with Ni Addition

Co-Re-Cr alloys are being developed for high-temperature application in gas turbines. In these alloys, the Cr2Re3-based σ phase is stable when the Cr content is higher than 20 atomic %. The addition of Ni is being studied to partially substitute Cr, which aims to suppress σ formation without sacrificing the benefit of Cr in the oxidation resistance of the alloy. The microstructure of the alloys with varying Cr (18–23%) and Ni (8–25%) was investigated by electron microscopy in the present study, primarily to look into the stability of the σ phase and its influence on the Co matrix phase transformation. The σ phase is mainly found in two morphologies in these alloys, where at high temperature only blocky σ phase is present at grain boundaries but cellular σ is formed through a discontinuous precipitation within the grains at lower heat treatment temperatures. The presence of fine cellular σ phase influences the alloy hardness. Moreover, the σ precipitation, which depletes the matrix in Re, also influences the allotropic transformation of the Co matrix

Additional Phases at High Boron Content in High-Temperature Co–Re–Cr Alloys

Boron largely increases the ductility of polycrystalline high-temperature Co–Re–Cr alloys. Therefore, the effect of boron addition on the alloy structural characteristics is of large importance for the stability of the alloy at operational temperatures. Along with the Co-solid solution matrix phase transformation from hcp to fcc structure, additional structural effects were observed in situ at very high temperatures (up to 1500 °C) using neutron diffraction (ND) in boron-containing Co–17Re–23Cr alloys. Increasing boron content up to 1000 wt. ppm lowers the temperature at which sublimation of Co and Cr from the matrix occurs. As a result, the composition of the matrix in the surface region is changed leading to the formation of a second and a third matrix hcp phases at high temperatures. The consideration on the lattice parameter dependence on composition was used to identify the new phases appearing at high temperatures. Energy-dispersive spectroscopy and ND results were used to estimate the amount of Co and Cr which sublimated from the surface region of the high-boron sample. In the sense of alloy development, the sublimation of Co and Cr is not critical as the temperature range where it is observed (≥1430 °C) is significantly above the foreseen operation temperature of the alloys (1200 °C)