Transition Materials Research Articles

Integrating State of the Art Zirconia Thermal Barriers with Ytterbium Silicate Environmental Barriers for Silicon-Based Ceramic Turbine Components

AbstractAs gas turbine firing temperatures continue to increase for the sake of improved operating efficiency, the material's transition from Ni-based superalloy components toward ceramic matrix composites (CMCs) is concurrently in progress. Due to the complex nature of the turbine operating environment (envisaged ultrahigh temperatures, presence of water vapor, etc.), coating solutions for these CMCs are still on the forefront of design optimizations. Typically, rare-earth (RE) silicate environmental barrier coatings (EBCs) have been utilized to protect the CMCs from impinging water vapor; however, they lack the thermal insulation properties to enable continued use of simple and/or easily accessible bond coat materials (i.e., silicon). Combined thermal-environmental barrier coatings (T-EBCs) are such a multifaceted surface solution. T-EBCs have been considered in the past, but to this point have not been demonstrated to be technologically robust either due to high implementation costs or complex processing. This study utilizes and combines straightforward and well-established processes—such as plasma-sprayed 7 wt.% yttria-stabilized zirconia—to demonstrate the feasibility of MultiLayered T-EBCs comprised of zirconia-based oxides and RE silicate EBCs in a single coating. The results show that despite high thermal mismatch strains, the structures cannot only be deposited, but also in certain circumstances sustain cyclic thermomechanical loading.

Self-assembly in a near-frictionless granular material: conformational structures and transitions in uniaxial cyclic compression of hydrogel spheres.

We use a Markov transition matrix-based analysis to explore the structures and structural transitions in a three-dimensional assembly of hydrogel spheres under cyclic uniaxial compression. We apply these methods on experimental data obtained from a packing of nearly frictionless hydrogel balls. This allows an exploration of the emergence and evolution of mesoscale internal structures - a key micromechanical property that governs self-assembly and self-organization in dense granular media. To probe the mesoscopic force network structure, we consider two structural state spaces: (i) a particle and its contacting neighbours, and (ii) a particle's local minimal cycle topology summarized by a cycle vector. In both spaces, our analysis of the transition dynamics reveals which structures and which sets of structures are most prevalent and most likely to transform into each other during the compression/decompression of the material. In compressed states, structures rich in 3-cycle or triangle topologies form in abundance. In contrast, in uncompressed states, transitions comprising poorly connected structures are dominant. An almost-invariant transition set within the cycle vector space is discovered that identifies an intermediate set of structures crucial to the material's transition from weakly jammed to strongly jammed, and vice versa. Preferred transition pathways are also highlighted and discussed with respect to thermo-micro-mechanical constitutive formulations.

Loading rate dependency of maximum nanoindentation depth in nano-grained NiTi shape memory alloy

We examined the effect of loading rate on maximum nanoindentation depth for nano-grained superelastic NiTi shape memory alloy during stress-induced phase transformation. The depth decreases significantly with increasing loading rate. It is argued that the observed rate dependency is attributed to the release and transfer of latent heat during indentation and the temperature dependence of the material's transition stress. Dimensional analysis further shows that the depth is mainly governed by the normalized average stress in the phase transition zone and the ratio of heat conduction time over loading time. Experimental results support the rationale.

Crystal-Field Level Inversion in Lightly Mn-DopedSr3Ru2O7

Sr3(Ru(1-x)Mnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3d(x2-y2) orbital instead of the expected out-of-plane 3d(3z2-r2). We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.

Product Stewardship in Wollastonite Production

In July 2002, NYCO Minerals, Inc., discovered a heretofore unknown contaminant in its wollastonite ore. The contaminant was first believed to be tremolite asbestos. Immediate efforts were made to eliminate this material. Additional studies were initiated to fully characterize the contaminant and its distribution in the ore body. Subsequent study by NYCO and their consultants led to the identification of the contaminant as a transition material (TM) intermediate between tremolite and talc. In vitro dissolution rate measurements indicated that the TM dissolved much more rapidly than tremolite asbestos. This article provides background information on wollastonite mineralogy and NYCO's product stewardship program (PSP). At present, NYCO Minerals uses selective mining to control the trace levels of TM in the ore and finished product verified by periodic monitoring of workplace air and finished product.

Regular features of the evolutionary behaviour exhibited by plastic flow localisation and fracture in metals and alloys

Considered is the generation of self-excited waves by non-homogeneous plastic flow observed for a range of alloys in a single-crystal and a polycrystalline state at the stage of linear work hardening. It is found that under such conditions, in plastically deforming single austenitic crystals and polycrystalline Zr base alloys there originate self-excited waves propagating in the material volume at a rate of ∼10 -5 m/s. It turns out that the motion velocity of self-excited waves is inversely proportional to the extent of work hardening, which sets them apart from the well-known plasticity waves. The effect of extension axis orientation on the kinetic parameters of self-excited waves observable in the single crystals tested has been examined. Plastic flow is regarded as a self-organization process occurring in a deforming medium, and the universality of the proposed approach to the description of plastic flow is substantiated. The evolution of self-excited waves in the deforming alloys during material's transition to parabolic work hardening and on to fracture is described.