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Solid solutioning of Mg2(Si, Sn) has been a promising approach in reducing thermal conductivity and leads to improvement of thermoelectric performance. In addition to the Mg2(Si, Sn) solid solutions, we have noticed a layered structure with a gradient composition, which is formed by nonequilibrium solidification and peritectic reaction process and can provide further reduction of thermal conductivity of the Mg2(Si, Sn) solid solutions. All layers of the layered structure have the same face-centered cubic-based structure but varying Sn/Si concentration ratios in each layer. The interfaces between the layers are semi-coherent, reticulating with different numbers of misfit dislocations. Such an interfacial structure brings large numbers of phonon-scattering sources, resulting in further reduction of thermal conductivity in the Mg2(Si, Sn) solid solutions. Consequently, the undoped Mg2Si0.75Sn0.25 containing a higher density of the layered structure has relatively lower thermal conductivity, 1.9 W m-1 K-1 at 523 K, than Mg2Si0.25Sn0.75 with a much lower density of the layered structure, 2.3 W m-1 K-1 at 523 K.

Thermoelectric composite alloys consisting of the β-FeSi2 matrix and SiO2 particles dispersion were fabricated by a so-called combined reactions sintering process using reduction and oxidation reactions between eutectoid Si decomposed from α-Fe2Si5 and added Fe-oxide powder. Typical microstructure may include some of residual eutectoid Si particles, intermediate product Fe2SiO4 particles, and/or remaining reduced Fe particles depending on the composite alloy compositions and the process conditions. Partitioning of doping element, n-type Co or p-type Mn, during the process plays an important role to control the optimum carrier concentration of the composite alloys. Thermal conductivity can be reduced, as expected, by the dispersion of SiO2 particles. The solubility of doping elements, Co, Mn, Al, and Ru was evaluated in α-Fe2Si5 at 1373 K and in β-FeSi2 at 1073 K being based on the isotherm determination. It is suggested that suitable dopants for the present process are n-type Co and p-type Mn, since they have sufficiently large solubility around 10 at% in both α-Fe2Si5 and β-FeSi2 phases.

Retained austenite plays a significant role in the strength-ductility balance of medium Mn steels. In this contribution, a new processing route (Flash-Austenite Reversion Treatment, Flash-ART) was proposed to design retained austenite with a compositional core-shell structure in an Fe-0.20C-7.76Mn-1.99Al (wt%) steel. It was interestingly found that the austenite core, which should transform into martensite based on its composition and size, was stabilized by the Mn enriched shell. Flash-ART allows us to obtain more retained austenite compared with the conventional ART.

The microstructure and thermoelectric properties of an off-stoichiometric quaternary (Ti 0.2 ,Zr 0.8 )Ni 1.1 Sn half-Heusler (HH) alloy was investigated in three heating cycles. A high density of coherent nanoprecipitates with an average diameter of ∼13 nm and an interprecipitate spacing of ∼6 nm was observed in the alloy. Formation of the extremely fine nanoprecipitates, most likely to be the ‘full’ Heusler (FH) phase, was not only strongly related to the degree of excess Ni concentration ratio in the alloy, but also appeared to be affected by the Ti–Zr substitutions. We noticed the behaviour of both electrical resistivity ( ρ ) and Seebeck coefficient ( S ) of the alloy was closely associated with the microstructure evolution of the FH-nanoprecipitates, which depended on their phase instability at elevated temperature and the cyclic heating process. The ρ and S reduced after the 1st heating cycle and stabilised thereafter in the subsequent heating cycles. Despite of the presence of the metallic FH-nanoprecipitates, the stabilised S maintained similar magnitudes to S of the ZrNiSn (without FH-nanoprecipitates) and did not show degradation of S as previously seen in the ZrNi 1.1 Sn containing relatively much larger FH-nanoprecipitates. The high density of FH-nanoprecipitates and the presence of Ti–Zr point defects were responsible for the significant reduction of thermal conductivity ( κ ) of the alloy, about 30% and 20% less than κ of the ZrNiSn and ZrNi 1.1 Sn alloys, respectively. Moreover, further reduction of κ was noticed due to formation of the diffuse HH/FH interfaced FH-nanoprecipitates from the cyclic heating process. Consequently, the alloy has shown a maximum dimensionless figure of merit ( ZT ) up to 0.81 at 870 K.

The microstructures of thermoelectric TiNiSn half-Heusler alloys have been studied in detail. For concentration ratios that are slightly rich in Ni, a high density of Heusler-phase nanosized precipitates tended to precipitate within a half-Heusler matrix. The morphology and average size of the Heusler nanoprecipitates were very sensitive to the Ni concentration ratio in the half-Heusler matrix of the alloys. Smaller Heusler nanoprecipitates with coherent ellipsoidal ( HH orientations. Interfacial defects between the Heusler and half-Heusler phases, as well as lattice point defects, Ni antisites and vacancies, were found to be closely related to the formation of the Heusler nanoprecipitates. A mechanism has been proposed in this study to describe the coarsening of the Heusler nanoprecipitates via the formation of lattice point defects and interfacial defects.

Single crystals of E21 (L12) Ni3AlC1-x were prepared by the unidirectional solidification using the optical floating zone melting method to determine their mechanical properties. Particularly the effects of interstitial carbon atoms on mechanical properties were evaluated by compression tests at room temperature. Operative slip system of E21 Ni3AlC is {111} type which is the same as that of L12 Ni3Al. Strength of Ni3AlC single crystals increases with carbon concentration due to the solid solution effect, though the stress relief of yielding behavior is enhanced at the intermediate carbon content at around 3at%. A large gap appears in the carbon concentration dependence of critical resolved shear stress (as well as yield stress) at almost the same carbon content. This discontinuity in strengthening is attributed to the interaction between multiple solute carbon atoms and mobile dislocations.

The concept of utilising honeycomb abradable seals to improve gas turbine engine performance has been under development for many years. Engine operating temperatures, in the region of the seals, have been restricted to below 950°C by the reliance on a chromia scale for degradation protection. The introduction of nickel brazed FeCrAlY based alloys within the honeycomb seal could facilitate a safe increase in operating temperatures to over 1100°C. This is aided by the formation of a more stable, α-alumina scale.These Fe–20Cr–5Al–0.5Y foils, including the commercially produced variant designated MI2100, have been designed for a service lifetime of up to 24,000 hours. However, burner tests and isothermal oxidation tests in laboratory air at 1100 and 1200°C have shown them to fail after much shorter times. The major degradation of the foils occurs adjacent to the brazed region and limits the lifetime of the honeycomb seal.Cross-sectional analysis in a scanning electron microscope of seals manufactured ...

The structure of the martensite (α″)/martensite (α″) and parent (β)/martensite (α″) interfaces in a series of binary Ti–Nb alloys with Nb content ranging from 20 to 24 at.% was investigated. Both the α″/α″ and β/α″ interfaces comprised a series of { 2 1 ¯ 1 } β / / { 1 1 0 } α ″ terraces and steps when viewed edge-on (close to [ 0 0 1 ] α ″ ). Interfacial defects, particularly the transformation disconnections (b, h) superimposed along the terrace–step interface structure, have been identified. They were responsible for accommodating most of the transformation strain along the α″/α″ and β/α″ interfaces. Using the parameters b and h, the prediction of the α″ habit plane based on the topological model agreed well with the prediction from the phenomenological theory as well with experimental observations. The α″ habit plane in Ti–20Nb alloy is close to { 7 5 ¯ 5 ¯ } β and moves towards { 4 3 ¯ 3 ¯ } β in Ti–22Nb and Ti–24Nb alloys. The remaining transformation strain along the β/α″ interface was found to be accommodated by Type 1 twinning on ( 1 1 1 ) α ″ with Burgers vector bLIS close to 〈 2 ¯ 1 ¯ 3 〉 α ″ ( ≡ 〈 1 ¯ 1 ¯ 2 ¯ 〉 β ) .

In stiff engineering materials, martensitic transformations are shear-dominant diffusionless processes, and their crystallographic features can be successfully predicted using the phenomenological theory of martensite crystallography (PTMC). Recently, an alternative approach has been developed wherein the interface structure is modelled in terms of interfacial dislocations. This model, referred to as the topological model (TM), provides insight into the transformation process and identifies a set of five criteria that must be met for a transformation to be diffusionless. In the case of stiff engineering materials, the crystallographic predictions of the PTMC and TM are very similar. However, the TM enables a broader range of transformations to be treated, and two examples of ‘unusual’ martensites are presented here. The first is a diffusionless transformation in a small elastically soft protein crystal. The second is a transformation in a prospective high-temperature engineering alloy which exhibits the characteristic crystallographic features of martensite but where concomitant diffusion occurs. In this case, four of the five criteria mentioned above that relate to conservation of substitutional atomic sites are satisfied.