Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts.

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Multi-quantum-well structures of Bi2Te3 are predicted to have a high thermoelectric figure of merit ZT. Bi2Te3 thin films and Bi2Te3∕Bi2(Te0.88Se0.12)3 superlattices (SLs) were grown epitaxially by molecular beam epitaxy on BaF2 substrates with periods of 12 and 6nm, respectively. Reflection high-energy electron diffraction confirmed a layer-by-layer growth, x-ray diffraction yielded the lattice parameters and SL periods and proved epitaxial growth. The in-plane transport coefficients were measured and the thin films and SL had power factors between 28 and 35μW∕cmK2. The lattice thermal conductivity varied between 1.60W∕mK for Bi2Te3 thin films and 1.01W∕mK for a 10nm SL. The best figures of merit ZT were achieved for the SL; however, the values are slightly smaller than those in bulk materials. Thin films and superlattices were investigated in plan view and cross section by transmission electron microscopy. In the Bi2Te3 thin film and SL the dislocation density was found to be 2×1010cm−2. Bending of the SL with amplitudes of 30nm (12nm SL) and 15nm (6nm SL) and a wavelength of 400nm was determined. Threading dislocations were found with a density greater than 2×109cm−2. The superlattice interfaces are strongly bent in the region of the threading dislocations, undisturbed regions have a maximum lateral sie of 500nm. Thin films and SL showed a structural modulation [natural nanostructure (nns)] with a wavelength of 10nm and a wave vector parallel to (1,0,10). This nns was also observed in Bi2Te3 bulk materials and turned out to be of general character for Bi2Te3. The effect of the microstructure on the thermoelectric properties is discussed. The microstructure is governed by the superlattice, the nns, and the dislocations that are present in the films. Our results indicate that the microstructure directly affects the lattice thermal conductivity. Thermopower and electrical conductivity were found to be negatively correlated and no clear dependence of the two quantities on the microstructure could be found.

The best p-type skutterudites so far are didymium filled, Fe/Co substituted, Sb-based skutterudites. Substitution at the Sb-sites influences the electronic structure, deforms the Sb 4 -rings, enhances the scattering of phonons on electrons and impurities and this way reduces the lattice thermal conductivity.In this paper we study structural and transport properties of p-type skutterudites with the nominal composition DD 0.7 Fe 2.7 Co 1.3 Sb 11.7 {Ge/Sn} 0.3 , which were prepared by a rather fast reaction-annealing-melting technique. The Ge-doped sample showed impurities, which did not anneal out completely and even with ZT > 1 the result was

Bi 2 Te 3 is known for its large thermopower, a low thermal conductivity, and thereby a large thermoelectric figure of merit ZT at room temperature. Particularly, the low thermal conductivity is attributed to a high structural disorder. Electron probe microanalysis, transmission electron microscopy (TEM), and energy dispersive x-ray spectrometry in the TEM were applied on n-type Bi2(Te,Se)3 and p-type (Bi,Sb)2Te3 bulk materials for structural and chemical analysis. Significant variations in stoichiometry were found on the micrometer scale. Bulk materials showed a texture, grain sizes of 1−10 μm, and dislocations in the basal plane with a density of 109 cm−2 having a high mobility at room temperature. A structural modulation [natural nanostructure (nns)] was found on the nanometer scale. This nns was also observed in Bi2Te3 thin films and Bi2Te3/Bi2(Te,Se)3 superlattices and turned out to be of general character for Bi2Te3 materials. The nns was analyzed in detail by stereomicroscopy in the TEM and by image simulation. The nns was found to be a pure sinusoidal displacement field with (i) a displacement vector parallel to ⟨5,−5,1⟩ and an amplitude of about 10 pm and (ii) a wave vector parallel to {1,0,10} and a wavelength of 10 nm. Bi2Te3 samples from different batches produced under similar conditions showed different characteristics with respect to the nns: none, one, or two superimposed nns were observed. The nns is present in all parts of the sample with the same orientation. The formation of the nns is either bound to a certain stoichiometry range or to the thermal history of a sample and is related to the tendency of Te compounds to form amorphous phases. An ordered network of dislocations a few nanometers apart and chemical fluctuations on the nanometer scale as origins of the nns were ruled out. The displacement field of the nns is superimposed to the average structure and should significantly affect the thermoelectric properties. Particularly, the lattice thermal conductivity should be decreased due to phonon scattering on the sinusoidal strain field of the nns. Also, the nns should yield a one-dimensional or zero-dimensional behavior of the phonons and anisotropic transport coefficients in the basal plane.

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Sb2Te3 and Bi2Te3 thin films were grown at room temperature on SiO2 substrates using MBE and were subsequently annealed at 250°C. The films were stoichiometric, polycrystalline, textured, and yielded strikingly low charge carrier densities of about 2.7 × 10(exp 19) cm-3. The in-plane transport properties were measured at room temperature, the thermopower was 130 µVK(exp -1) for Sb2Te3 and -153 µVK(-1) for Bi2Te3 thin films. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing