The paper describes in principle how information about textures can be obtained through the application of laser-ultrasonics (LUS) which can be carried out at elevated temperatures, for example in connection with hot rolling. The benefits from getting a measure of texture in this way are explained together with the proposed methodology which is based on the elastic anisotropic properties of the textured material. Measurements are made using only a single laser pulse and in real-time. Two approaches are presented to modelling the propagation of elastic waves, ray tracing and finite difference modelling. These give consistent results but the latter provides a more complete prediction of the ultrasonic spectrum that can be quantitatively related to measured signals through a cross-correlation procedure. Some experimental results are presented for room temperature measurements on a sheet of interstitial-free steel. Agreement between experimental data and modelling results is good and allows estimation of the 4th order coefficients of the orientation distribution function.

The microstructure and chemical composition of white layers (WLs) formed during hard turning of AISI 52100 steel were studied using atom probe tomography (APT) and transmission electron microscopy (TEM). APT analyses revealed a major difference in the re-distribution of the carbon (C) atoms between WLs formed above and below the A_c1 temperature, i.e. T-WL and M-WL, respectively. In T-WL, the C-atoms segregate to grain boundaries (GBs) forming interconnected or isolated C-rich clusters, ∼5 nm, with a concentration of 9.8 ± 0.3 at.%C. Apart from the GB segregation, in M-WLs, large C-rich regions were found with 24.8 ± 0.4 at.%C. Owing to the chemical composition (stoichiometry) and element partitioning of such regions, they were assigned as θ-carbides (cementite). The APT results reveal that the original θ-carbides remain un-dissolved in the M-WLs, but might be plastically deformed due to the excessive strain that exists in hard machining process. The obtained results are in good agreement with the temperatures that are reached during formation of M-WLs. The isolated nano-sized C-clusters were assigned as off-stoichiometric carbides whereas the interconnected C-rich clusters were attributed to Cottrell atmospheres, evident by the linear shape of the C-enrichment as observed in the APT reconstructions. The C-contents in the nano-sized martensitic and ferritic grains were estimated to 0.50 ± 0.06 at.%C and ∼0.46 ± 0.02 at.%C, respectively. The C-content in the ferritic grains, beyond the C-solubility limit in ferrite (<0.1 at.%) is governed by the high dislocation density inside the grains, supported by the favorable binding energy between dislocations and C-atoms compared to C-atoms and Fe in carbides. No other evidence of redistribution of the substitutional alloying elements was observed. TEM analyses showed that T-WLs comprises of an equiaxed and nano-sized grains with well-defined cell boundaries, whereas the structure in the M-WLs comprised of elongated sub-grains formed via re-orientation of the original martensite followed by breakage/partitioning into elongated sub-grains.

In recent literature the gradual yielding of quenched martensitic steels has been attributed to either heterogeneous microstructures having different strength levels or to the presence of small scale, Type II, residual stresses. Using in-situ tensile testing in synchrotron diffraction experiments in combination with crystal plasticity finite element modelling (CPFEM) we show that the dominant influence on yielding derives from the residual stresses which are a product of the displacive transformation from austenite during quenching. As plastic straining proceeds, the measured diffraction peaks become narrower and asymmetric, as predicted by the CPFEM calculations. The model predictions are generally in good agreement with published results showing large variations in local strains in different microstructural elements.

Four commercial steels with carbon contents in the range 0.1-0.5 wt.% have been examined in the as-quenched condition using electron microscopy, X-ray diffraction and atom probe tomography. The austenite had been deformed 0%, 10% and 30% prior to brine quenching. No influence of this deformation was evident on the martensite hardness or in any of the microstructure measurements. Increasing carbon content showed a well-known marked effect on the hardness but resulted in little refinement in the grain structure of the martensite. All crystal structures were cubic; no evidence of tetragonality was seen even at the highest carbon level but some systematic changes in grain boundary misorientations existed. The content of carbon in true interstitial solid solution deduced from X-ray line shifts was small (∼0.02 wt.%), and was independent of the total carbon content in the steel. Atom probe tomography showed that carbon was almost completely segregated to lath boundaries and dislocations but with an increasing density of segregates in the higher carbon steels. Calculations of diffusion distances confirmed that the segregation patterns were compatible with autotempering of the martensite during quenching. Analysis of different possible contributions to strength leads to the conclusion that segregated carbon atoms at defects behave similarly to carbon in true solid solution and that this is the largest single factor controlling the strength of as-quenched martensite. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Plastic deformation processes in hexagonal metals are complex and are best analyzed using procedures such as visco-plastic self-consistent crystal plasticity modelling. These involve a large number of adjustable parameters and make limited use of independent input data. Using physical arguments, the authors show that several of the parameters can be replaced by experimentally measured values of critical resolved shear stresses from the literature. A further simplification derives from the argument that all deformation modes interact with the same substructure, and so a common work-hardening behaviour can be assumed as a reasonable first approximation. Furthermore, many microstructural contributions to the strength can be introduced through a single constant term. In these ways, the twelve or more adjustable parameters in the model are reduced to only three. This new approach is tested critically by applying it to a sheet magnesium alloy for which the plastic strain ratio varies markedly during the test. Its complex plastic behaviour, which arises from changes among the active deformation modes, is successfully predicted. A benefit of the present approach is that the effect of metallurgical variables such as grain size or precipitation strengthening can be readily investigated. Although tested here for a magnesium alloy, the same principles should be applicable to other hexagonal close-packed materials.

Ceramics and their composites are in general brittle materials because they are predominantly made up of ionic and covalent bonds that avoid dislocation motion at room temperature. However, a remarkable ductile behavior has been observed on newly developed 11 mol.% ceria-stabilized zirconia (11Ce-TZP) composite containing fine alumina (8 vol.% Al2O3) and elongated strontium hexa-aluminate (8 vol.% SrAl12O19) grains. The as-synthesized composite also has shown full resistance to Low Temperature Degradation (LTD), relatively high strength and exceptionally high Weibull modulus, allowing its use in a broader range of biomedical applications. In this study, to deepen the understanding of plastic deformation in Ce-TZP based composites that could soon be used for manufacturing dental implants, different mechanical tests were applied on the material, followed by complete microstructural characterization. Distinct from pure Ce-TZP material or other zirconia-based ceramics developed in the past, the material here studied can be permanently strained without affecting the Young modulus, indicating that the ductile response of tested samples cannot be associated to damage occurrence. This ductility is related to the stress-induced tetragonal to monoclinic (t-m) zirconia phase transformation, analogue to Transformation-Induced Plasticity (TRIP) steels, where retained austenite is transformed to martensite. The aim of this study is to corroborate if the observed plasticity can be associated exclusively to the zirconia t-m phase transformation, or also to microcraking induced by the transformation. The t-m transformed-zones produced after bending and biaxial tests were examined by X-ray refraction and SEM/TEM coupled with Raman. The results revealed that the observed elastic-plastic behavior occurs without extensive microcracking, confirming a purely elastic-plastic behavior driven by the phase transformation (absence of damage).