Laser Powder bed fusion (L-PBF) has attracted a lot of interest in recent years, not only for its profound advantage of producing metallic components of complex geometries but also for the possibility of manipulating microstructures and crystallographic textures. Additionally, recent observations on wrought austenitic steels have revealed the strong dependence of the transformation induced plasticity (TRIP) effect in metastable stainless steels on the crystallographic texture [1–3]. Taking the aforementioned observations into consideration, we can now process TRIP steels such as 304L by L-PBF, in order to produce differently textured specimens and manipulate the TRIP effect. In this contribution, in situ uniaxial tension and compression tests with neutron diffraction, are utilized for monitoring of the microstructural evolution during deformation. The present study highlights how different microstructures, produced by L-PBF, lead to different deformation behavior in austenitic stainless steels and paves the way for tailored microstructures in different types of steels and for studies under different loading conditions.

References

[1] E. Polatidis et al., “The interplay between deformation mechanisms in austenitic 304 steel during uniaxial and equibiaxial loading,” 2019, doi: 10.1016/j.msea.2019.138222.

[2] E. Polatidis et al., “Suppressed martensitic transformation under biaxial loading in low stacking fault energy metastable austenitic steels,” Scr. Mater., vol. 147, pp. 27–32, Apr. 2018, doi: 10.1016/j.scriptamat.2017.12.026.

[3] E. Polatidis, J. Čapek, A. Arabi-Hashemi, C. Leinenbach, and M. Strobl, “High ductility and transformation-induced-plasticity in metastable stainless steel processed by selective laser melting with low power,” Scr. Mater., vol. 176, pp. 53–57, Feb. 2020, doi: 10.1016/j.scriptamat.2019.09.035.

Phase transformations in a single crystal of a metastable β titanium alloy (Ti-15Mo in wt %) were investigated in situ during heating by synchrotron x-ray diffraction. Metastable β titanium alloys contain such type and amount of alloying elements that the high‑temperature β phase (body-centred cubic) can be retained in a metastable state during fast cooling to room temperature; i.e. the formation of low-temperature α phase (hexagonal close-packed) is prevented. Ti alloys from this class generally undergo a wide range of phase transformations due to their metastable nature. First, nano-sized particles of metastable ω phase form in this class of Ti alloys during fast cooling by a difusionless displacement mechanism, which can be characterized as a collapse of neighbouring (111)_β planes into their intermediate position. During ageing or heating, ω particles grow by a combined displacement and diffusion process which is accompanied by rejection of alloying elements from the ω phase into the surrounding β matrix. At higher temperatures, lamellae of the thermodynamically stable α phase precipitate in the material; this process can be assisted either directly or indirectly by the previous β+ω microstructure.

In situ x-ray diffraction was measured using 60 keV photons at the high-energy beamline ID11, ESRF, Grenoble, France. This experiment was performed using an oriented single crystal of Ti-15Mo prepared in an optical floating zone furnace. A slice of the single-crystalline material with the [100]_β crystallographic axis parallel to the primary beam was placed in a special quartz chamber furnace which allowed measuring in a high vacuum. X-ray diffraction patterns were acquired in situ during heating with a constant heating rate of 5 °C/min.

Fitting of the temperature dependence of intensity of selected representative single-crystalline diffraction spots showed that at the beginning of linear heating, up to approximately 350°C, the volume of ω phase decreased, which is likely connected with displacement-accompanied ω to β reversion. Between 350°C and 420°C, the volume fraction of ω particles increased which is the consequence of diffusion-driven coarsening of ω phase particles. Subsequently, as the temperature approached the stability limit of the ω phase, the volume of ω decreased. A complete dissolution was observed at 560°C. Finally, a rapid growth of the α phase commenced at about 580°C. It was also verified that during linear heating, none of the crystallographic variants of ω and α phase is preferred.

Many mechanical structures are submitted to repeated loadings during their life span and can break under stress lower than the ultimate tensile stress. This phenomenon, called fatigue of materials, has attracted the scientific community attention due to its effect in many industrial sectors, such as the transport, aeronautic and energy. Fatigue design is thus crucial in engineering and it requires the accurate characterization of material behavior under cyclic loadings to ensure the safety and reliability of structures throughout their life. It is presently common to find mechanical systems subjected to several billion cycles, in what is called the gigacycle fatigue domain or very high cycle fatigue (VHCF) domain [1]. The characterization of the fatigue behavior of materials have been largely investigated with fatigue tests requiring long testing time with standard laboratory. To overcome this inconvenient new approaches using ultrasonic fatigue machines have been developed during the last decades. In particular, the present research group developed recently a new method for the fast determination of fatigue behavior interpreting diffraction patterns with a temporal resolution of ∼1 µs during an ultrasonic fatigue test and loading frequency of about 20 kHz. The present study points on the estimation of the amount of energy stored by the specimen during its deformation due to an ultrasonic fatigue loading. This energy is a crucial parameter as it is strictly related to the fatigue damage and can be estimated from the intrinsic dissipation and the mechanical work supplied to the specimen. X-ray diffraction analysis were performed to measure the supplied work by integrating over one fatigue cycle of the product of the strain rate by the stress. In particular, pure copper and steel specimens were loaded using a 20 kHz ultrasonic fatigue machine mounted on the six-circle diffractometer available at the DiffAbs beamline on the SOLEIL synchrotron facility in France. Then, in order to obtain the mechanical work: 1) from the shift of Bragg peaks is possible to estimate the total stress applied to the sample, 2) from both the broadening and shift of peaks one can measure the mean elastic lattice strain distribution, and 3) from the peak broadening the fluctuation of elastic strain is deduced, providing information about intragranular strain heterogeneity and dislocation density.

[1] Bathias, C. & Paris, P. (2005). Gigacycle Fatigue in Mechanical Practice. New York: Marcel Dekker.

Important problem studied in this work is the anisotropy of mechanical properties for textured polycrystalline materials. The mechanical behaviour during in-situ loading tests for magnesium AZ-31 alloy was studied using neutron diffraction. The lattice strains were measured during tensile by using angle-dispersive neutron diffraction (TKSN 400 at NPI in Řež, Czech Republic) and changing sample orientation with respect to the scattering vector. The measurements were done for sets of poles corresponding to different orientations of the grains in strongly textured Mg alloy. Subsequent experiment was performed using time of flight (TOF) neutron diffraction at the pulsed reactor IBR-2 in Joint Institute for Nuclear Research (Dubna, Russia), using EPSILON-MDS instrument equipped with 9 detectors. The experiments allowed to develop an experimental methodology based on the so-called crystallite group method in order to determine the evolution of the stresses localised in polycrystalline grains having different crystallographic orientations. The components of stress tensor were determined directly from measured lattice strains corresponding to chosen orientations of crystallite lattice.It was found that the crystallites having two main orientations, named A and B, are harder when compared with other ones. For these orientations the basal slip system cannot be activated because the load is applied in direction parallel to the basal plane. Orientation B was completely transformed to twins (having T orientation) during the compression test. In the case of the soft orientations C and D, the direction of the load is inclined from the basal plane, i.e. the basal system can be activated. Using the experimental data the evolution of stress tensor and von Mises stress were determined for selected groups of grains. A large difference in the hardness of crystallites having different lattice orientations was found. The highest von Mises stress appeared on twins, which was compensated by low stresses localised on soft orientations C and D.

The novelty of our study is in original methodology used for direct determining of stress tensor for groups of polycrystalline grains having different orientations (especially for preferred texture orientations). The stress evolution measured during sample loading allowed us to find out the critical resolved shear stress (CRSS) values for different slip systems and twinning process.

Research on lead-free piezoceramics is a trending topic [1]. A significant component of this search is the characterization of the effect of texture on the properties of polycrystalline electroceramics. The present contribution describes an integrated methodology, systematized in a software package, to solve the following tasks: (a) interpretation by numerical simulation of XRD patterns produced by textured samples; (b) forecast of the effective elasto-electrical properties of piezoceramics, starting from the knowledge of the corresponding single-crystal tensors and the texture determined in (a).

Part (a) considers 1D and 2D diffraction experiments, with Bragg-Brentano, grazing incidence and transmission geometries. The inverse pole figure of the symmetry axis of fiber-textured piezoceramics is proposed and refined by a Rietveld-type procedure [2].

The calculations in part (b) are performed using a variant of the Voigt-Reuss-Hill approximations. Particular precautions are taken with regard to the selection of the quantities considered as independent variables [3].

The computer programs developed to solve the proposed tasks are shown, the use of the MPOD database [4] in this type of work is described, and representative case studies are presented.

Fig. 1 shows as an example the computerized modelling of the variation of the representative longitudinal surfaces of the elastic compliance s(h) and the charge constant d(h) of the lead-free piezoceramic 0.95(Na_0.5Bi_0.5)TiO₃-0.05BaTiO₃ (BNBT5) as the texture evolves from relatively sharp to a random distribution.

Figure 1. Modelled effect of axial texture on elastic compliance and piezoelectric charge constant of lead-free BNBT5 piezoceramic. As the width of the orientation distribution (Ω) increases, the elasticity tends to isotropic and the piezoelectricity collapses to zero.

[1] Villafuerte-Castrejón, M. E., Morán, E., Reyes-Montero, A., Vivar-Ocampo, R., Peña-Jiménez, J. A., Rea-López, S. O., & Pardo, L. (2016). Materials 9, 21. [2] Burciaga-Valencia, D. C., Villalobos-Portillo, E. E., Marín-Romero, J. A., Del Río, M. S., Montero-Cabrera, M. E., Fuentes-Cobas, L.E. & Fuentes-Montero, L. (2018). J. Mater. Sci: Mater. Electron. 29, 15376. [3] Villalobos-Portillo, E. E., Fuentes-Montero, L., Montero-Cabrera, M. E., Burciaga-Valencia, D. C. & Fuentes-Cobas, L. E. (2019). Mater. Res. Express 6, 115705. [4] Fuentes-Cobas, L. E., Chateigner, D., Fuentes-Montero, M. E., Pepponi, G & Grazulis, S. (2017). Adv. Appl. Ceram. 116, 428.

Sponsorship by the Consejo Nacional de Ciencia y Tecnología (México), Projects 257912 and 270738, is appreciated. Support from the Project MAT2017-86168-R“Piezocerámicas ecológicas para la generación de ultra-sonidos” (CSIC, Spain), is acknowledged.

Production of nanostructures of extended covalent systems has remained a long-standing challenge, mainly due to the elevated activation energies required for their crystallization.[1] Such solids tend to exhibit outstanding mechanical properties, i.e. superhardness, the most illustrative case being diamond. Moreover, if nanostructuration is achieved (ideally in the ≈10 nm size range), further enhancement of the hardness can be obtained. For instance, diamond nanorods show an increase of the hardness by 86% compared to the bulk (from 80 GPa to 150 GPa).[2] Superhard materials are of great industrial importance, with applications as cutting and polishing tools, coatings and abrasives. Diamond is indeed the traditional choice for such purposes, but it has well-known limitations: it is brittle, oxidizes to carbon dioxide at 800–900 °C in air and reacts with Fe‑containing solids during cutting, not to mention the difficulty and cost of its production associated to the high-pressure machinery needed.

While several possible diamond substitutes have been suggested, boron carbide (B_4+δC) stands as one of the few superhard phases that can be reached at room pressure. Boron carbide crystallizes in the R-3m spacegroup and its network is based on B icosahedra linked to each other through both direct B-B bond and CBC chains, as depicted in Figure 1. B_4+δC exhibits an intrinsic hardness of 38 GPa, yet far from the industrially profitable range. Plenty of effort has been devoted to the optimization of boron carbide’s particle size and consequent amelioration of its mechanical properties. Approaches using different reactants, lower temperatures (down to 600°C) and/or liquid-phase reactions have not been able to enable further lower the B_4+δC particle size. In this work, instead of using pristine reagents, we demonstrate the capacity to produce 10 nm B_4+δC nanoparticles from a nano-precursor, namely NaB₅C. The structure of this cubic compound (space group Fd-3c) resembles that of perovskites, where B₅C octahedra form an anionic network that leaves cavities filled by Na⁺ cations (Figure 1 left). 5-7 nm NaB₅C nanoparticles were synthesized by using a high temperature liquid-phase procedure in molten salts.[3] The intrinsic carbon and boron mixture in a composition lying well within the range of the B_4+δC solid solubility makes it an interesting precursor to yield boron carbide. Indeed, upon calcination, the NaB₅C nanostructures are transformed to B_4+δC with nanostructuration preservation at circa 10 nm. After hot-pressing densification, the synthesized powders show enhancement of their mechanical properties above any previous record. We have used powder X-ray diffraction to shed light on the transformation from NaB₅C to B_4+δC at the atomic level. The implications of the new morphology of B_4+δC on the mechanical properties will be discussed as well as the importance of the templating effect remaining from the original NaB₅C nanostructures.