Effect of the heat treatment on the microstructure, magnetism and magnetocaloric effect in Fe-rich (Mn,Fe)_y(P,Si) melt-spun ribbons

The effect of the heat treatment on the magnetism, magnetocaloric effect and microstructure formation has been systematically studied in Fe-rich (Mn,Fe)y(P,Si) melt-spun ribbons (1.80 ≤ y ≤ 2.00). XRD, SEM and EDS measurements demonstrate that a metal deficiency prompts the stable (Mn,Fe)Si phase, whereas in the metal-rich region the (Mn,Fe)3Si phase is formed. It is found that the annealing temperature influences the composition and lattice parameters of the (Mn,Fe)y(P,Si) alloys, which greatly affects the Curie temperature (TC). For the optimal metal/non-metal ratio y the magnetic entropy change (|ΔSm|) is found to increase from 5.5 to 15.0 Jkg−1K−1 in a magnetic field change of 2 T by varying the annealing temperature from 1313 to 1433 K, indicating an enhancement of the first-order magnetic transition (FOMT). The presented results reveal that the secondary phase and magnetic properties in the (Mn,Fe)y(P,Si) system can be tuned by varying the annealing temperature and by adjusting the metal/non-metal ratio y.RST/Fundamental Aspects of Materials and EnergyTeam Maria Santofimia Navarr

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Surface Oxidation and Wettability of Fe–Mn and Fe–Mn–Si-Alloyed Steel After Annealing

The surface oxidation and wettability of Mn and Si-alloyed steel after annealing at different conditions are studied with scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and a so-called de-wetting method. After exposure at 950 °C for 1 hour in an Ar + 5 vol pct H2 gas atmosphere with dew points (DP) ranging from – 40 °C to 10 °C, oxides were observed along the grain boundaries or dispersed on the surface for the Fe–1.8 Mn steels while a continuous oxides layer was formed on Fe–1.9 Mn–0.94 Si steels (composition in weight fractions). The oxides formed at different DPs were predicted based on thermodynamic calculations. (Fe,Mn)O was formed on Fe–1.8 Mn steel at the whole range of DPs, while the oxide phase on Fe–1.9 Mn–0.94 Si steel depends on the DP. At low-DP SiO2 were formed and with increasing the DP (Fe,Mn)SiO3 or (Fe,Mn)SiO3 + (Fe,Mn)2SiO4 were formed and finally (Fe,Mn)2SiO4 were formed. An increase of the fraction of Fe in the oxide with increasing DP for both steels was observed with XPS analysis. As a measure for the surface wettability, the contact angle of Pb droplets on the annealed steels surfaces was determined with SEM and image analysis software. Also, the contact angle of Pb on pure Fe and on the Mn and Si alloyed steels free of surface oxides was measured for comparison. The results show that the contact angle of Pb on the steel surfaces after annealing decreases with increasing DP. This improved wettability with increasing dew point is related to the Fe fraction of the oxides formed on the surface.Team Jilt SietsmaTeam Maria Santofimia NavarroTeam Marcel Herman

Microstructural development during the quenching and partitioning process in a newly designed low-carbon steel

This paper presents a detailed characterization of the microstructural development of a new quenching and partitioning (Q&P) steel. Q&P treatments, starting from full austenitization, were applied to the developed steel, leading to microstructures containing volume fractions of retained austenite of up to 0.15. The austenite was distributed as films in between the martensite laths. Analysis demonstrates that, in this material, stabilization of austenite can be achieved at significantly shorter time scales via the Q&P route than is possible via a bainitic isothermal holding. The results showed that the thermal stabilization of austenite during the partitioning step is not necessarily accompanied by a significant expansion of the material. This implies that the process of carbon partitioning from martensite to austenite occurs across low-mobility martensite–austenite interfaces. The amount of martensite formed during the first quench has been quantified. Unlike martensite formed in the final quench, this martensite was found to be tempered during partitioning. Measured volume fractions of retained austenite after different treatments were compared with simulations using model descriptions for carbon partitioning from martensite to austenite. Simulation results confirmed that the carbon partitioning takes place at low-mobility martensite–austenite interfaces.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.OLD Metals Processing, Microstructures and PropertiesOLD Surface and Interface Engineerin

Determination of fracture strength and fracture energy of (metallo-) ceramics by a wedge loading methodology and corresponding cohesive zone-based finite element analysis

A wedge loaded testing methodology to determine the fracture energy and strength of (semi-) brittle (metallo-)ceramics is presented. The methodology combines a tailored specimen geometry and a comprehensive finite element analysis based on cohesive zone modelling. The use of a simulation-based approach to extract both fracture strength and energy from experimental data avoids the inherent inaccuracies found in closed-form expressions that rely on a priori assumptions about the deformation field. Results from wedge splitting tests on Ti3SiC2 and Ti2AlC (MAX phase) materials are used to illustrate the procedure. The simulation-based approach is further validated by comparing the fracture strength and fracture energies predicted by the proposed method and those indicated by a conventional four-point bending fracture toughness test (ASTM standard). The new protocol offers the possibility to measure not only the fracture properties of brittle material in its pristine state but also in the healed state.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-1Aerospace Structures & Computational MechanicsNovel Aerospace Material

Multiple crack-healing and strenght recovery in MAX phase ceramics (abstract)

Delft University of Technolog

Kinetics of zircon formation in yttria partially stabilized zirconia as a result of oxidation of embedded molybdenum disilicide

Recently MoSi2 sacrificial particles embedded in yttria partially stabilized zirconia (YPSZ)have been proposed as attractive healing agents to realize significant extension of the lifetime of the thermally loaded structures. Upon local fracture of the YPSZ, the embedded healing particles in the path and in the vicinity of the crack react with the oxygen atoms transported via the crack and first fill the crack with a viscous glassy silica phase (SiO2). The subsequent reaction between this freshly formed SiO2 and the existing tetragonal ZrO2 of the YPSZ leads to the formation of rigid crystalline zircon (ZrSiO4), which is key in the crack-healing mechanism of YPSZ based materials. The isothermal kinetics of the self-healing reaction and the mechanism of zircon formation from the decomposing MoSi2 and the surrounding YPSZ were assessed via X-ray diffraction (XRD). The obtained results revealed that at 1100 °C the reaction between amorphous SiO2 and YPSZ is completed after about 10 h. For a more accurate determination of the kinetics of the self-healing reaction, bilayer samples of YPSZ – MoSi2 (with and without boron addition)were annealed in air over a temperature range of 1100–1300 °C. This led to the formation of a MoSi2/amorphous (boro)silica/zircon/YPSZ multi-layer, which was investigated with scanning electron microscopy (SEM)and electron probe X-ray microanalysis (EPMA). Kinetic modeling of the growth of zircon and silica or borosilicate layers showed that zircon growth was dominated by the diffusion of Si4+ in zircon whereas the growth of the silica or borosilicate layer was controlled by oxygen diffusion. Moreover, a significant increase in the rate of ZrSiO4 formation was observed due to the presence of B in the MoSi2 particles.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-1Novel Aerospace Material

Selective oxidation of aluminium in Mo(Al,Si)₂

Mo(AlxSi1-x)2 alloy with x in the range of 0.35–0.65 were prepared by a one-step spark plasma sintering. To study the exclusive formation of an α-Al2O3 scale, oxidation experiments were conducted in low and high oxygen partial pressure ambient at 1373 K; viz.: 10−14 and 0.21 atm. The oxidation kinetics follows a parabolic rate law after a transient period. A counter-diffusion process of O and Al along grain boundaries of Al2O3 scale is responsible for the equiaxed and columnar grain growth based on a two-layered microstructure. The formation of a dense equiaxed α-Al2O3 layer contributes to excellent oxidation resistance.Team Marcel HermansTeam Maria Santofimia NavarroTeam Vera PopovichTeam Jilt Sietsm

Mo(Al_xSi₁₋_x)₂ healing particles for high temperature ceramics and encapsulation by selective oxidation of aluminium

To prevent premature triggering of the healing reaction in Mo-Si containing self-healing thermal barrier coating system, an oxygen impenetrable shell (α-Al2O3) around the sacrificial healing particles (MoSi2) is desired. Here an encapsulation method is presented through selective oxidation of Al in Mo(AlxSi1-x)2 particles. Healing particles of Mo(AlxSi1-x)2 is designed in terms of alumina shell thickness, particle size and fraction Al dissolved. By replacing Si by Al in MoSi2 up to the maximum solubility (x = 0.65) a strong crack healing ability is maintained (relative volume expansion ≥ 40 %). The formed exclusive α-Al2O3, featuring a two-layered structure, results from a counter-diffusion process along the grain boundaries, and its oxidation kinetics fits well with the 3D diffusion-Jander model. After 16 h exposure in gaseous ambient with a pO2 of 5 × 10-10 atm. at 1100 °C, a closed and dense shell of α-Al2O3 is formed with a thickness of about 1.3 µm. The oxide shell produced under this condition provided healing particles with significantly improved stability upon exposure to high pO2 of 0.2 atm. at 1100 °C for 50 h. The particles after exposure feature an inner core of MoSi2 with Al completely consumed and an oxide shell of α-Al2O3.Team Marcel HermansTeam Maria Santofimia NavarroTeam Vera PopovichTeam Joris Di