Characterisation of L21-ordered Ni2TiAl precipitates in FeMn maraging steels

The precipitates formed in a new series of Fesingle bondMn maraging steels when aged at 500 °C were identified as the L21-ordered Ni2TiAl phase. The precipitate formed a coherent-coplanar microstructure analogously to γ/γ' Ni-based superalloys and maintained a high number density and homogeneous dispersion within α′-martensite matrix even after aging for 10,080 min. An increase in the Mn content of the alloy led to faster precipitation kinetics and thus rapid hardening kinetics

Microstructural evolution of Mn-based maraging steels and their influences on mechanical properties

The microstructural evolution in a set of Mn-based maraging steels (7–12 wt% Mn) when aged at 460–500 ºC for various durations up to 10,080 min and the influences on mechanical properties are systematically investigated. The improved yield strength of peak-aged samples is attributed to the formation of Ni2TiAl precipitates and the precipitation strengthening is governed by Orowan mechanism. Segregation of Mn at grain boundaries in the initial aging stage resulted in severe intergranular brittleness. During further aging, accumulated Mn segregation leading to the formation of ductile lath-like reverted austenite removed the embrittlement and significantly improved the ductility. In the overaged condition, the steady work hardening after yielding compensates the loss of yield strength resulting from the coarsening of precipitates and softening of α′-martensite matrix. There was only limited evidence of the TRIP effect in the reverted austenite, indicating that work hardening was associated with other deformation mechanisms. Increasing the aging temperature or the Mn content of alloy that promotes austenite reversion was demonstrated to accelerate the improvement of ductility

Microstructural evolution and wear mechanism of Ti3AlC2 – Ti2AlC dual MAX phase composite consolidated by spark plasma sintering (SPS)

In this work, we report the synthesis, deformation and tribological behaviour of a novel Ti3AlC2 – Ti2AlC MAX phase composite metallo-ceramic. The dual MAX phase composite was synthesized by spark plasma sintering (SPS) under a vacuum environment using Ti, Al, and C precursor powders. The deformation mechanism and the tribological behaviour were studied and analyzed by SEM, TEM, and Raman spectroscopy. The transition in friction and wear as well as the operative wear mechanisms involved were further discussed. Detailed analyses of the worn surface showed that Ti3AlC2 – Ti2AlC dual MAX phase composite is intrinsically self-lubricating

On the use of cryomilling and spark plasma sintering to achieve high strength in a magnesium alloy

Bulk nanostructured magnesium alloy AZ31 has been produced by spark plasma sintering at four different temperatures from 350 to 450 °C. The effect of sintering temperature on microstructural evolution and compression behaviour was studied in detail. It was concluded that the sample consolidated at 400 °C exhibited the highest strength. Higher sintering temperature (450 °C) improved the compressive strain of the bulk sample but at the sacrifice of strength. However, samples consolidated at 350 °C displayed brittle behaviour with low strength. All consolidated samples had a bimodal microstructure with nanocrystalline and coarse grains. The nanocrystalline microstructure formed by cryomilling was retained after consolidation and a maximum microhardness was approximately 150 HV. The bulk samples consolidated at 400 °C with an average grain size of 45 nm showed exceptional average true compressive yield strength of 400.7 MPa, true ultimate compressive strength of 499.7 MPa, which was superior to published results for most of conventional magnesium alloys. Although nanostructured materials usually have high strength but poor ductility, the material in this study exhibited high strength and a true compressive strain of 0.036

The lubricating properties of spark plasma sintered (SPS) Ti3SiC2 MAX phase compound and composite

MAX phase composites Ti3SiC2–TiCx and Ti3SiC2–(TiCx + TiC) were synthesized and consolidated via a powder metallurgy spark plasma sintering (SPS) technique. The bulk compositions and microstructural evolution of the resulting SPS discs were analyzed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM) paired with an energy-dispersive spectroscopy (EDS) system. The tribological behavior of the synthesized discs was investigated at room temperature under dry sliding conditions using an Al2O3 ball by employing a ball-on-disc tribometer configuration. Postmortem analyses of the worn surfaces showed that the Ti3SiC2 MAX phase exhibited intrinsic self-lubricating behavior due to the evolution of easily sheared graphitic carbon at the sliding surface. The addition of stoichiometric TiC delayed the oxidation kinetics of Ti3SiC2, which favors the evolution of graphitic carbon in lieu of rutile and oxycarbide films. Thus, this work shows comprehensively the existence of an intrinsic self-lubricating behavior of Ti3SiC2 and the important role of secondary-phase TiC in the Ti3SiC2 matrix in its tribological behavior. The wear mechanisms in both composites are dominated by tribo-oxidation triggered by frictional heating. This is then followed by deformation-induced wear upon friction transition

Ramification of thermal expansion mismatch and phase transformation in TiC-particulate/SiC-matrix ceramic composite

This article presents a microstructural study on the role of incipient residual stress relaxation in TiC-particulate/SiC-matrix ceramic composite toughened by thermal expansion mismatch and phase transformation toughening. Exhaustive microstructural studies was undertaken using scanning electron microscopy and transmission electron microscopy following a wear test. It was found that the superposition of hydrostatic tensile stress induced at the surface following the sliding contact on the inherent residual stresses locked in the composite led to a relaxation and/or reduction in the residual stresses. Stress relaxation presented a wider implication for the tribological properties of this ceramic matrix composite (CMC) in the form of a grain-scale rippling microstructural phenomena

Phase stability of a standing-wave free-electron laser

The standing-wave free-electron laser (FEL) differs from a conventional linear-wiggler microwave FEL in using irises along the wiggler to form a series of standing-wave cavities and in reaccelerating the beam between cavities to maintain the average energy. The device has been proposed for use in a two-beam accelerator because microwave power can be extracted more effectively than from a traveling-wave FEL. A simplified numerical simulation indicates that, with appropriate prebunching, the standing-wave FEL can produce an output signal that is effectively the same in all cavities. However, changes in the beam energy of less than 1% are found to introduce unacceptably large fluctuations of signal phase along the device. Analytic calculations and single-particle simulations are used here to show that the phase fluctuations result from beam synchrotron motion in the initial signal field, and an approximate analytic expression for the signal phase is derived. Numerical simulations are used to illustrate the dependence of phase fluctuations on the beam prebunching, the beam-current axial profile, and the initial signal amplitude

Enhancing ductility and strength of nanostructured Mg alloy by in-situ powder casting during spark plasma sintering

Due to internal processing defects, bulk nanostructured Mg alloys have high strength but extremely poor ductility. A novel and facile process was designed and in-situ powder casting was initiated during spark plasma sintering. This process significantly reduced processing induced defects, enhanced inter-particle bonding and introduced significant precipitation without extra ageing treatment, leading to improvement of the compressive strength and ductility. The compressive strain of bulk sample consisting of pure cryomilled powder was 3.6% with an ultimate strength of 500 MPa, while cryomilled powder mixed with eutectic Mg-Zn alloy powder obtained a compressive strain of 6.6% and ultimate strength of 506 MPa. The ductility of the sample with mixed powder was increased by 83% without any sacrifice of strength compared to the sample consisting of only pure cryomilled powder

Improving the oscillating wear response of cold sprayed Ti-6Al-4V coatings through a heat treatment

Cold spray (CS) coating technique is being studied as a potential solution for repairing aircraft Ti-6Al-4V components. This work is focused on the restoration of damaged components due to wear induced by vibrations. It is known that Ti-6Al-4V CS deposition shows difficulties to obtain non-porous coatings due to the high strength of this material, that is detrimental for wear resistance. In this sense, performing a post-heat treatment leads to lower porosity CS Ti-6Al-4V coatings and improves their mechanical properties, and thus, a better tribological behaviour is also expected. Therefore, the objective of this study was to determine the effect of a post-heat treatment on the wear resistance of Ti-6Al-4V coatings deposited by the CS technique. Ti-6Al-4V CS coatings were used, which have been sprayed with nitrogen as process gas at a temperature of 1100 °C and a pressure of 50 bar. The coatings were subjected to a solution heat treatment followed by a precipitation heat treatment. Oscillating and unidirectional sliding wear experiments were conducted on the coatings at room temperature and at 450 °C. A pin on disc configuration was used with a bearing steel counterbody. The results were compared to those obtained on the substrate (which represents the material to be repaired) and on the as-sprayed coating, which were derived from a previous work. The heat treated coating presented improved wear behaviour as compared to the substrate as well as to the as-sprayed coating, particularly during the high temperature tests. Wear at high temperature was dominated by material transference from the counterbody to the Ti-6Al-4V coating

Spinel–rock salt transformation in LiCoMnO4−δ

The transformation on heating LiCoMnO4, with a spinel structure, to LiCoMnO3, with a cation-disordered rock salt structure, accompanied by loss of 25% of the oxygen, has been followed using a combination of diffraction, microscopy and spectroscopy techniques. The transformation does not proceed by a topotactic mechanism, even though the spinel and rock salt phases have a similar, cubic close-packed oxygen sublattice. Instead, the transformation passes through two stages involving, first, precipitation of Li2MnO3, leaving behind a Li-deficient, Co-rich non-stoichiometric spinel and, second, rehomogenization of the two-phase assemblage, accompanied by additional oxygen loss, to give the homogeneous rock salt final product; a combination of electron energy loss spectroscopy and X-ray absorption near edge structure analyses showed oxidation states of Co2+ and Mn3+ in LiCoMnO3. Subsolidus phase diagram determination of the Li2O-CoOx-MnOy system has established the compositional extent of spinel solid solutions at approximately 500°C