Molybdenum (Mo) alloys are essential for applications requiring outstanding mechanical properties at high temperatures across various industrial sectors. Understanding and predicting the creep properties of Mo alloys is crucial for service safety and the design of new materials. This study introduces a physics-based crystallographic creep model dedicated to the characteristic hierarchical microstructure of Mo–La2O3 alloys. By sourcing most parameters from existing literature and calibrating others within recommended ranges, the model efficiently predicts creep behavior beyond its initial calibration scope. Through the integration of microstructure descriptors, we systematically explored the impact of different microstructural features on creep behavior and identified underlying mechanisms. This analysis yielded two pivotal concepts: the minimum acceptable grain size and the necessary nanoparticle number density. These metrics, readily obtainable from the model, quantify the requisite grain size and nanoparticle content to achieve the target steady-state creep rates for operational demands, thus providing essential insights for the creep condition-oriented design of Mo–La2O3 alloys. The model is also expected to be adaptable for developing other Mo alloys reinforced by second phase particles, aimed at achieving desired creep properties under specified conditions, assuming that relevant parameters are accessible through literature or lower-scale simulations.

Precipitation at grain boundaries is typically not regarded as an efficient method for strengthening materials since it can induce grain boundary embrittlement, which detrimentally affects ductility. In this research, we developed a multi-principal element alloy (MPEA) with the composition Cr30Co30Ni30Al5Ti5 (at.%), incorporating both intragranular and intergranular nanoprecipitates. Utilizing multiscale, three-dimensional, and in-situ electron microscopy techniques, coupled with computational simulations, we established that intergranular nanoprecipitation in this material plays a crucial role in enhancing strength and promoting dislocation plasticity. The structure of intergranular nanoprecipitation comprises multiple phases with varying composition and structure. Despite the diversity, the crystal planes conducive to the easy glide of dislocations are well-matched, allowing for the sustained continuity of dislocation slipping across different phase structures. Simultaneously, this structure generates an undulated stress field near grain boundaries, amplifying the strengthening effect and facilitating multiple slip and cross-slip during deformation. Consequently, it promotes the proliferation and storage of dislocations. As a result, our material exhibits a yield strength of approximately 1010 MPa and an ultimate tensile strength of around 1500 MPa, accompanied by a significant fracture elongation of 41%. Our findings illuminate the potential for harnessing intergranular nanoprecipitation to optimize the strength-ductility trade-off in MPEAs, emphasizing the strategy of leveraging complex compositions for the design of sophisticated functional microstructures.

Hexagonal Mo2C particles with semi-coherent phase interfaces were introduced within the Mo matrix via in-situ reactions. The strong hindering effect of Mo2C particles on dislocations enables the composite to reach an ultimate compressive strength of 262 MPa at 1100 degrees C, which is 1.4 times that of pure Mo. Moreover, these Mo2C particles not only undergo plastic deformation by the activation of pyramidal dislocations to alleviate stress concentration at phase interfaces, but also promote dynamic recovery and recrystallization in Mo matrix, endowing the composite with excellent high-temperature plasticity.

It is now well established that, upon decreasing system sizes down to a few m or below, the nature of plasticity of metallic materials is changing. Two important features of this small-sizes plasticity are two size effects, which can be summed up as “smaller is stronger” and “smaller is wilder”, this last observation meaning that the jerkiness of plastic deformation becomes prominent at small enough system sizes. In FCC and HCP materials, this is now rather well understood within the framework of obstacle-controlled plasticity, from the key role of a scaling ratio between the system size L and an internal scale l mainly dictated by dislocation patterning in pure materials, or by the nature of extrinsic disorder in alloys. The situation is more complex in BCC materials, for which screw dislocation motion becomes lattice-controlled, i.e. is thermally activated, below a transition temperature T_a . Therefore, in small-sized BCC systems, temperature, size and strain-rate effects combine to give rise to a complex landscape. We show, from an analysis of the literature as well as micropillar compression tests on Molybdenum performed with different sample sizes, under different temperatures and different applied strain-rates, that (i) near or above T_a , the plasticity of pure BCC metals is athermal and obstacle-controlled, much like at bulk scales, therefore mimicking that of pure FCC metals; (ii) below T_a and for sample sizes larger than ∼ 1 m, BCC plasticity becomes lattice-controlled, this damping dislocation avalanches and thus reducing wildness; but (iii) for very small systems, still below T_a , the role of screw dislocations on plasticity vanishes, i.e. is no more lattice-controlled, opening again the door for wild plastic fluctuations and jerkiness.

Molybdenum and its alloys are known for their superior strength among body-centered cubic materials. However, their widespread application is hindered by a significant decrease in ductility at lower temperatures. In this study, we demonstrate the achievement of exceptional ductility in a Mo alloy containing rare-earth La2O3 nanoparticles through rotary-swaging, a rarity in Mo-based materials. Our analysis reveals that the large ductility originates from substantial variations in the electronic density of states, a characteristic intrinsic to rare-earth elements. This characteristic can accelerate the generation of oxygen vacancies, facilitating the amorphization of the oxide-matrix interface. This process promotes vacancy absorption and modification of dislocation configurations. Furthermore, by inducing irregular shapes in the La2O3 nanoparticles through rotary-swaging, incoming dislocations interact with them, creating multiple dislocation sources near the interface. These dislocation sources act as potent initiators at even reduced temperatures, fostering diverse dislocation types and intricate networks, ultimately enhancing dislocation plasticity.

Molybdenum and its alloys typically exhibit superior strength compared with other body centered cubic materials, while the pronounced decrease in ductility at lower temperatures often imped their widespread applications. In this study, we demonstrate the attainment of extraordinary ductility by utilizing rotary-swaging to process a Mo alloy containing rare earth La2O3 nanoparticles —a rarity within the domain of Mo-based materials. Our atomic structure analysis elucidates that the exceptionally large ductility is originated from the substantial variations in electronic density of states, a characteristic intrinsic to rare-earth elements, which can expedite the generation of oxygen vacancies. This, in turn, facilitates the amorphization of the oxide-matrix interface under precise processing control, which then exhibits a propensity for vacancy absorption and modification of dislocation configurations. Furthermore, by imparting irregular shapes to the La2O3 nanoparticles through rotary-swaging, we succeeded in engendering multiple dislocation sources in the vicinity of the interface as incoming dislocations interact with these La2O3 nanoparticles. The newly generated dislocation sources persistently operate as potent dislocation initiators under applied stress even at reduced temperatures, resulting in the formation of diverse dislocation types and intricate dislocation networks and ultimately leading to superior dislocation plasticity.

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 & DEG;C, 400 & DEG;C, 500 & DEG;C, and 600 & DEG;C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be & GE; 400 & DEG;C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 & DEG; to 400 & DEG;C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 & DEG;C (free of DSA), while complex dislocation structure in the samples ratcheted at & GE; 400 & DEG;C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 & DEG;C to 600 & DEG;C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior.

The intermittent plasticity of pure Mo microcrystals with diameters from 500 to 3500 nm was studied using micro-pillar compression methodology at temperatures ranging from 25 °C to 200 °C, in order to explore its dependence on external size and temperature. The technological background is the potential value of BCC metals in the manufacture of high-temperature components in modern fields requiring miniaturization, such as microelectronics. We analyzed plastic fluctuations in terms of wildness, avalanche size distribution, and burst peak velocity. This reveals coupled size and temperature effects on a mild-to-wild transition: the transition temperature decreases with decreasing sample diameter, while the transition diameter increases with rising temperature. In addition, strain-rate sensitivity tests were conducted, implying that mild plasticity is associated to screw dislocation motions controlled by thermally activated nucleation of kink-pairs, whereas wild plasticity is nearly athermal. This experimental observation is interpreted by a damping effect of the thermally activated motion of screw dislocations on the propagation of avalanches. From this, we propose a controlling parameter based on a simple dislocation source model, which unifies the coupled sample size and temperature effects on the mild-to-wild transition, and which could be of practical significance for microscale applications.

In 2011, a serious nuclear accident occurred in Fukushima, Japan, and one of the major causes was the destruction of the cladding material mainly made of Zr alloy due to a violent chemical reaction with high-temperature water vapor. Since then, scholars in various countries have been trying to find a better nuclear fuel cladding material. Because of its high temperature strength, ferritic/martensitic(F/M) steel is used in thermal power units. Over the past hundred years, the high temperature performance of ferritic/martensitic steel has developed simultaneously with the operating temperature of thermal power system. F/M steel is widely used in nuclear power system as well as thermal power, The fatigue life of nuclear fuel cladding material is an important parameter to study its service life, due to the special shape of the cladding material, especially for the pipe with a thickness of less than 0.5mm, it is difficult to directly measure its mechanical properties, this paper through special experimental materials and fixtures F/M pipe steel at room temperature and high temperature tensile test, as well as low cycle fatigue performance test, and its fracture port, surface crack analysis. Through the analysis and processing of experimental data, typical stress-cycle life curves, total strain amplitude-life curves, elastic strain amplitude-life curves, hysteresis curves with different strain amplitudes, and fatigue life prediction curves were obtained.

Swaged Mo-xLa2O3 alloys (x = 0, 0.3, and 0.6 wt.%) were subjected to different annealing temperatures (Ta), in order to demonstrate the Ta-sensitive La2O3 effect on microstructural evolution and tensile ductility. Although coarse La2O3 particles were detrimental to ductility, nanosized La2O3 particles were found to increase the ductility by highly stabilizing the deformation-induced hierarchical microstructures of lamellar elongated grains with low-angle grain boundaries inside. A competition between the two opposite effects derived from the La2O3 addition led to an unexpected x-dependence of ductility upon different Ta.

Sc solute clusters with a high number density were produced in an Al-0.3 wt.%Sc alloy when aged at250°C,while fine Al 3 Sc precipitates were predominantly formed in the same alloy aged at 300°C.The alloy strengthened by Sc solute clusters displayed higher yield strength and simultaneously greater ductility than its counterpart strengthened by Al 3 Sc precipitates.This clearly demonstrates a superior strengthductility synergy promoted by the Sc solute clusters in Al-Sc alloys.The effects of Al 3 Sc precipitates and Sc solute clusters on ductility were discussed in comparison by using a micromechanics fracture model.Since the Sc clusters were stabilized at 250°C,the Al-Sc alloys strengthened by Sc solute clusters should find extensive application fields within a wide temperature range,due to their high temperature resistance.

Al-Zn-Mg series aluminum alloys have important applications in aerospace, transportation etc, for their excellent properties of low density and high strength. Further optimizing the microstructure to obtain higher mechanical properties and better corrosion resistance is the development direction of Al-Zn-Mg alloys. Microalloying has become an important means of improving the properties of aluminum alloys, owing to the limited space for alloy composition optimization and heat treatment processes improvement. The effects of microalloying elements on the mechanical properties, hot deformation behavior and corrosion resistance of Al-Zn-Mg alloys were briefly summarized, focusing on the different effects of the second phase particles formed by microalloying elements in different process stages, such as effectively refine grains and strongly hinder the movement of dislocations. The effects of pin grain boundaries, sub-grain boundaries and inhibiting recrystallization during hot deformation were discussed. The internal mechanism of improving the corrosion resistance of the alloy was explained. In addition, the further research direction of microalloying of Al-Zn-Mg aluminum alloy was prospected, understanding the interaction mechanism of microalloying elements and dual alloying-microalloying elements to realize the precise and accurate addition of microalloying elements will be one of the main research contents in the future. Clarifying the regulation effect of microalloying elements on deformation structures and dislocation configurations during hot working will provide a reference for improving the corrosion resistance of alloys.

Low-temperature decomposition of supersaturated solid solution into unfavorable intergranular precipitates is a long-standing bottleneck limiting the practical applications of nanograined aluminum alloys that are prepared by severe plastic deformation. Minimizing the vacancy concentration is generally regarded as an effective approach in suppressing the decomposition process. Here we report a counterintuitive strategy to stabilize supersaturated solid solution in nanograined Al-Cu alloys via high-density vacancies in combination with Sc microalloying. By generating a two orders of magnitude higher concentration of vacancies bonded in strong (Cu, Sc, vacancy)-rich atomic complexes, a high thermal stability is achieved in an Al-Cu-Sc alloy that precipitation is nearly suppressed up to ~230 °C. The solute-vacancy complexes also enable the nanograined Al-Cu alloys with higher strength, greater strain hardening capability and ductility. These findings provide perspectives towards the great potentials of solute-vacancy interaction and the development of nanograined alloys with high stability and well-performed mechanical properties.

Although the evolution of dislocation substructure during ratcheting deformation has been previously studied in 316LN austenitic steel, studies on ratcheting-induced twinning and de-twinning behaviors are very scarce. In this paper, microstructural evolutions, especially the twinning/de-twinning behaviors, were systematically investigated in a 316LN austenitic steel under ratcheting cycling. Different peak stresses (320, 400, 450, and 500 MPa) were deliberately designed to demonstrate a potential correlation between the twinning/de-twinning and the dislocation pattern. The experimental results showed that, at the low peak stress of 320 MPa, annealing twins were quite stable and the dislocation pattern mainly consisted of planar dislocation arrangement. Raising the peak stress to 400 MPa, most of the annealing twins were gradually de-twinned and the dislocation pattern displayed a different feature prevailed by dislocation veins, walls and cells. As the peak stress reaches 450 MPa and in particular up to 500 MPa, the annealing twins get quickly de-twinned, while stacking faults and deformation twins were induced besides the dislocations. The critical stress to trigger deformation twins in the 316LN austenitic steel was evaluated to be around 500 MPa, which is in broad agreement with the experimental results. This indicates that the nucleation of deformation twins under ratcheting cycles should be controlled by the peak stress rather than the accumulation strain. An additional cycling hardening stage was observed in the ratcheting cycling response under the peak stress of 450 and 500 MPa, which should be closely related to the formation of stacking faults and deformation twins. Moreover, the stress-dependent de-twinning of annealing twins were discussed in terms of dislocation interaction with annealing twins.

The corrosion behavior of pure Molybdenum (Mo) and Mo doped with 0.3 wt.% La2O3 was investigated in aerated 3.5 wt.% NaCl at 25 degrees C. Compared with the pure Mo, the doped Mo exhibits significantly increased corrosion resistance, with a smaller current density during anodic polarization and a 2 similar to 3 times larger charge transfer resistance. Such an enhancement originates from the refinement of grains and purification of the grain boundary due to the addition of La2O3, which facilitates the formation of a compact and protective oxide film. Our results provide a recipe to improve the corrosion resistance of Mo alloys.

The near-eutectic Al-12 wt.% Ce alloy has been alloyed with 0.4 wt.% Sc to improve its creep resistance. A bidirectional influence between Sc and Ce has been identified: (i) the Ce element suppresses the Sc partition between the liquid and solid phases during solidification, as implied by the reduction of Sc microsegregation in the as-cast ingot; (ii) the Sc atoms migrate towards the Al11Ce3 interface to form Al3Sc thin layer or particles upon aging. The former effect promotes the spatial uniformity of Al3Sc precipitate distribution in the Al matrix and the latter one introduces additional lattice misfit strain to the Al11Ce3/Al-alpha phase boundary inhibiting matrix dislocations to climb over the Al11Ce3 lamellae. Both effects can improve the creep resistance of the aged Al-12Ce-0.4Sc alloy showing a high tensile stress threshold for dislocation creep of similar to 60 MPa at 300 degrees C. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Al-2.5 wt% Cu alloys with different Sc additions (0, 0.1, 0.3, 0.5 wt%) are studied in comparison to reveal the Sc microalloying effect on precipitation, room temperature mechanical properties, and creep resistance. The results show that the Sc addition into the Al-Cu alloys can effectively promote the precipitation of theta'-Al2Cu, reducing the size and narrow the size distribution. However, the Sc-dependences of mechanical properties at room and at high temperatures are much different. Although the 0.3 wt% Sc addition results in the densest homogeneous theta'-Al2Cu precipitation and hence the highest room temperature strength, the 0.5 wt% addition leads to the most improved creep resistance at 300 degrees C that is derived from a nanostructural Sc-based hierarchy, i.e., Al3Sc dispersoid/heterogeneous theta'-Al2Cu precipitate units, homogeneous theta'-Al2Cu precipitates, strongest Sc segregation at theta'/matrix interfaces, and Sc clusters. The nanostructural hierarchy with thermally resistant nanostructural features at different length scales provides a new way to develop advanced Al alloys with excellent high-temperature stability and mechanical properties. The strengthening mechanisms at room and high temperatures are respectively discussed. (c) 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Envelope tracking (ET) and envelope elimination and restoration (EER) are techniques that have gained in importance in the last decade in order to obtain highly efficient radio frequency power amplifier that transmits signals with high peak-to-average power ratio. In this study, a multilevel multiphase buck converter is presented as a solution for the envelope amplifier used in ET and EER. The presented multiphase buck converter generates multilevel voltage using “node” duty cycles and nonlinear control. In this way, the multilevel is implemented using only one simple power stage. However, the complexity of the multilevel converter implementation has been shifted from complicated power topologies to complicated digital control. Detailed discussion regarding the influence of the design parameters (switching frequency, output filter, and time resolution of the digital control) on the performance of the proposed envelope amplifier is presented. The design of the output filter is conducted fulfilling the constraints of the envelope slew rate and minimum driver pulse that can be reproduced. In the cases when these two constraints cannot be fulfilled, they may be relieved by the modified control that is presented and experimentally validated. Finally, in order to validate the concept, a prototype has been designed and integrated with a nonlinear class F amplifier. Efficiency measurements showed that by employing EER, it is possible to save up to 15% of power losses, comparing to the case when it is supplied by a constant voltage. Additionally, adjacent channel power ratio has been measured. The obtained results showed the value higher than 30 dB for signals up to 5 MHz of bandwidth, without using a predistortion technique.

The combination of minimum time control and multiphase converter is a favorable option for dc-dc converters in applications where output voltage variation is required, such as RF amplifiers and dynamic voltage scaling in microprocessors, due to their advantage of fast dynamic response. In this paper, an improved minimum time control approach for multiphase buck converter that is based on charge balance technique, aiming at fast output voltage transition is presented. Compared with the traditional method, the proposed control takes into account the phase delay and current ripple in each phase. Therefore, by investigating the behavior of multiphase converter during voltage transition, it resolves the problem of current unbalance after the transient, which can lead to long settling time of the output voltage. The restriction of this control is that the output voltage that the converter can provide is related to the number of the phases, because only the duty cycles at which the multiphase converter has total ripple cancellation are used in this approach. The model of the proposed control is introduced, and the design constraints of the buck converter's filter for this control are discussed. In order to prove the concept, a four-phase buck converter is implemented and the experimental results that validate the proposed control method are presented. The application of this control to RF envelope tracking is also presented in this paper.

Citation Cheng, Pengming (2014). Wide bandwidth envelope trackers with reduced switching frequency for RF power amplifier. Thesis (Doctoral), E.T.S.I. Industriales (UPM). https://doi.org/10.20868/UPM.thesis.30885.