Materials Characterization最新文献

Three micro mechanical testing geometries including micro-pillars, micro-cantilevers and micro-shear specimen, were employed to measure the twinning stress of the Cantor high entropy alloy. This study presents a comparative analysis of the challenges in specimen preparation, effectiveness in activating deformation twinning and complexity of post mortem analyses associated with each geometry. Our findings demonstrate that micro-shear testing represents the optimal approach for twinning studies, offering direct activation of the twinning system with the superior reproducibility compared to other geometries. Micro-pillar compression is also suitable for twinning investigations, providing statistical analysis of mechanical data with relatively faster sample preparation. However, micro-cantilever testing presents challenges with a lower success rate in twinning activation and the stress gradients, which complicates quantitative interpretation, resulting in the least favorable geometry for twinning studies.

The microstructural evolution of NiTi shape memory alloy (SMA) with Cu interlayer joints fabricated by ultrasonic spot welding (USW) was thoroughly investigated using transmission electron microscopy (TEM) and Transmission Kikuchi Diffraction (TKD). Furthermore, an analysis was conducted on how these microstructural changes influence the behavior of superelasticity. Significant plastic deformation and element diffusion occur during the USW process, leading to the formation of dislocation entanglements, Ti(Ni_0.5Cu_0.5), Ni_3-xCu_xTi phases, and NiTiCu₂O₄ nano-oxide in the vicinity of the interface. According to high-resolution transmission electron microscopy (HRTEM) results, there exists a crystallographic orientation relationship (ORs) between Cu and Ti(Ni_0.5Cu_0.5): [011]Cu//[100]Ti(Ni_0.5Cu_0.5). Additionally, semi-coherent interfaces form at Cu/Ti(Ni_0.5Cu_0.5) and NiTiCu₂O₄/R-phase interfaces. However, during cyclic tensile testing, the deterioration of superelastic response in NiTi joints is primarily attributed to the hindrance caused by the USW-induced room-temperature stabilized martensite, dislocation entanglements, and NiTiCu₂O₄ during martensite reverse transformation.

The microstructure and mechanical properties of Cu-12.5Ni-5Sn-xY alloys with different Y addition amounts prepared using spark plasma sintering (SPS) were investigated, and the relationship was discussed. Results indicate that the addition of Y could significantly improve the mechanical properties. When 0.4 wt% Y is added, the maximum hardness and yield strength are approximately 329.5 HB and 691.9 MPa, which are approximately 31 % and 48 % higher than that without Y, respectively. The addition of appropriate amount of Y can significantly refine the grains, promote the transformation of γ-phase in the α + γ coexisting structure from lamellar-shape to needlelike-shape, and significantly inhibit the nucleation and growth of discontinuous precipitation (DP). Especially, when 0.4 wt% Y is added, the nano-scale NiY₃ particles precipitated at the grain boundary can hinder the migration of the grain boundary and occupy the nucleation sites of the γ-phases. In general, grain refinement, nano-precipitation strengthening and inhibition of DP are the main reasons for the improvement of mechanical properties.

In this paper, the tensile properties and microstructural responses of Al-containing Fe-Mn-C high manganese austenitic steel at room temperature down to ultra-cryogenic temperatures were investigated by uniaxial tensile tests and microstructural characterization. The addition of 3.3 wt% Al effectively increased the stacking fault energy (SFE) and remained it within the range of the twin-dominated deformation mechanism, even at the ultra-cryogenic temperature of 4.2 K. The studied steel demonstrates excellent tensile properties at 4.2 K, the yield strength and ultimate tensile strength reached 1248 MPa and 1620 MPa, respectively, while preserving a satisfactory total elongation of 32.1 %. The contributions of different strengthening mechanisms to yield strength and strain hardening were estimated. The lattice friction stress to yield strength significantly increased at cryogenic temperatures, from 211 MPa at 298 K to 995 MPa at 4.2 K. While dislocation strengthening remained the primary source of strain hardening across all testing temperatures, the contribution of planar faults increases from 255 MPa at 298 K to 335 MPa at 4.2 K. These results offer valuable insights for designing high manganese steel for ultra-cryogenic temperature environments and enriching the understanding of its deformation mechanism at cryogenic temperature.

High entropy carbide ceramics showed outstanding high-temperature oxidation resistance and high-temperature mechanical property, showing a great application potential in nuclear reactor cladding materials. This work proposed a brazing technology to realize the connection of (TiZrTaNbCr)C/Zr-4 brazed joint by Ni-based filler, which exhibited an outstanding high-temperature oxidation resistance and mechanical property. The interface microstructure and phase compositions of the (TiZrTaNbCr)C/Zr-4 joint were investigated. The brazing seam was primarily composed of Zr(s,s), Zr₂Ni and ZrCr₂. The ZrC and Cr₂₃C₆ interface reaction layer and the diffusion of Ni elements ensured the interface bonding. Different to previous researches, the second phase of Cr₂₃C₆ was observed in the ZrC reaction layer, improving the strength of interface reaction layer. As a result, a highest shear strength of 105 MPa was achieved at the (TiZrTaNbCr)C/Zr-4 brazed joint. Furthermore, the high temperature shear strength at 800 °C of (TiZrTaNbCr)C/Zr-4 joint was 91 MPa, maintaining 87 % of the room-temperature shear strength. Compared to Zr-4 alloy, the (TiZrTaNbCr)C and BNi-2 filler showed an outstanding oxidation resistance, and the shear strength of the joint was maintained 67 % after oxidized at 900 °C for 8 h.

The γ' precipitate coarsening kinetics in additively manufactured Ni-based superalloys needs to be investigated to predict the microstructure stability at high temperature. Therefore, we investigate the γ' precipitate coarsening kinetics in a laser powder bed fusion IN738LC Ni-based superalloy at 900 °C. We find that the initial bimodal γ' precipitate size distribution at 24 h of ageing time changes to unimodal distribution at 168 h of ageing time at 900 °C. We perform the mean-field modelling to understand the transition physics using γ-γ' composition and γ' volume fraction obtained using atom probe tomography and find that the modelling results are consistent with the experimental observation. Based on the modelling, we found that the γ' volume fraction is an important parameter in the mean-field modelling, which is slightly higher than the equilibrium value due to the non-equilibrium composition of γ-γ' phases in the additively manufactured superalloy. Furthermore, the γ/γ' interfacial energy is predicted to increase from bimodal to unimodal distribution due to an increase in Ti/Al ratio in γ' precipitate, which increases the γ' precipitate coarsening kinetics. It is also found that the γ/γ' interfacial energy and z factor have more influence on the γ' precipitate coarsening kinetics than the mobility parameter in additively manufactured IN738LC. In conclusion, the γ' precipitate coarsening kinetics in additively manufactured Ni-based superalloy is predicted to be slightly faster than the alloy with equilibrium composition of phases.

Characterizing graphite shape in cast irons is of importance for control of cast components. This is still done based on standard charts and has been more and more supported by quantitative image analysis during the last decades. Despite the significant progresses made by image analysis techniques and software packages, the results of quantitative image analysis are still moderately satisfactory, and the question raised of the capabilities of machine learning to supplement this approach.

This work first presents a review on graphite shape analysis by standard image analysis and also lists the very few works that have used machine learning approach. Then a series of challenging images in which both lamellar, compacted and spheroidal graphite coexist was submitted to standard image analysis and to a convolutional neural network, a deep learning model. The comparison of the two methods shows that machine learning produced very encouraging results when compared to standard image analysis, in particular for what concerns the presence of lamellar graphite.

The cracking behaviors of VCoNi medium-entropy alloys with and without short-range order (SRO) structures under different durations of hydrogen charging were investigated. The findings demonstrate that the presence of short-range order in the VCoNi alloy effectively traps hydrogen within the grain interior, thereby mitigating hydrogen segregation at grain boundaries. Consequently, the SRO structure within the VCoNi alloy prolongs the incubation period for hydrogen-induced grain boundary cracking and results in a higher density of grain interior cracks after 12 h of hydrogen charging, indicating that SRO impedes the diffusion of hydrogen into the alloy's interior compared to the alloy lacking SRO. Grain boundary cracking is a consequence of stress concentration and hydrogen segregation at grain boundaries. The initiation of cracks within grains is primarily ascribed to the generation of nanovoids at the intersection of intersecting slip bands.

This article proposes a method for achieving the connection of SiC and 7A52 aluminum alloy using femtosecond laser surface modification of SiC ceramics. The influence of laser power on the morphology, chemical composition, and wettability of the SiC ceramic surface and the liquid-solid composite bonding mechanism of SiC and 7A52 after femtosecond laser modification was investigated. After femtosecond laser modification, the ceramic structure was constructed with a periodic groove structure of appropriate size, which increased the bonding area between the solder and the ceramic surface. Additionally, thermal decomposition occurred on the SiC surface, resulting in a recasting layer, which improved the wetting effect of the solder on the ceramic surface and enhanced the diffusion of Al and Mg elements to the joint interface. The existence of periodic groove structures changes the fracture path of the joint and improves the strength of the liquid-solid composite joint, which is approximately 45.9 % higher than the original joint strength.

During the creep deformation, damage mainly occurs in the tertiary creep stage. Thus, creep life is essentially controlled by the tertiary stage. To find out the impact of the dislocation microstructure on the creep rate in this stage, and in particular its thickness-dependence, creep tests of Ni-based single crystal superalloy are carried out at 1100 °C/130 MPa with sheet samples of varying thickness. The creep rate in tertiary creep clearly increases and rises faster in thinner sample sheets. The observed microstructures demonstrate that the γ/γ’ phase distribution shows no topological inversion. It is found that the variety of dislocations in γ’ rafts decrease with decreasing sample thickness. The γ’ rafts in thin samples are sheared by only two types of superdislocation involving individual screw superdislocations and double Lomer-Cottrell structures, while the γ’ rafts in thick samples are sheared by three types of superdislocations, again individual screw superdislocations and double Lomer-Cottrell structures, but also Kear-Wilsdorf structures. Also the density of disclocations varies significantly. A higher density of individual screw superdislocations and of mobile dislocation pairs, but lower density of locked non-planar structures promotes the creep of thinner samples.