Materials Characterization最新文献

To meet the harsh thermal shock (−50–250 °C) requirements of power device packaging, a novel sintered Ag-based die attach material based on the sustained release effect of Al was proposed. In this study, Al particles with natural core-shell structure covered with Al₂O₃ layer were selected as the ideal doping-phase, and micron Al particles doped sintered Ag composite paste (AgAl) was prepared. First, the microstructural evolution of sintered AgAl joints during thermal shock was studied. The results showed that Al particles effectively suppressed the cracks generation, which attributed to the sustained release effect of Al element. To clarify sustained release mechanism of Al, the interfaces evolution between Ag and Al particles was investigated. In the initial stage, Al was covered by the Al₂O₃ layer, preventing inter-diffusion between Ag and Al. As the thermal shock proceeds, the Al₂O₃ film was fractured, achieving direct contact between metal Al and Ag. Diffusion thus takes place. For the sintered Ag5Al joint, the shear strength was 17.3 MPa after 1000 cycles, which is 1.57 times that of pure sintered Ag joint. These results indicate the successful development of low-cost, high-reliability die attachment materials for use in harsh thermal shock environments.

Aluminum-quasicrystal composites are attractive candidate alloys for additive manufacturing, but the cost of evaluating such systems can be prohibitive. Recently, surface laser glazing studies have been used to show that desirable composite microstructures can be formed in an Al₈₅Cu₆Fe₃Cr₆ alloy under a wide range of laser processing parameters. Here the thermal stability of these laser track microstructures has been evaluated by performing in situ scanning transmission electron microscopy heating experiments on specimens produced by focused ion beam milling from the center of the tracks. The initial track microstructures exhibited a uniform phase mixture with 70 % by volume of I-phase quasicrystalline dispersoids in a FCC Al matrix with a film of Al₂Cu at the interface. Preliminary ramped heating experiments were used to identify the temperatures at which transformations occurred and then isothermal experiments on fresh samples were used to monitor the progress of these processes. For temperatures of up to 450 °C the only significant changes were a redistribution and coarsening of the Al₂Cu phase with a gradual increase in the Fe content of the phase. At higher temperatures, the I-phase decomposed to form a mixture of crystalline ω-Al₇Cu₂Fe and μ-Al₄Cr phases. The I-phase decomposition proceeded by ejection of Cu and Fe with the remaining Cr-rich regions transforming to the Al₄Cr approximant phase. The implications of these phenomena for use of the alloy in additive manufacturing are discussed.

The effect of heat treatment temperature on the microstructure and mechanical properties of Y₂O₃/316 L composites prepared by laser powder bed fusion was studied. The cellular substructures formed in the as-built samples remained stable when heat-treated at 1000 °C and disappeared as the temperature increased to 1100 °C. Compared with the as-built sample, a larger number of fine particles precipitated in the sample at 1000 °C, and the particles were significantly coarsened at 1100 °C. Under the stress-induced grain boundary migration mechanism, the grains undergo coarsening. The samples heat-treated at 1000 °C had the best combination of strength and ductility compared to the other samples. The high strength of the samples can be attributed to the retention of the cellular substructure and the enhancement of the Orowan strengthening mechanism. The high ductility of the sample is brought about by the formation of the twinned structure. The heat treatment developed in this work for the additive manufacturing of steel matrix composites to tailor its microstructure and mechanical properties for its various applications is critical.

The quenching and partitioning (QP) process has proven to be an effective technique for stabilizing retained austenite (RA). The N element partitioning process has replaced the traditional C element partitioning method, resulting in improved stability of RA in super martensitic stainless steel. To enhance these benefits, an innovative combination of intercritical annealing (A) and QP has been developed, known as the QAP process. This technique utilizes interstitial N atoms and substitutional Ni, Mn atoms to co-stabilize RA, leading to excellent comprehensive mechanical properties. The findings indicate that the QAP samples display a yield strength (YS) that is 75–121 MPa higher than the QP sample. At intercritical annealing temperatures of 500 °C and 550 °C, the QAP steel demonstrates a product of strength and elongation (PSE) of 36.61 GP·% and 31.51 GP·%, respectively, surpassing the QP steel (26.52 GP·%). Furthermore, the presence of secondary martensite increases with higher intercritical annealing temperatures in QAP samples. However, excessive secondary martensite significantly diminishes toughness.

Cycling lifespan/stability is one of the most important limitations for multivalent transition metal sulfides (TMSs) used in secondary batteries. However, the chaos reaction of TMSs is an important obstacle that restricts their electrochemical stability in all-enclosed batteries. Herein, taking FeS anode and lithium-ion batteries as the example, the phase-lithiation correlation is studied through TEM and DFT analysis. It's found that the pristine hexagonal FeS converts to tetragonal FeS phase accompanied by severe and rapid capacity decay. However, it converts to orthorhombic FeS with enhanced capacity and nice lifespan when introducing trace amount amorphous Al₂O₃ upon FeS power's surface. DFT calculations further reveal that Al₂O₃ induces the phase conversion from low-acitve tetragonal FeS to high-active orthorhombic FeS, which promotes the lithiation-delithiation reversibility and stability. Based on the above result, it is expected to provide useful guidelines for the in-situ structure engineering of TMSs-based anodes in all-enclosed secondary batteries.

Titanium alloys are generally characterized by low surface hardness, poor thermal conductivity, high friction coefficients and susceptibility to adhesive wear, which significantly hinder their industrial applications. To address this issue, we enhance the wear resistance of titanium alloys by incorporating nano WC particles. However, the optimal amount of WC to be added to titanium alloys remains unexplored. In this study, we prepared Ti6Al4V-xWC (wt%) (x = 0, 10, 20) coatings on Ti6Al4V substrates using laser melting deposition, specifically examining the effects of WC content on the microstructure and properties of the coating. The experimental findings indicate that the introduction of WC particles resulted in the formation of a TiC reinforced phase within the composite coating which promoted the equiaxialization of lamellae α phase. The hardness of the coatings increased significantly with the increase of the mass fraction of WC nanoparticles. Notably, the Ti6Al4V-10WC and Ti6Al4V-20WC coatings exhibited wear resistance that was 2.5 and 3.4 times greater, respectively, compared to the Ti6Al4V coatings. This enhancement in wear resistance can be attributed to the reinforcing phase (TiC) formed by the addition of WC. This experiment demonstrates a viable approach to improving the wear resistance of the Ti6Al4V titanium alloy through surface treatment.

A systematic investigation of the development of shear textures during hot rolling of Fe – 3 wt% Si steel has been carried out in the present work with a special focus on the influence of austenite in preferential orientation selectivity. This orientation selectivity was anticipated owing to the fact that texture evolution during thermomechanical processing is entirely an effect of crystallographic rotations and such rotations may experience constraining effects because of surrounding austenite. The influence of austenite on shear texture evolution was studied over a range of finish rolling temperatures (FRTs). The evolution of microstructure and microtexture within the surface-subsurface zone was studied adopting EBSD. Sharp Goss orientation was observed at lower FRT, while texture intensities of Brass and Copper were observed to be high at higher FRT. Additionally, it was inferred that in the surface-subsurface zone, next to the austenite, the ease of rotation about ND contributed to the higher observed frequency of the Brass orientation despite of its overall lesser fraction as compared to Copper orientation. The less observed frequency of Copper orientation near the austenite has been attributed to the resistance offered by the austenite to its adjacent ferrite grains while rotating about TD. This constraining effect of austenite has been shown to be consistent over the entire range of studied FRTs. Moreover, it has been shown that at higher rolling temperatures, the lower constraining effect by austenite paves the way for Goss orientation formation, which acts as the precursor to the formation of Brass at the FRT.

In engineering applications where high temperatures are a concern, the quest for structural components exhibiting robust thermomechanical stability is paramount. This study delves into the thermomechanical stability of 316 L steel fabricated via laser powder bed fusion (LPBF), with a particular emphasis on understanding the role of dislocation cell structures. In this study, deliberate rolling deformation followed by annealing at elevated temperatures was performed. The rolling deformation resulted in the introduction of dislocations and deformation twins in LPBF 316 L. Notably, deformation twins were observable in LPBF 316 L annealed at 800 °C, but they vanished at 1050 °C. Dislocation recovery was evident in pre-deformed samples following annealing at both temperatures. However, even after annealing at 800 °C, the grain and dislocation cell structures of heavily pre-deformed samples exhibited considerable stability. Conversely, during annealing at 1050 °C, substantial recrystallization occurred in pre-deformed samples, with the degree of recrystallization correlating with the extent of plastic deformation. In contrast, the grain structure of non-deformed samples remained unchanged after annealing at 1050 °C. This suggests that the dislocations induced by rolling serves as dislocation-stored energy, thus promoting recrystallization. The enduring stability observed in the grain and cell structures of heavily pre-deformed samples at 800 °C, as well as the high stability of these structures in non-deformed samples at 1050 °C, can be attributed to the presence of dislocation cell structure with low-energy contributing to retarded recrystallization.

The effective connection of magnesium and steel is of great significance to the weight reduction of vehicles. Mg-3.66Al-2.29Ce-1.32La (wt%) magnesium alloy (AE44) and DP340 steel was successfully jointed by friction stir lap welding (FSLW) with significant plunge depth of the tool. The joint shear strength exceeds 396 N/mm, which derived from multi-scale strengthening mechanisms. The microstructure and formation mechanism of the macroscopic Mg and steel hooks embedded into the other's plates were thoroughly analyzed. Multivariate linear regression of geometrical parameters of hooks shows that a small depth of Mg and steel hook, narrow width of steel hook, large width of Mg hook and small aspect ratio of magnesium hook can increase joint strength. Simultaneously, microscopic interlocks like sawtooth formed on the interface makes the Mg/steel interface fully contact and limits the initialization of deformation, resulting in an effective joining. Due to deep plunge depth, considerable element diffusion occurred at the interface and an interlayer composed of Fe₂Al₅ phase was formed. Fe₂Al₅ phase and adjacent steel grains exhibited an orientation relationship of [111]_Fe//[100]_Fe2Al5 and (002)_Fe2Al5//(10$\overline{1}$)_Fe. Furthermore, the semi-coherent interplanar mismatches occurred at Fe₂Al₅/Fe interface.