Precipitation location of secondary phase and microstructural evolution during static recrystallization of as-cast Ti-25V-15Cr-0.3Si titanium alloy

The microstructural evolution and precipitation location of the secondary phase of an as-cast Ti-25V-15Cr-0.3Si titanium alloy were investigated via isothermal compression experiments and heat treatment. The average aspect (length-to-width) ratio, average area and size of the grains at different heat treatment temperatures and holding time were analyzed and the effects of deformation and annealing time on the grain area and size were considered. It was found that the grain size was strongly influenced by the height reduction and holding time. Grain growth was significant when annealing time increased from 10 min to 2 h at 950 °C and height reduction of 30%; however, grain growth was minimal at annealing time between 2 and 4 h. Many dispersion particles were observed to form in continuous chains; the precipitation location was confirmed to be along initial grain boundaries, and the dispersion particles were identified to be Ti₅Si₃ phase by TEM.     

The control of abnormal grain growth in low-voltage SnO2 varistors by microseed addition

In this research, the addition effects of three different quantities of micron-sized seeds (microseeds) to a SnO₂ varistor prepared from nanomaterials on the microstructure and electrical properties were studied. Moreover, surge-withstanding capability of low-voltage SnO₂ varistors was investigated. The X-ray diffraction pattern disclosed a single phase SnO₂ for microseed grains. The morphological features of samples were characterized using scanning electron microscopy. The abnormal distribution of grain size with elongated grains of SnO₂ in fine grains matrix was observed in sintered samples without microseeds. The low content of microseed addition (0.3 wt%) had not controlled abnormal grain growth, however, it increased mean grain size to 37 µm. Although the high content of microseeds (7.5 wt%) stopped abnormal grain growth, it had a negative effect on relative density and mean grain size. The normal grain size distribution with maximum mean grain size (45 µm) was obtained in samples containing 1.5 wt% microseeds. These samples showed the lowest breakdown field (240 V/cm) and the highest surge-withstanding capability (1.5 kA/cm²). Furthermore, the standard deviation of the electrical parameters of these samples was improved due to normal grain-size distribution.     

Predicting Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) populations and associated grain damage in smallholder farmers’ maize stores: A machine learning approach

Prostephanus truncatus is a notorious pest of stored-maize grain and its spread throughout sub-Saharan Africa has led to increased levels of grain storage losses. The current study developed models to predict the level of P. truncatus infestation and associated damage of maize grain in smallholder farmer stores. Data were gathered from grain storage trials conducted in Hwedza and Mbire districts of Zimbabwe and correlated with weather data for each site. Insect counts of P. truncatus and other common stored grain insect pests had a strong correlation with time of year with highest recorded numbers from January to May. Correlation analysis showed insect-generated grain dust from boring and feeding activity to be the best indicator of P. truncatus presence in stores (r = 0.70), while a moderate correlation (r = 0.48) was found between P. truncatus numbers and storage insect parasitic wasps, and grain damage levels significantly correlated with the presence of Tribolium castaneum (r = 0.60). Models were developed for predicting P. truncatus infestation and grain damage using parameter selection algorithms and decision-tree machine learning algorithms with 10-fold cross-validation. The P. truncatus population size prediction model performance was weak (r = 0.43) due to the complicated sampling and detection of the pest and eight-week long period between sampling events. The grain damage prediction model had a stronger correlation coefficient (r = 0.93) and is a good estimator for in situ stored grain insect damage. The models were developed for use under southern African climatic conditions and can be improved with more input data to create more precise models for building decision-support tools for smallholder maize-based production systems.     

加热工艺参数对12%Cr钢晶粒长大行为的影响

研究了不同加热工艺参数下(加热温度1050~1300 ℃,保温时间0.25~24 h)12%Cr超超临界转子钢的奥氏体晶粒长大行为,并通过光学显微镜(OM)观察晶粒尺寸的变化规律,建立晶粒长大数学模型。结果表明:随着加热温度增加,晶粒尺寸逐渐增加,加热温度低于1150 ℃时,晶粒尺寸增加明显,而温度高于1150 ℃后,晶粒尺寸逐渐趋于稳定;随着保温时间的增加,晶粒尺寸逐渐增加,保温时间增加到3 h后,晶粒尺寸增加趋势放缓。采用非线性回归方法和Arrhenius晶粒长大模型,建立了该钢的晶粒长大数学模型。     

哈氏合金X电子束表面处理微观组织研究

哈氏合金因其优良性能被广泛应用于航空工业,利用强流脉冲电子束(HCPEB)轰击哈氏合金X表面对其进行改性,并对其显微组织进行分析和讨论。利用扫描电子显微镜(SEM)观察腐蚀后试样,利用电子背散射衍射(EBSD)观察未腐蚀试样。结果表明：从扫描电镜照片可以看到,未轰击试样晶界被腐蚀,轰击后试样表面存在约为2μm的重熔层,腐蚀程度减轻,晶界处出现颗粒状碳化物；从EBSD结果可知,轰击5次试样晶粒尺寸最小,轰击5次和10次试样晶粒取向性更好,多为[101]和[110]。实验表明强流脉冲电子束轰击能够使哈氏合金X表面出现重熔层,并改变其晶粒大小和晶体取向。     

Strategy to design high performance TiB2-based materials: Strengthen grain boundaries by solid solute segregation

TiB₂ exhibits a unique combination of excellent properties that make it promising candidate for applications in extreme environments, where retention of strength at high temperatures is essential. Tailoring grain boundary properties by segregation is believed a prominent way to design high-temperature performance of ceramics. In this work, segregation tendencies of solute elements, including Sc, Y, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, in TiB₂ grain boundaries and the strengthening/weakening effects induced by segregations are investigated by first-principles calculations. The results reveal that small atoms tend to segregate to grain boundary sites with local compression strains, while large atoms prefer grain boundary sites with local expansion strains. Deteriorated grain boundary strength is usually caused by additional expansion strain induced by segregation, while improved grain boundary strength results from either enhanced local bonding induced by segregation of small atoms or increased fracture strain due to segregation of large atoms. Cr and V, especially Cr, exhibit strong segregation tendency and improvement on grain boundary strength, which provides useful guidelines for the design of high performance TiB₂-based materials.     

Phase Field Modelling Allotropic Transformation of Solid Solution

Based on multiphase field conception and integrated with the idea of vectorvalued phase field, a phase field model for typical allotropic transformation of solid solution is proposed. The model takes the non-uniform distribution of grain boundaries of parent phase and crystal orientation into account in proper way, as being illustrated by the simulation of austenite to ferrite transformation in low carbon steel. It is found that the misorientation dependent grain boundary mobility shows strong influence on the formation of ferrite morphology comparing with the weak effect exerted by misorientation dependent grain boundary energy. The evolution of various types of grain boundaries are quantitatively characterized in terms of its respective grain boundary energy dissipation. The simulated ferrite fraction agrees well with the expectation from phase diagram, which verifies this model.     

Effects of SiC content on the densification,microstructure, and mechanical properties of HfB2–SiC composites

To investigate the effects of SiC on microstructure, hardness, and fracture toughness, 0, 10, 20, and 30 vol% SiC were added to HfB₂ and sintered by SPS. Upon adding SiC to 30 vol%, relative density increased about 4%; but HfB₂ grain growth had a minimum at 20 vol% SiC. This may be due to grain boundary silicate glass, responsible for surface oxide wash out, enriched in SiO₂ with higher fraction of SiC. By SiO₂ enrichment, the glass viscosity increased and higher HfO₂ remained unsolved which subsequently lead to higher grain growth. Hardness has increased from about 13 to 15 GPa by SiC introduction with no sensible variation with SiC increase. Residual stress measurements by Rietveld method indicated high levels of tensile residual stresses in the HfB₂ Matrix. Despite the peak residual stress value at 20 vol% SiC, fracture toughness of this sample was the highest (6.43 MPa m^0.5) which implied that fracture toughness is mainly a grain size function. Tracking crack trajectory showed a mainly trans-granular fracture, but grain boundaries imposed a partial deflection on the crack pathway. SiC had a higher percentage in fracture surface images than the cross-section which implied a weak crack deflection.     

Enhancing the intergranular corrosion resistance of high Mg-alloyed Al–Mg alloy by Y addition

Microalloying is thought to improve the performance of Al–Mg alloys commonly used in transport applications. The effect of Y addition (0–0.4%) on the microstructure, mechanical properties, and corrosion resistance of Al–9.2Mg–0.7Mn alloy is investigated for potential use in engineering applications. The generation of the β-Al₃Mg₂ phase along the grain boundaries is suppressed in the as-cast alloy due to the formation of the AlMgY ternary phase. The average intergranular corrosion mass loss of the alloy with 0.1% Y addition decreases about 53.1% almost at no expense of mechanical performance in the as-rolled alloy after annealing. Moreover, the alloy with 0.1% Y addition shows the corrosion mass loss about 30.2% lower than the Y-free alloy in the sensitized state. The enhanced corrosion resistance of the alloy can be ascribed to the reduced β-Al₃Mg₂ precipitation along the grain boundaries associated with Y addition.