An overview and critical assessment of the mechanisms of microstructural refinement during ultrasonic solidification of metals

A refined, equiaxed grain structure and the formation of finer primary intermetallic phases are some of the notable benefits of ultrasonic processing of liquid/solidifying melts. Ultrasonic treatment (UST) has been widely explored in Al and Mg-based alloys due to its operational versatility and scalability. During UST, the refinement of grain and primary intermetallic phases occurs via cavitation-induced fragmentation mechanisms. In addition, UST improves the efficiency (activation of particles) of the conventional grain refinement process when potent particles are added through master alloys. Though the UST’s ability to produce refined as-cast structures is well recognized, the understanding of the refinement mechanisms is still debated and unresolved. Significant efforts have been devoted to understanding these mechanisms through the use of sophisticated techniques such as in-situ/ real-time observation, lab-scale and commercial-scale casting processes. All these studies aim to demonstrate the significance of cavitation, fragmentation modes, and alloy chemistry in microstructure refinement. Although the physical effects of cavitation and acoustic streaming (fluid flow) are primary factors influencing the refinement, the dominant grain refinement mechanisms are affected by several solidification variables and casting conditions. Some of these include melt volume, solute, cooling rate, potent particles, grain growth (equiaxed, columnar or dendritic), and the cold zones of the casting where the onset of nucleation occurs. This review aims to provide a better insight into solidification variables emphasizing the importance of cold zones in generating fine structures for small- and large-volume (direct chill) castings. Another important highlight of this review is to present the relatively less explored mechanism of (acoustic) vibration-induced crystallization and discuss the role of cavitation in achieving a refined ingot structure.     

Influences of Bi2O3 on microstructure and magnetic properties of MnZn ferrite

MnZn ferrites were prepared by conventional oxide ceramic process. The effects of Bi₂O₃ on microstructure and magnetic properties of MnZn ferrites were investigated by means of characterizing the fracture surface micrograph, composition of grain boundary, magnetic properties and density by scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS), B-H analyzer and Archimedes method, respectively. The results indicate that Bi₂O₃ mainly segregates and concentrates in the grain boundary regions, promotes solid-state reaction and grain growth, reduces porosity and enhances density. Optimum addition of Bi₂O₃ increases the permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability.     

The effects of annealing at 1000 °C for 24 h on the extrinsic properties of Pr-Fe-B and Nd-Fe-B-type sintered magnets

Recent studies have shown the effects of a post sintering heat treatment at 1000 °C for 24 h on the microstructure and magnetic properties of Pr-Fe-B/Nd-Fe-B magnets based on Nd₁₆Fe₇₆B₈ and Pr₁₆Fe₇₆B₈. In an attempt to understand the influence of environmental factors, an investigation into the effects of annealing under different degrees of vacuum for both types of sintered magnets has been carried out. The effect of annealing the Pr-Fe-B magnets at 1000 °C for 24 h resulted in a general increase in the magnetic properties, especially the intrinsic coercivity, although the degree of improvement appeared to be dependent on the initial annealing conditions (ambient pressure). Oxygen analysis of sintered and annealed magnets indicates a change in the nature of the grain boundary phases after the annealing treatment. The effect of annealing the Nd-Fe-B magnets at 1000 °C for 24 h resulted in a general decrease in the magnetic properties, especially the intrinsic coercivity.     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

A two-dimensional simulation of grain structure growth within the substrate and the fusion zone during direct metal deposition

In this paper, a predictive multi-scale model based on a cellular automaton (CA)-finite element (FE) method has been developed to simulate thermal history and microstructure evolution during metal solidification for the Direct Metal Deposition (DMD) process. The macroscopic FE calculation that is validated by thermocouple experiment is developed to simulate the transient temperature field and cooling rate of single layer and multiple layers. In order to integrate the different scales, a CA–FE coupled model is developed to combine with thermal history and simulate grain growth. In the mesoscopic CA model, heterogeneous nucleation sites, grain growth orientation and rate, epitaxial growth, re-melting of pre-existing grains, metal addition, grain competitive growth, and columnar to equiaxed phenomena are simulated. The CA model is able to show the entrapment of neighboring cells and the relationship between undercooling and the grain growth rate. The model predicts the grain size, and the morphological evolution during the solidification phase of the deposition process. The developed “decentered polygon” growth algorithm is appropriate for the non-uniform temperature field. Finally, the single and multiple-layer DMD experiment is conducted to validate the characteristics of grain features in the simulation.     

不同氮肥处理下小麦冠层和叶片光谱特征及产量分析

利用遥感光谱无损、快速分析出氮肥的施用时期和施用模式,对于保护环境、产量及氮肥利用率的提高具有重要意义。利用FieldSpec 4 Wide-Res Field Spectrum radiometer便携式地物光谱仪,测定了不同氮水平下小麦冠层和叶片两种模式光谱特征及红边参数变化规律;提出一个新指数--归一化差异最大指数（normalized difference maximum index,NDMI）,并分析其与叶面积指数（leaf area index,LAI）、SPAD（soil and plant analyzer development）值、MDA（malondialdehyde）含量、旗叶氮含量和产量的相关性。结果表明,小麦叶片原始光谱在开花后26 d起800～1 330 nm区间的光谱反射率以N3（1/3底施+1/3冬前追肥+1/3拔节期追肥）处理为最高,N1处理（1/2底施+1/2冬前追肥）次之。主要原因是由冬前和拔节期两个时期均施三分之一氮肥,增强了叶片光合能力。小麦冠层原始光谱,在400～700 nm波段,N2（1/2底施+1/2拔节期追肥）处理最低;在760~1 368 nm波段区间,由于群体结构不同,在开花期至灌浆中期N1处理的光谱反射率最高,N3处理次之;N3处理的冠层光谱反射率在开花后26和33 d最高。建议用400～700和760～1 368 nm波段的冠层原始光谱数据,分别来辨别小麦旗叶含氮量的高低及施肥模式。叶片模式下一阶微分光谱在500～750 nm区间出现两个“峰”,通过峰的位置偏移程度和偏移时期来估测施氮的模式。在670～740 nm区间冠层一阶微分光谱值在开花期最高,开花后10 d的一阶微分光谱值最低。在开花期至开花后10 d N1处理的一阶微分光谱值高于N3处理;灌浆中期至开花后33 d N3处理的一阶微分光谱值高于N1处理。可以通过一阶微分最大值来推测小麦所处的生育期和施肥的方式及施肥时期。在开花期至灌浆中期,冠层反射率一阶导数最大值（FD-Max）N1处理最高,N3处理次之;在开花后26～33 d,N3处理的群体结构较其他处理密,导致其一阶导数最大值一直最高。四个处理叶片一阶导数最大值变化趋势不如冠层显著。四个处理的反射率一阶导数最大值对应的红边位置（REP_FD-Max）中,N1和N3冠层REP_FD-Max在灌浆中期后偏移显著;在开花后26～33 d,N3处理的群体上层结构密,叶片宽且厚,冬前追施氮肥影响REP_FD-Max偏移程度。基于NDVI基础上,筛选出一个新指数--归一化差异最大指数。冠层归一化差异最大指数（CNDMI）与农化参数的相关系数高于叶片归一化差异最大指数（LNDMI）,且CNDMI与产量的相关性比LNDMI显著。冠层归一化差异最大指数与旗叶氮含量、SPAD值和MDA含量有着显著的相关性,相关系数r分别为0.812 88,0.928 21和-0.722 17。综上所述,借助光谱数据和红边参数可以推测小麦含氮量的高低,所处的生育期和施氮肥的模式,进而为田间施肥管理及施肥诊断提供依据。CNDMI与小麦产量有着更好的相关性,符合我国资源卫星的光谱波段范围,具有可实际操作性。     

Improving the property of calcium ferrite using a sonochemical method

Power ultrasonic vibration was applied to the solidification of calcium ferrite (CF) melt in this study. The results indicated that power ultrasound can promote the formation of CF by accelerating the solidification process. Ultrasonic vibration greatly refined the CF grains, resulting the grain size decreased from 1893 to 437 μm. Meanwhile, ultrasonic vibration significantly enhanced the compressive strength, reduced the reduction time and improved the reducibility of CF slags. With ultrasonic treatment, the ultimate compressive strength of samples increased from 37.5 to 67.8 MPa, and the reduction time decreased from 225 to 136 min.