Papers by Keyword: Nucleation

Abstract: Solidification behavior of SiCw/Al-18Si metal matrix composites (MMCs) was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimeter (DSC) in order to reveal the effects of strontium addition and whisker content. The results show that the Si phase does not nucleate on SiC whisker surface. With the increasing of SiC whisker content, solidification onset and peak temperatures of primary Si decrease. Sr addition lowers solidification onset and peak temperatures of primary Si, and reduces its size. Whisker content has larger effects on solidification onset and peak temperatures of primary Si without Sr addition than that of primary Si with Sr addition.But solidification onset and peak temperatures of eutectic are barely affected by whisker content and Sr addition.

Abstract: The feasibility of bubble templating method to prepare hollow particles was tested in this paper by passing a bubble into a silica sol. The silica sol was prepared by mixing TEOS, water, alcohol and hydrochloric acid. To control the gel point, the reactants and their concentrations were varied in each experiment. After the sol was prepared, an ammonia bubble was introduced into the prepared sol through a capillary tube. With the catalysis of ammonia, the silica sol gelled on the surface of bubble and formed a hollow gel particle. The nucleation of silica gel on the surface of bubble was discussed in this paper.

Abstract: The effect of Pt and Ge on the stability of NiSi films has been examined. The addition of a small amount of Pt (5 at%) in the Ni film increases the disilicide nucleation temperature to 900oC leading to a better stability of NiSi at high temperatures. For Ni films on Si1-xGex with x=0.29 and 0.58, no NiSi2 was found after annealing at 850°C. The increase in thermal stability of NiSi has been explained in terms of nucleation concept. Calculated ternary phase diagrams allow to understand the effect of the third element (Pt or Ge) on the driving force for nucleation. The redistribution of this element can also be explained with the ternary phase diagrams.

Abstract: A molecular dynamic model for instantaneously undercooled iron melt has been applied to model the isothermal annealing of the stable percolation clusters. They contain mutual penetrating and contacting icosahedra with atoms at the vertices and centers. Such stable clusters form only below the critical temperature Tg ~1180 K. We identify Tg as the glass transition temperature. It is established that the time when a homogeneous nucleation starts is minimal for Tg. The stable icosahedral percolation clusters do not form above Tg. The proposed quantitative model describes the atomic mobility in metal glasses, where root-mean-square atomic displacement is the sum of contributions from both linear (Einsteinian) and logarithmic terms, associated to irreversible structure relaxation. The activation parameters of the model change abruptly at the icosahedral percolation transition.

Abstract: Six years have passed since the international workshop “Investigations and Applications of Severe Plastic Deformation” held 2-8 August 1999 in Moscow, Russia. This workshop focused on severe plastic deformation (SPD) processing to produce bulk nanostructured metals and alloys. Since 1999 the field has expanded from 200 to over 2000 publications that have addressed the microstructures and properties that can be produced by a growing number of SPD techniques. In view of this expansion, the outlook for ongoing development of severely deformed materials is updated. Special attention is given to factors influencing the manufacturing and commercialization of SPD-processed metals, including barriers to their widespread application. Recommendations are made for future SPD research that will facilitate more rapid commercialization of SPD-processed metals and enhance the competitiveness of SPD processing with respect to alternative technologies for producing bulk nanostructured metals.

Abstract: Deformation twins have been oberved in nanocrystalline (NC) Al synthsized by cryogenic ball-milling and in NC Cu processed by high-pressure torsion under room temperature and at a very low strain rate. They were found formed by partial dislocations emitted from grain boundaries. This paper first reviews experimental evidences on deformation twinning and partial dislocation emissions from grain boundaries, and then discusses recent analytical models on the nucleation and growth of deformation twins. These models are compared with experimental results to establish their validity and limitations.

Abstract: The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Abstract: While electron backscatter diffraction (EBSD) has become an established technique within materials characterization labs around the world, the technique is still relatively young and new applications are continuing to emerge. Automated EBSD or Orientation Imaging Microscopy (OIM) systems are being used in combination with other equipment within the scanning electron microscope (SEM) to perform in-situ measurements. This includes tensile stages for observing changes in local orientation during deformation and heating stages for studying orientation changes arising during recrystallization and grain growth as well as phase transformations. In addition to these temporally three-dimensional studies, spatially three-dimensional studies can be performed by in-situ serial sectioning in microscopes equipped with both electron and focused ion beams. These in-situ techniques are briefly reviewed. The review is followed by a detailed analysis of in-situ heating experiments on copper. The movement of grain boundaries during recrystallization and subsequent grain growth are tracked. The effect of orientation relationships on grain boundary mobility and nucleation are explored. No special relationship with grain boundary mobility was observed. However, twins appear to play a significant role in the nucleation process.

Abstract: Ni with columnar grain was cold rolled and then recrystallized. The orientations at triple junctions before and after annealing were characterised using the EBSD technique, and recrystallization nuclei at triple junctions were identified. A comparison was made between the orientations of the nuclei and the orientations of the grains in the same area before annealing. Both nuclei with orientations within the orientation spread of the deformed grains as well as away from that were found.