Y. K. Wu

RELAXED SI0.7GE0.3 LAYERS GROWN ON LOW-TEMPERATURE SI BUFFERS WITH LOW THREADING DISLOCATION DENSITY

Si0.7Ge0.3 epilayers with low threading dislocation density have been grown on Si (001) substrates by introducing a low temperature Si buffer. Such a structure can be used as the buffer for the growth of device structures. In comparison with the conventional compositionally graded buffer system, it has the advantages of having lower threading dislocation density, smaller thickness for required degree of relaxation, and smoother surface. Experimental evidence suggests that an anomalous relaxation mechanism has been involved.

Allotropic transformation of cobalt induced by ball milling

We have found that phase transformation can occur in cobalt when subjected to ball milling. The phase formation of cobalt was found to depend on the milling intensity. Under different milling intensity or different milling time, the phase transformations follow the routes of h.c.p. + f.c.c. + h.c.p., h.c.p. + f.c.c. --> h.c.p. --> f.c.c. + h.c.p. and h.c.p. + f.c.c. --> h.c.p. --> f.c.c. + h.c.p. --> f.c.c., respectively. Our results indicate that the phase formation of cobalt induced by ball milling was determined by the accumulation of structural defects and not by the local temperature rise. Different milling intensity may adjust the rate and level of the accumulation of defects. The as-obtained f.c.c. cobalt is stable and it did not return to the h.c.p. state after annealing at different temperatures. While the h.c.p, phase is not stable, and it partly converted to the f.c.c. phase after annealing at 450 degrees C. The fact was interpreted as being caused by the grain size effect, and it was further proved that the small grain size tends to stabilize the f.c.c. structure of cobalt.

PHASE-TRANSFORMATION OF COBALT-INDUCED BY BALL-MILLING

We have found that phase transformation can occur in cobalt when subjected to ball milling. The two modifications of cobalt, i.e., face‐center‐cubic (fcc) and hexagonal close‐packed (hcp) phases, which usually coexist at room temperature and are often difficult to be separated from each other, can now be easily separated by using the simple ball milling technique. The phase formation of cobalt was found to depend on the mill intensity. Under different mill intensity or different milling time, the phase transformations follow the routes of hcp+fcc→hcp, hcp+fcc→hcp→fcc+hcp, and hcp+fcc→hcp→fcc+hcp→fcc, respectively. Our results indicate that the phase formation of cobalt induced by ball milling was determined by the accumulation of structure defects. Different mill intensity may adjust the rate and level of the accumulation of defects.

Microstructure investigations on explosive TiNi(or Ni)/TiC-composite-formation reaction during mechanical alloying

The microstructure of TiNi(or Ni)/TiC composite prepared by mechanical alloying (MA) of elemental Ti, Ni and C powders has been investigated in detail. The result shows that large agglomerates with size 3-10 mm were abruptly formed for Ti50Ni20C30, Ti40Ni40C20 and Ti30Ni50C20 powders at the milling duration of 3 h 30 min-3 h 35 min, which suggests melting of the powders and subsequent quenching has occurred during MA. Transmission electron microscopy shows that spherical TiC grains, lath twin martensite (M) and B2 phase are directly formed after 3 h 35 min MA of Ti50Ni20C30 powders. It is also found that there exists definite orientation relationships between M and B2 phases which are very close to those obtained by Otsuka et al. in equiatomic TiNi martensite. The resultant phases are predominantly TiC and M phases with a small amount of B2 phase for Ti40NI40C20; and Ni and TiC phases for Ti30Ni50C20 after 3 h 35 min of MA. The microstructure characteristics of the as-milled materials are very similar to those of melted and solidified ones, which proves that melting of the powders and subsequent quenching has really occurred during the MA process. We concluded that MA in Ti50Ni20C30, Ti40Ni40C20 and Ti30Ni50C20 is not governed by a gradual diffusional reaction, but by a self-sustained high-temperature synthesis (SHS) one. The SHS reaction is believed to be triggered by the release of heat of formation of the TiC phase and ignited by the mechanical collisions. The thermodynamic and kinetic conditions for the SHS reaction is discussed, and it shows that MA is a versatile method in inducing SHS reaction in systems with large heat of formation.

Microstructure and nanoscale composition analysis of the mechanical alloying of FexCu100 − x (X = 16, 60)

The microstructures of Fe16Cu84 and Fe60Cu40 (atomic percent) during mechanical alloying (MA) were studied by high resolution electron microscopy (HREM). Nanoscale composition distribution in Fe16Cu84 was determined using a HF 2000 FEG TEM. In the Fe16Cu84 specimen, a number of deformation twins were observed. In the Fe60Cu40 specimen, shear band and generation of nanocrystals in the shear band were observed, which is shown to be a typical mechanism for grain size reduction during MA. In both specimens, the b.c.c. grains tend to be very small (<5 nm) before alloying, which is shown to be a prerequisite condition for the dissolution of Fe in Cu and is also direct evidence to support the thermodynamical model proposed by Yavari et al. Nanoscale composition analysis in Fe16Cu84 specimen shows that the average Fe contents in both the interior of grains and the grain boundaries (GBs) are close to the designed composition, thus proving that a supersaturated solid solution has really formed. However, the Fe contents in both cases are rather inhomogeneous, indicating that the mixing of Fe and Cu during MA is inhomogeneous. The process of MA is suggested to be divided into two stages: at the early stage, the grain sizes reduce quickly to a steady value due to the mobility of dislocations; further deformation can be fully accommodated by GBs. As a result, very fast volume diffusion and GB diffusion are achieved. NC-structure and greatly enhanced diffusion coefficients allow the formation of supersaturated solid solutions in immiscible systems with positive enthalpy of mixing. Copyright (C) 1996 Acta Metallurgica Inc.

HREM observations of the synthesized process of nano-sized SiC by ball milling of Si and C mixed powders

Abstract The synthesis of nano-sized SiC through ball milling (BM) of elemental Si and graphite mixed powders at room temperature has been reported, and detailed reaction process has been characterized by high-resolution electron microscopy (HREM). High-resolution electron microscopy (HREM) observations presented herein suggest that amorphous graphite (a-graphite), amorphous silicon (a-Si) and nano-sized crystalline Si (c-Si) with many defects are produced during BM, which is prerequisite to the reaction. In some areas, SiC is synthesized through a diffusion of C atoms into the a-Si/and c-Si. In the former, a-Si(C) forms, and then mechanically-driven crystallization of the a-Si(C) occurs to form SiC. In the latter, C atoms directly replace Si atoms to form SiC with an orientation relationship of (111)SiC//(111)Si. In other areas, localized self-sustained reaction occurs to form slightly larger SiC grains.

BALL-MILLING OF DUCTILE METALS

Pure copper powder was employed to study the effects of ball milling on the development of the structure and properties of ductile metals. The results indicate that larger spheres with diameters of about 2-2.5 mm are created after 20 h of ball milling. The formation of such spheres is mainly due to sphere-to-flake or sphere-to-sphere welding. This welding is not complete, leaving large pores and curved voids in the spheres. The average grain size of such spheres is 10-100 nm. The increase in lattice strain is about 0.2%. The microhardness increases from 45 MPa (unmilled) to 220 MPa (milled for 20 h). High-resolution transmission electron microscopy (HRTEM) investigations show the following: (a) the deformation of ball-milled copper proceeds by [112](11

Microstructure and homogeneity of nanocrystalline Co-Cu supersaturated solid solutions prepared by mechanical alloying

(

Microstructure investigations of ball milled materials.

) over bar 1) twinning or high-order twinning; (b) the [112](11