H. Q. Ye

A NEW ICOSAHEDRAL PHASE WITH M35 SYMMETRY

Abstract A metastable phase with the point group symmetry m35 but no ordinary translation symmetry has been found in rapidly quenched (Ti 1−xVx)2Ni alloys with × = 00-0-3 by electron diffraction and high-resolution electron microscopy.

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.

High-resolution electron microscopic study of manganese silicides MnSi2−x

Abstract Several members of the family of Nowotny phases with compositions MnSi 2− x are studied by means of electron diffraction and high-resolution electron microscopy. The diffraction patterns exhibit spacing as well as orientation “anomalies.” From the corresponding high-resolution images it is concluded that the compounds exhibit a particular type of disorder. The spacing anomalies result from the fact that the manganese and silicon arrangements have different and to some extent independent periods along the c direction of the tetragonal structure. The orientation anomaly is due to the fact that the silicon helices can be shifted longitudinally along the c axis of the manganese sublattice. Dislocation-like arrangements of lattice fringes can consistently be explained by means of this model.

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.

Calculations of single-crystal elastic constants made simple

[Yu, R.; Zhu, J.] Tsinghua Univ, Beijing Natl Ctr Electron Microscopy, Dept Mat Sci & Engn, Beijing 100084, Peoples R China. [Yu, R.; Zhu, J.] Tsinghua Univ, Adv Mat Lab, Dept Mat Sci & Engn, Beijing 100084, Peoples R China. [Yu, R.; Zhu, J.] Tsinghua Univ, State Key Lab New Ceram & Fine Proc, Dept Mat Sci & Engn, Beijing 100084, Peoples R China. [Ye, H. Q.] Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, Shenyang 110016, Peoples R China.;Yu, R (reprint author), Tsinghua Univ, Beijing Natl Ctr Electron Microscopy, Dept Mat Sci & Engn, Beijing 100084, Peoples R China;ryu@tsinghua.edu.cn

Si-induced twinning of TiC and formation of Ti3SiC2 platelets

Abstract Effects of silicon in titanium carbide (TiC) have been investigated using high-resolution electron microscopy combined with high-spatial-resolution analytical electron microscopy. The results demonstrate that Si can reduce the twin boundary energy of TiC, leading to the formation of a lot of two-dimensional (2D) defects in TiC grains containing Si. These defects were identified as microtwins of four (111) spacings thick and structural-related Ti 3 SiC 2 platelets of only one unit cell thick. The twin-stabilizing effect of Si is discussed in terms of coordination environments of Si. Very thin Ti 3 SiC 2 platelets are formed, accompanied by the segregation of Si atoms and carbon vacancies to the twin boundaries. Microtwins and Ti 3 SiC 2 platelets were found to grow in 2D with Si totally confined in the defects.

Effects of Si and Al on twin boundary energy of TiC

Effects of aluminum and silicon on the twin boundary energy (TBE) of titanium carbide (TiC) have been investigated using transmission electron microscopy and first-principles calculations. The results demonstrate that Al is more effective relative to Si to reduce the twin boundary energy of TiC. By electronic structure analysis, the high TBE of pure TiC can be attributed to the energy penalty accompanying the formation of trigonal prismatic coordination of Ti atoms at twin boundaries. For the SUM doped TiC, the lower TBEs are partly due to the absence of the energy penalty accompanying the formation of trigonal prismatic coordination of Si/Al atoms, and are partly due to the stabilizing effect via the formation of pi bonding between carbon neighbors of Si/Al atoms. The pi bonding between carbon neighbors of Al is stronger than that of Si, providing an explanation for the larger twin-stabilizing effect of Al

Microstructure, interface and mechanical property of the DS NiAl/Cr(Mo,Hf) composite

A NiAl–Cr(Mo) composite containing Hf was directionally solidified under Ar atmosphere in Al2O3–SiO2 ceramic mold by the standard Bridgman method. The microstructure in the as-grown and hot isostatic pressing (HIP) conditions was studied using SEM, HREM and TEM equipped with EDXS. The composite was mainly composed of three phases, i.e. lamellar Cr(Mo), NiAl matrix and semi-continuously distributed Ni2AlHf. The common growth direction of NiAl and Cr(Mo) is , and the interface between them was atomically flat with no transition layer. Fine G-phase precipitates were formed in the presence of Si which was from the ceramic shell molds used during the directional solidification. The G-phase precipitates were cuboidal in shape and coherent with NiAl matrix. After the HIPed and aged treatment, the G-phase precipitates completely disappeared and the amount of intercellular Ni2AlHf phase was partially reduced. The mechanical properties were measured by tensile and compressive tests within the temperature range of 25∼1100°C. The BDTT of the HIPed and aged materials is significantly higher than the as-grown composite. The yield stress of the composite was higher than that of many NiAl-based alloys and possible strengthening mechanism were discussed.

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.

A cellular automaton investigation of the transformation from austenite to ferrite during continuous cooling

A cellular automaton model has been employed to investigate the transformation from austenite to ferrite in low carbon steels during continuous cooling. An important aspect of this approach is the implementation of incorporating local concentration changes into a nucleation or growth function, which is utilized by the automaton in a probabilistic fashion. The modeling gives a visual insight into the effect of cooling conditions on this transformation. The final nucleation number, the number of ferrite grains per austenite grain, ferrite grain size and the kinetics of ferrite formation are obtained as a function of the cooling rate or the undercooling temperature