Jhewn-Kuang Chen

Interfacial structure and growth mechanisms of lath-shaped precipitates in Ni-45 wt° Cr

Abstract The interfacial structure of bcc laths precipitated from a nickel-rich fee matrix were studied using transmission electron microscopy and high-resolution electron microscopy (HREM). 50 precipitates and more than 200 defects in their vicinity were analysed. HREM images indicate the parallel conjugate planes of the Kurdjumov-Sachs orientation relationship are continuous across the broad face of the laths. In agreement with earlier studies, the (121)f habit plane contains structural ledges, irregularly spaced dislocations but no regular array of misfit dislocations. 87° of dislocations in the habit plane have Burgers vectors in the conjugate plane. These dislocations are found in the risers of growth ledges and must climb as the ledges migrate. The dislocations accommodate misfit in the conjugate planes of the orientation relationship while misfit perpendicular to these planes is compensated by elastic strain. Stacking faults found extending into the matrix from the precipitate habit plane accommoda...

The role of atomic matching and lattice correspondences in the selection of habit planes

Abstract A procedure for predicting precipitate facet planes is demonstrated. The approach is based upon finding interphase boundaries with a high density of near-coincidence sites. Such near-coincidence site boundaries are identified by analyzing computer-generated lattice correspondences. For f.c.c./b.c.c. transformations, numerous lattice correspondences yield a similar degree of matching in 3-dimensions but substantially different matching in two-dimensions. The (1 2 1) f habit plane of precipitates in NiCr is shown to be a near-coincidence site boundary in which each near-coincidence site lattice cell contains three atoms. This boundary is identified using a non-Bain type of lattice correspondence. However, the observed interfacial structure, growth direction, and facet planes of these precipitates suggest the selection of the habit plane results from geometric matching rather than the existence of a lattice correspondence during the growth process. A lattice correspondence is also proposed that explains the 4 9 4 habit plane of martensite in Fe8Cr1C.

Effects of alloying elements upon austenite decomposition in Low-C steels

The kinetics of austenite decomposition were studied in high-purity Fe-0.1C-0.4Mn-0.3Si-X (concentrations in weight percent;X represents 3Ni, 1Cr, or 0.5Mo) steels at temperatures between 500 °C and 675 °C. The transformation stasis phenomenon was found in the Fe-C-Mn-Si-Mo and Fe-C-Mn-Si-Ni alloys isothermally transformed at 650 °C and 675 °C but not in the Fe-C-Mn-Si and Fe-C-Mn-Si-Cr alloys at any of the temperatures investigated. The occurrence of transformation stasis was explained by synergistic interactions among alloying elements. The paraequilibrium model was applied to calculate the metastable fraction of ferrite in each alloy. This fraction was shown to coincide with cessation of transformation in the Mo alloy transformed at 600 °C. Transformation stasis was found in both the Ni and the Mo alloys isothermally reacted at 650 °C and 675 °C. The interactions among Mn, Si, and Mo, as well as interactions among Mn, Si, and Ni, appear to decrease the threshold concentrations for transformation stasis in Fe-C-Mn-Si systems. Segregation of Mn and Mo to the α/yγ boundary, assisted by the presence of Si, was suggested to enhance the solute draglike effect (SDLE) and lead to transformation stasis. In the Ni alloy, a lower driving force for ferrite formation resulting from the Ni addition could be responsible for the occurrence of transformation stasis.

The elastic strain energy of growth ledges on coherent and partially coherent precipitates

The formation rate of growth ledges on a faceted precipitate strongly affects the growth kinetics and the shape of the precipitate. An Eshelby-type model is used to compare the strain energy associated with the nucleation of a ledge on different facet planes of a body-centered cubic (bcc) precipitate in face-centered cubic (fcc) matrix. Ledge nucleation is only likely at facet areas where the interaction energy between the ledge and the precipitate is negative. The strain energy for ledge formation is not symmetric on any of the facet planes, but it is symmetric about the center of the precipitate. For coherent precipitates comparable to those observed in the Ni-Cr system, ledges form with the lowest strain energy on the broad facet of the precipitate implying that precipitate thickening should occur faster than lengthening and widening. A procedure for modifying the Eshelby model is suggested in order to allow strain-energy calculations of partially coherent precipitates. The strain energy for ledge formation on at least one type of partially coherent lath is lowest for a ledge located on the facet perpendicular to the crystallographic invariant line (IL). This situation favors precipitate lengthening in the invariant line direction.

Effects of Zinc Oxide and Porosity on Permittivity of Sintered Zinc Sulfide-Silicon Dioxide

The dielectric constants of hot-pressed ZnO-containing 80 at. % ZnS–20 at. % SiO2 dielectric ceramics are analyzed at a frequency range varied from 1 MHz to 1 GHz. Their relative densities are controlled to range from 85 to 99% by varying the hot press pressures. A linear relationship is observed between dielectric permittivity and sintering density. The measured permittivity at 13.56 MHz frequency is found to range from 6.3 to 8.03, which is higher than the estimated values for composite materials using the Bruggeman and Lichtenecker equations. The large grain boundary area arising from nanosized grains contributes to the increase in permittivity. Additions of 0.5–5 wt % zinc oxide then increase the permittivity by forming nanosized Zn2SiO4 willemite phase on the surfaces of SiO2 particles. The presence of ZnO also stabilizes the high-temperature ZnS wurtzite phase to room temperature.

Effects of Nd and rotary forging on mechanical properties of AZ71 Mg alloys

Abstract The effects of Nd addition on the mechanical properties and plastic deformability of AZ71 Mg alloys were investigated. 0.5%–2.0% (mass fraction) Nd was added to AZ71 Mg alloys. The grain size and the amount of brittle β;-Mg 17 Al 12 phase reduce with increasing the Nd addition, while nanosized Al x Nd y precipitates form. In combination with 32% rotary forging and subsequent annealing, the grain size of Nd-added AZ71 Mg alloys reduces greatly from over 350 μm to below 30 μm. Both tensile strength and ductility increase with the Nd addition up to 1.0%. The addition of Nd beyond 1.0% leads to the aggregations of rod-shaped Al 11 Nd 3 and blocky Al 2 Nd precipitates, thereby deteriorating both strength and ductility. The 1.0% Nd-added AZ71 Mg alloy shows tensile strength up to 253 MPa and elongation of 10.7%. It is concluded that adding 1.0% Nd to AZ71 Mg alloy yields the optimum toughness, whether under as-cast or rotary forging and annealing conditions.

Optimization of Ge-Sb-Sn-O Films for Thermal Lithography of Submicron Structures

Ge–Sb–Sn inorganic resist materials are developed to fabricate submicron structures that have a pattern size smaller than 150 nm and a depth over 100 nm via thermal lithography. The materials are sputtered under Ar:O2 atmospheres by varying O2 flow rate and Ge level. Both increasing O2 flow rate and Ge level can decrease the extinction coefficient, k, at 405 nm wavelength to within a range of 0.58–0.89. Films with appropriate absorption of laser power are chosen to optimize exposure and development processes. A continuous 2.0 mW laser power is used to crystallize the Ge–Sb–Sn–O films. 175-nm-wide and 107-nm-deep grooves are formed after development using alkaline solutions. The 175 nm width is well below half of the 380 nm diffraction limit. A laser pulse strategy is also developed to fabricate discrete dot patterns. By shortening the write 1 time to 0.5 T (7.8 ns), the dot patterns change from oval to round shape. A pit pattern as small as 140 nm diameter and 100 nm depth is achieved.

Phase transformations and phase equilibria in Ti -25V -N system at 1200°C

Abstract The nature of phase transformations and phase equilibria in the Ti- V - N system with about 20 at.-%V and 11 -15 at.-%N has been studied at 1200°C. The results obtained for the titanium rich corner of the Ti - V- N phase diagram provide new information on phase coexistence and the composition of particular phases in this system. Thermodynamic calculations, based on the use of thermodynamic parameters from the three binary subsystems, together with newly assessed thermodynamic parameters published previously, were carried out. Reasonable agreement was obtained between the experiment and the calculations.

The Effects of Adding TiC Powders to VANADIS 4 Tool Steel by HIP Treatment

In this study, the commercial VANADIS 4 (V-4) tool steel powders with sifting classification below 25 μm to be the matrix with fine titanium carbide (TiC) powder to produce a new material with high hardness and wear resistance, via powder metallurgy, sintering and HIP (Hot Isostatic Pressing) process. Experimental results showed that the TRS of original V-4 steel powder was 678.5 MPa, but below 25 μm of V-4 steel powder adding 35 wt% TiC enhanced to 868.6 MPa through 1673 K sintered. Beside, the hardness increased to HRA 86.2, TRS reached 1059.3 MPa, and porosity decreased to 1.0% of the V-4 steel powders (below 25 μm) added 35 wt% TiC after 1673 K sintered and heat treatments. Furthermore, HIP treatment can improve the microstructure and mechanical properties of V-4 composite material. TRS of V-4 composite steel increased to 1180.4 MPa and hardness was HRA 87.4 (HRC 71.7), porosity decreased to 0.71% after 1673 K sintered and HIP (1523 K, 150 MPa, 1 hour) treatments.

Microstructural changes of the hsp phase in Ti-25V-N alloys annealed at 1273K

The increasing practical importance of mixed Ti and V compounds (usually in the form of nitrides, carbides and carbonitrides) has focused attention to the Ti-V-N system and its subsystems. A comprehensive review of the Ti-N system was given by Wriedt and Murray. In the latest overview of the ternary Ti-V-N system the lack of sound experimental data in the Ti-rich corner of the phase diagram was noted. The overall microstructure evolution of Ti-25V-N system at 1,473K together with phase diagram modeling is reported. The aim of the present work is to report on the interesting internal microstructure of the hcp phase resulting from the annealing of Ti-25V-N alloys at 1,273K.