Materials Science Forum Vol. 879

Abstract: Cubic boron nitride (c-BN) is a wide bandgap III-V compound semiconductor potentially useful for solar-blind photodetectors. This paper describes work on the use of Sulphur doping to adjust the bandgap of c-BN films prepared by plasma-enhanced chemical vapor deposition (PECVD). An S-doped c-BN film based metal-semiconductor-metal (MSM) solar-blind ultraviolet (SBUV) photodetector was successfully fabricated and its electro-optical properties were characterized. The photocurrent shows peak responsivity at 254nm with sharp cutoff wavelengths at 220 and 300 nm, respectively, which is appropriate for use in solar-blind detection. The maximum response reached 1.55×10^-7 A/W/cm² with a rejection ratio of more than three orders of magnitude. The high solar-blind region UV response could be attributed to the successful substitution of boron by Sulphur and the suppression of B vacancies. The experimental results show the same peak in response at around 254nm as is found in the theoretical analysis.

Abstract: In this work, the fourth element Cu was introduced to substitute Ni in polycrystalline Ni-Mn-Sn alloys. It was shown that Cu doping did not change the crystal structure of the martensite in Ni_50-xCu_xMn₃₉Sn₁₁ (x=0, 1, ......,7) alloys, but resulted in the decrease of martensitic transformation temperatures. Due to the higher atomic radius of Cu with respect to that of Ni, the lattice volume of martensite unit cell increases with the gradual substitution of Ni by Cu. For the alloys with the Cu content of 0-4%, the martensitic transformation is from weak magnetic (paramagnetic) austenite to weak magnetic (paramagnetic) martensite. When the Cu content is higher than 4%, the paramagnetic to ferromagnetic transition of austenite was introduced. The temperature interval between magnetic transition and structural transformation was enlarged with the increase of Cu content. Due to the relatively smaller magnetization difference between austenite and martensite, the field induced inverse martensitic transformation behavior is not significant in the present Cu-doped alloys.

Abstract: A novel technique called interdendritic-melt solidified control (IMSC) was developed to manufacture equiaxed superalloy in the efforts of reducing porosity. The effect of process parameter, such as withdrawal rate, of the IMSC on the porosities and mechanical properties of IN792 alloy was investigated, compared with conventional investment casting (CC) technique. The IMSC and CC samples were characterized by optical metallography and scanning electron microscopy. In addition, the minor phases, such as MC carbides and trace amount of η phase, were identified by synchrotron X-ray diffraction. The results indicate that proper withdrawal rate for IMSC can produce castings with much reduced porosity and higher stress rupture properties at elevated temperature compared to conventional investment casting. However, fast withdrawal rate will produce much severe porosity and lower mechanical property.

Abstract: With the growing demand of miniaturization of cell cultivation a new approach towards measuring and sensing bio-analytes needs to be made due to the problem of small volumes (less than 150μl) containing small amounts of analytes. Most of the available glucose sensors monitor the glucose concentration with the help of enzymes, which become very inaccurate in terms of long time measurement and uses (i.e. consumes) glucose during the measurement becoming not available anymore for the cells. Therefore, we focused on applying an enzyme-free glucose sensor based on a microelectromechanical system (MEMS).

Abstract: The determination of mechanical properties can provide insight into complex biochemical processes such as the cell cycle and even the progression of diseases. However, this field of biomechanical research strongly depends on a high-resolution measurement technology and a gentle stimulation method in order to examine these mostly biological samples in their original condition. In terms of this work, a different approach utilizing a modified ESPI setup and two types of ferro-fluids is presented. By exposing the prepared specimen to an electro-magnetic field it has been possible to induce a local displacement and to monitor the deformation with the adapted ESPI setup.

Abstract: We studied three series of Z-phase strengthened steels using scanning electron microscopy, transmission electron microscopy, and atom probe tomography to reveal the detailed microstructure of these steels. In particular, the phase transformation from M(C,N) to Z-phase (CrMN) was studied. Carbon content in the steels is the governing factor in this transformation. The impact toughness of some test alloys was rather low. This is attributed to the formation of a continuous W-rich film along prior austenite grain boundaries. Cu and C addition to the test alloys changed Laves phase morphology to discrete precipitates and improved toughness dramatically. BN particles were found in some steels. Formation of BN is directly linked to the B concentration in the steels.

Abstract: Morphological evolution of carbides in DZ125 superalloy during heat treatment was investigated by using SEM and EDS. The results show that in DZ125 directionally solidified superalloy, the MC carbide is either smooth platelet or bone-shaped structure with nodes in morphology in as-cast state, which is rich in Ta, Ti and Hf. During solid solution treatment, parts of primary MC carbides dissolve, so that the morphology of MC carbide changes from smooth and regular surface to scattered configuration. During aging heat treatment and long term aging, the secondary MC carbides enriched in Hf element precipitated from the matrix. Moreover, there is some M₆C carbide enriched in W element precipitated on the grain boundary. With the process of heat treatment, the content of Hf element in MC carbides increases obviously.

Abstract: This paper aimed to study the quasi-static and dynamic properties of a titanium alloy plate with nominal composition Ti-3.5Al-2.5V-1.5Fe-0.25O (ATI 425^TM titanium alloy with lower limit of Al element) in thickness of 8 mm. The quasi-static tensile properties were investigated by MTS^TM testing system at strain rate of 10^-3 s^-1, while dynamic compression properties by Split Hopkinson Pressure Bar system at strain rate of 3000±200 s^-1. The results show that the quasi-static prperties of the Ti-3.5Al-2.5V-1.5Fe-0.25O plate are comparable to the commercial Ti-6Al-4V plate in 8mm-thick. The Ti-3.5Al-2.5V-1.5Fe-0.25O plate exhibats good dynamic strength. The average dynamic flow stress is 100MPa higher than that of Ti-6Al-4V plate. However, the maximum strain during homogeneous plastic deformation of Ti-3.5Al-2.5V-1.5Fe-0.25O plate is only 50~60% of that of Ti-6Al-4V plate.

Abstract: Hot tearing sensitivity of MgZn_2.5Y_xZr_0.5(x=0.5,1,2,4,6) alloys were predicted by using thermodynamic calculations based on Clyne-Davies’ model, and verified experimentally by using a "T"-type hot tearing permanent-mold equipment. The hot tearing mechanism with different Y content of MgZn_2.5Y_xZr_0.5 alloys was investigated. The results demonstrated that the predicted values are consistent with that of measured values. The hot tearing susceptibility of the alloys from high to low are: MgZn_2.5Y₂Zr_0.5 > MgZn_2.5Y_0.5Zr_0.5 > MgZn_2.5Y₄Zr_0.5 > MgZn_2.5Y₁Zr_0.5 > MgZn_2.5Y₆Zr_0.5. Among the studied alloys, the hot tearing sensitive of the alloys of MgZn_2.5Y₁Zr_0.5 and MgZn_2.5Y₆Zr_0.5 is less than that of the commercial alloy AZ91.

Abstract: Semi-empirical models for predicting the austenite static recrystallization behavior are widely used in designing thermomechanical treatments to improve final mechanical properties. However, a problem with these models is that their utility can be limited to the range of deformation conditions and chemical compositions they were developed for. This work focuses on the study of the applicability of current recrystallization models to the range of low strain conditions and/or high Nb microalloying additions (≈0.1%). To do so, the recrystallization behavior of two low carbon Nb-Ti microalloyed steels (0.04 and 0.11% Nb and ≈0.01% Ti) has been investigated by torsion tests. Experimental results for recrystallization time and recrystallized grain size have been compared to previously developed equations. It has been observed that at low strains (ε = 0.1) the predictions fail. A dependence of the n Avrami exponent both on temperature and applied strain was also found.