Post-mortem Microstructure Research Articles

Preparation of Glass-Ceramics of Li<sub>2</sub>O-Al<sub>2</sub>O<sub>3</sub>-ZnO-SiO<sub>2</sub> System Coating Toughed by ZrO<sub>2</sub> Basic on Metal Substrate

Glass-ceramics of Li2O-Al2O3-ZnO-SiO2 system coating on Q235 steel surface is prepared by sintering process, and improved hardness and fracture toughness of the coating through add appropriate ZrO2 in glass-ceramics. Post-mortem crystal process, sintering technology and microstructure of the coating were investigated by differential scanning calorimetry (DSC), X-Ray powder diffraction(XRD) and scanning electron microscope(SEM). Maximum bending force and fracture deflection were measured by three point bending method. Results showed that main crystal phase of the coating were Li2Al2Si3O10 and Li8Zn10Si7O28. Crystallization temperature was about 670°C, optimized sintering temperatures was about 810°C, vickers hardness increased from 550 to 898 with an increase of 60%, maximum bending force increased from 1468 N to 1736 N with an increase of 18%, fracture deflection increased from 0.54mm to 0.65mm with an increase of 20%, and have a good combining ability between coating and metal substrate.

Pristine-to-pristine regime of plastic deformation in submicron-sized single crystal gold particles

Pristine single crystalline gold particles with sizes ranging from 300 to 700nm have been fabricated through high-temperature (1150°C) liquid de-wetting of gold thin films atop a specially designed SiO2/Si substrate for in situ transmission electron microscopy testing. Quantitative compression tests showed that these particles display cataclysmic structural collapse immediately following elastic loading to very high stresses (over 1GPa), resulting in a nearly pristine postmortem microstructure despite the large plastic deformation experienced by the particle. This distinct class of dislocation plasticity behavior is attributed to the very high degree of structural perfection of the initial sample, resulting from high-temperature formation or annealing around the melting point. Temporally correlated dislocation nucleation from the contact interface together with the inability to form stable junctions inside is proposed to explain the pristine-to-pristine structural collapse. Upon further compression, once the contact diameter d increases to above a critical value (∼250nm), continuous plastic deformation begins to set in under relatively low flow stress with the postmortem microstructure containing a high density of tangled dislocations, suggesting that a critical dislocation tangling volume under multiple slip is needed for the onset of dislocation storage (robust dislocation jamming) and more conventional plasticity.

Thermal cycle stability of a novel glass–mica composite seal for solid oxide fuel cells: Effect of glass volume fraction and stresses

A novel glass–mica composite seal was developed based on a previously reported concept of “infiltrated” mica seals for solid oxide fuel cells. Ba–Al–Ca silicate sealing glass powder and Phlogopite mica flakes were mixed at glass volume fractions of 10–50 vol.% to make the glass–mica composite seals. The seals were leak tested for short-term thermal cycle stability as a function of glass volume fraction. Composite seals with 10 and 20 vol.% glass were also leak tested under compressive stresses from 3 to 100 psi and helium pressures of 0.2 or 2 psi. Post-mortem microstructure analyses were used to characterize the fracture (leak path) of the glass–mica composite seals and were related to the high temperature leakages. Open circuit voltage tests on dense 8YSZ electrolyte with the glass–mica composite seal showed very good thermal cycle stability over 250 cycles with minute (<1%) voltage drop.

Localized hydrogen cracking in the austenitic phase of a duplex stainless steel

The effects of hydrogen on the ductility loss and fracture behavior of a nitrogen-doped duplex stainless steel were investigated via tensile testing after electrochemical hydrogen pre-charging. Post-mortem microstructure analyses were performed to characterize the microstructural damage evolution and fracture surface. Hydrogen charging of the steel resulted in quasi-cleavage and intergranular fractures, which were associated with transgranular austenite cracking and ferrite/austenite interface cracking, respectively. The crucial factors that resulted in brittle-like cracking were deformation twinning in austenite and nitrogen-related solution hardening.

Elimination of loading reverberation in the split Hopkinson torsional bar

The loading reverberation is a multiple wave effect on the specimen in the split Hopkinson torsional bar (SHTB). Its existence intensively destroys the microstructure pattern in the tested material, and therefore, interferes with the study correlating the deformed microstructure to the macroscopic stress–strain response. This paper discusses the problem of the loading reverberation and its effects on the post-mortem observations in the SHTB experiment. The cause of the loading reverberation is illustrated by a stress wave analysis. The modification of the standard SHTB is introduced, which involves attaching two unloading bars at the two ends of the original main bar system and adopting a new loading head and a couple of specially designed clutches. The clutches are placed between the main bar system and the unloading bars in order to lead the secondary loading wave out of the main bar system and to cut off the connection in a timely manner. The loading head of the standard torsional bar was redesigned by using a tube-type loading device associated with a ratchet system to ensure the exclusion of the reflected wave. Thus, the secondary loading waves were wholly trapped in the two unloading bars. The wave recording results and the contrasting experiments for examining the post-mortem microstructure during shear banding both before and after the modification highly support the effectiveness of the modified version. The modified SHTB realizes a single wave pulse loading process and will become a useful tool for investigating the relation between the deformed microstructure and the macroscopic stress–strain response. It will play an important role especially in the study of the evolution of the microstructure during the shear banding process.