Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

We have investigated the mechanical behavior of a composite material consisting of a Zr57Nb5Al10Cu15.4Ni12.6 metallic glass matrix with 60 vol pct tungsten particles under uniaxial compression over a range of strain rates from 10−4 to 104 s−1. In contrast to the behavior of single-phase metallic glasses, the failure strength of the composite increases with increasing strain rate. The composite shows substantially greater plastic deformation than the unreinforced glass under both quasi-static and dynamic loading. Under quasi-static loading, the composite specimens do not fail even at nominal plastic strains in excess of 30 pct. Under dynamic loading, fracture of the composite specimens is induced by shear bands at plastic strains of approximately 20 to 30 pct. We observed evidence of shear localization in the composite on two distinct length scales. Multiple shear bands with thicknesses less than 1 µm form under both quasi-static and dynamic loading. The large plastic deformation developed in the composite specimens is due to the ability of the tungsten particles both to initiate these shear bands and to restrict their propagation. In addition, the dynamic specimens also show shear bands with thicknesses on the order of 50 µm; the tungsten particles inside these shear bands are extensively deformed. We propose that thermal softening of the tungsten particles results in a lowered constraint for shear band development, leading to earlier failure under dynamic loading.

The effect of treatment time on the microstructure of AISI 304 austenitic stainless steel during liquid nitrocarburizing (LNC) at 703 K (430 °C) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Experimental results revealed that the modified layer was covered with the alloy surface and the modified layer depth increased extensively from 2 to 33.4 μm with increasing treatment time. SEM and XRD showed that when the 304 stainless steel sample was subjected to LNC at 703 K (430 °C) for less than 4 hours, the main phase of the modified layer was expanded austenite. When the treatment time was prolonged to 8 hours, the abundant expanded austenite was formed and it partially transformed into CrN and ferrite subsequently. With the increased treatment time, more and more CrN precipitate transformed in the overwhelming majority zone in the form of a typical dendritic structure in the nearby outer part treated for 40 hours. Still there was a single-phase layer of the expanded austenite between the CrN part and the inner substrate. TEM showed the expanded austenite decomposition into the CrN and ferrite after longtime treatment even at low temperature.

The deformation and fracture behavior of simulated heat-affected zones (HAZ) within HSLA-100 and HY-100 steel weldments has been studied as a function of stress state using notched and unnotched axisymmetric tensile specimens. For the case of the HSLA-100 steel, the results for fine-grained, as well as coarse-grain HAZ (CGHAZ) material, show that, despite large differences in the deformation behavior when compared to base plate or weld metal, the failure strains are only weakly dependent on the thermal history or microstructure. Ductile microvoid fracture dominates the failure of the HSLA-100 steel with small losses of ductility occurring in the HAZ conditions only at high stress triaxialities. In contrast, the HY-100 steel is susceptible to a large loss of ductility over all of the stress states when subjected to a severe, single-pass simulation of a CGHAZ. The ductility loss is greatest at the high stress triaxiality ratio in which case failure initiation occurs by a combination of localized cleavage and ductile microvoid fracture.

Các đĩa của hợp kim magiê AZ31 được ép đùn đã được xử lý bằng phương pháp xoắn áp lực cao (HPT) ở áp suất 6.0 GPa qua 1/4, 1 và 5 vòng quay, ở cả nhiệt độ phòng (296 K (23 °C)) và ở nhiệt độ cao hơn là 463 K (190 °C). Các mặt cắt ngang của các đĩa đã được kiểm tra sau khi chế biến, và cho thấy có một mức độ không đồng nhất cao trong tất cả các mẫu. Sự không đồng nhất này được thể hiện qua tính chất của các mẫu chảy, qua sự phân bố kích thước hạt, và qua các phép đo độ cứng vi mô toàn diện. Các kết quả cho thấy có thể tăng gấp đôi độ bền của hợp kim ở một số vùng của đĩa sau khi chế biến qua 5 vòng quay ở nhiệt độ phòng.

Fully austenitic Fe-28Mn-10Al-1.0C steel with high stacking fault energy exhibited exceptionally high uniform elongations (85 to 100 pct) and total elongations (100 to 110 pct) at room temperature. The origin of such exceptional room-temperature ductility was rationalized in terms of strain accommodation mechanisms of reduction of glide plane spacing in Taylor lattice (TL) formation at low strains and TL rotation forming domain boundaries (DBs) and microbands (MBs) at high strains.

An aluminum metal matrix composite (MMC) brake drum was tested in fatigue at room temperature and extreme service temperatures. At room temperature, the hybrid composite did not fail and exceeded estimated vehicle service times. At higher temperatures (62 and 73 pct of the matrix eutectic), fatigue of a hybrid particle/fiber MMC exhibited failure consistent with matrix overloading. Overaging of the A356 matrix coupled with progressive fracture of the SiC particles combined to create the matrix overload condition. No evidence of macro-fatigue crack initiation or growth was observed, and the matrix–particle interface appeared strong with no debonding, visible matrix phases, or porosity. An effective medium model was constructed to test the hypothesis that matrix overloading was the probable failure mode. The measured particle fracture rate was fit using realistic values of the SiC Weibull strength and modulus, which in turn predicted cycles to failure within the range observed in fatigue testing.

The hot compression deformation behavior and microstructural evolution of an Fe-Cr-W-Mo-V-C steel have been investigated by hot compression deformation experiments carried out at 900 °C to 1150 °C and under strain rates varying from 10 to 0.1 s−1. The results revealed that the flow stress decreased with decreasing strain rate, while increasing deformation temperature led to a lower flow stress. An Arrhenius-type equation was used to analyze the effects of the strain rate and deformation temperature on the plastic flow behavior of the steel. Based on this equation and the experimental results, the average activation energy was calculated to be 747.7 kJ/mol. The tested samples were subjected to careful microstructural examinations, with a focus on determination of the dynamic recrystallization (DRX) grain sizes. A straightforward contour map correlating the DRX grain sizes with the different deformation conditions was drawn. According to this map and the microstructural examination results, the optimum hot working parameters enabling us to obtain appropriate DRX microstructures have been identified at 0.1 s−1 for the strain rate and 1100 °C for the deformation temperature.