E. A. Trillo

A TEM investigation of M23C6 carbide precipitation behaviour on varying grain boundary misorientations in 304 stainless steels

Transmission electron microscopy (TEM) along with electrochemical potentiokinetic reactivation (EPR) testing was performed on different grades of 304 stainless steel (0.01, 0.025, 0.05, and 0.07%C) in order to assess the sensitization and precipitation behaviour on different grain boundary misorientations. The materials were heat treated at 670°C for 50 h to subject the materials to the sensitization regime. The EPR data and TEM observations revealed that when the amount of carbon was increased the degree of sensitization increased along with the density of precipitates. Large angle misorientations (Θ>15°) were prevalent in all the carbon content materials and the {1 1 0} grain surface orientation was found to be the major texturing orientation. The steels with lower carbon contents nucleated a few small precipitates on high angle grain boundaries, while larger amounts of carbides were observed on lower angle grain boundaries for the higher carbon contents. It was deemed that higher carbon contents required lower energies to nucleate and grow precipitates. A carbon content threshold was found (above 0.05% C) in which precipitates fully saturate the grain boundary. Precipitation followed the energies of different types of boundaries. The highest energy boundary (general random grain boundary) nucleated precipitates first, then precipitation followed on non-coherent twin boundaries, and was not observed on coherent twin boundaries. A “critical nucleation energy”, γgb(crit.), was therefore found to exist at which precipitation will occur on a boundary. This value was found to be in the range of 16 mJ m-2<γgb(crit.)<265 mJ m-2 which corresponds to the energies of special boundaries (coherent and non-coherent portions of twins respectively) at the ageing temperature of 670 °C.

Adiabatic shear bands and examples of their role in severe plastic deformation

The accommodation of severe plastic deformation in impact crater formation, ballistic rod flow and penetration in thick targets, shaped charge formation, and a variety of friction-stir welding and processing is shown to occur by microstructure refinement, particularly dynamic recrystallization in the development of localized or overlapping adiabatic shear bands. Optical metallography and transmission electron microscopy observations of solid-state flow in adiabatic shear zones are compared to illustrate these mechanisms which can often involve intermixed microstructural regimes composed of recovery, recrystallization, and grain growth phenomena.

Combined effects of deformation (strain and strain state), grain size, and carbon content on carbide precipitation and corrosion sensitization in 304 stainless steel☆

Abstract In this investigation we have examined the combined effects of temperature, aging time, carbon content, grain size, strain, and strain state (uniaxial versus rolling deformation) on sensitization development in 304 stainless steel. In general, increasing the carbon content, strain, and strain state (uniaxial → rolling) decreases the sensitization time. Reducing the grain size also accelerates the rate of sensitization development. In addition, for sufficiently small grain sizes, even with superimposed straining, the grain boundaries become dominant in Cr diffusion kinetics.

Copper deposition during the corrosion of aluminum alloy 2024 in sodium chloride solutions

Copper and copper-rich particle clusters were observed to deposit on aging aircraft skin material (Al 2024 sheet coupons) after corrosion immersion experiments for 5 days in acidic (pH 3) neutral (pH ∼ 6), and basic (pH 11) 0.6 M NaCl solutions. SEM analysis employing an EDX spectrometer showed a propensity of large Cu particle clusters on Fe-rich or Fe-containing areas while a TEM inventory of second-phase particles in the alloy sheet showed a propensity of Al-Fe-Cu-Mn and Al-Cu-Si particles along with Al-Cu-Mg and Al-Cu-Fe-Mn-Si particles and particle clusters. A modified replication technique was used to lift particles from the corroded coupon surfaces. TEM analysis employing an EDX spectrometer showed a wide range of copper deposits exhibiting microdendritic morphologies in basic and neutral environments, and botryoidal (or nodular) morphologies in acidic environments. The plating or cementation of copper from solution as an electrochemical displacement reaction appears to be a major contributor to the pitting corrosion of 2024 aluminum alloy.

Evaluation of mechanical and corrosion biocompatibility of TiTa alloys

As-received and heat-treated Ti40Ta and Ti50Ta alloys were evaluated to determine their corrosion as well as mechanical performances and compared to Ti6A14V, a common material utilized for orthopedic (surgical) implants. Anodic potentiodynamic tests performed in PlasmalyteTM showed that all samples, except for the Ti50Ta specimen aged at 400 °C for 3 h gave a curve similar to that of Ti6A14V. Optical and TEM microscopy was performed to determine as-received and heat-treated microstructures. As-received materials showed an α precipitate in an α+β and martensite matrix. Samples that were aged at 400 °C increased in the density and the length of the α precipitate. Vickers hardness measurements were performed to get an approximation of the tensile strengths. Aged Ti40Ta and Ti50Ta specimens produced the highest tensile values when compared to the Ti6A14V material, representing a 31% and 56% increase for the 3 h samples and an 18% and 58% increase for the 10 h samples. Of all the materials studied the Ti50Ta specimen aged for 10 h exhibited the best biocompatibility showing excellent corrosion resistance combined with the highest tensile strength (1089 MPa and 58% harder/stronger than Ti6A14V).

Utilization of Selected Area Electron Diffraction Patterns for Characterization of Air Submicron Particulate Matter Collected by a Thermophoretic Precipitator

Abstract A thermophoretic precipitator (TP) that uses a novelty of direct sampling of ambient air particulate matter (PM) onto transmission electron microscopy (TEM) grids was designed and utilized to determine its potential applicability for the collection and consequent qualitative analyses of representative PM in the air, especially those with aerodynamic diameter less than 1 µm (PM1.0 ). After a calibration process, preliminary field tests were performed under different weather conditions, locations, and time frames. TEM, selected area electron diffraction (SAED), and electron energy-dispersive X-ray spectrometry (EDS) analyses were performed on individual samples, and chemical species were analyzed. During this investigation, individual air PM with different sizes ranging from 10 [H9262]m to 10 nm for TEM analysis was collected. Two observations were made: (1) a large fraction of collected particulates were aggregates of very small particles of both organic and inorganic origin, and (2) a large fraction of the collected particulates were crystalline or polycrystal-line. This study has demonstrated, by utilization of SAED patterns from TEM on air particles collected by a TP, the potential to analyze and identify individual air PM in a nanometer regime qualitatively by combining SAED and EDS data.

Comparison of residual microstructures associated with impact craters in fcc stainless steel and bcc iron targets: the microtwin versus microband issue

Abstract Deformation microtwins characteristic of Neumann bands dominate the residual microstructures beyond a dynamic recrystallization and highly deformed regime below the crater wall of impact craters in polycrystalline bcc iron targets impacted by 3.2 mm diameter iron projectiles at velocities ranging from 0.5 to 3.8 km s −1 . Corresponding impact craters in polycrystalline fcc 304L stainless steel targets impacted by 3.2 mm diameter stainless steel projectiles at velocities ranging from 0.5 to 3.9 km s −1 were observed to have a more limited dynamic recrystallization zone at the crater wall followed by a highly deformed transition into a region of mostly microtwins, but with some intermixing of grains containing microbands.

Dynamic recrystallization-induced flow phenomena in tungsten–tantalum (4%) [001] single-crystal rod ballistic penetrators

Abstract Deformation-flow microstructures associated with [001] W–4% Ta penetrator fragments in a rolled homogeneous steel armor target exhibit dynamic recrystallization. The equiaxed, recrystallized grain structure observed in the deformed penetrator is also associated with soft zones in corresponding microhardness maps. Microstructure evolution is also examined by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED).

Materials science and metallurgy of the Caribbean steel drum. Part I Fabrication, deformation phenomena and acoustic fundamentals

Steel-drum fabrication, especially the sinking of the drum head (also referred to as the “pan”) by hand with a hammer, has been examined in detail utilizing light metallography (LM) and transmission electron microscopy (TEM) to characterize residual microstructures corresponding to reductions in thickness of up to 50 % at the bottom of the drum head. Dislocation densities in the low-carbon (0.01–0.05 wt % C), ferritic steels can exceed 1010 cm-2. Simulations of simple, ideal, free circular notes utilizing 316 stainless-steel plates (0.05 wt % C), cold rolled to reductions up to 40%, revealed that deformation (per cent cold reduction) has an important effect on the acoustic spectrum, especially harmonic spectra. Harmonic-node splitting was observed for thin circular plates (0.076 cm thick); the frequency difference was 60 Hz at 20% cold reduction and 160 Hz at 40% cold reduction. These dispersion effects, due to deformation-induced microstructures, as well as irregularities in the note geometries and thicknesses, point to the complex and non-linear acoustic features that contribute to the unique sounds of the Caribbean steel drum.

Deformation-induced microstructure and martensite effects on transgranular carbide precipitation in type 304 stainless steels

Abstract Plastic deformation of 304 stainless steel (SS) induces transgranular (TG) carbide precipitation, which is critically dependent on deformation-induced microstructural changes occurring during thermal treatment of the SS. Uniaxial deformation of the 304 SS to 40% strain produces a high density of intersecting micro-shear bands composed of heterogeneous bundles of twin-faults and about 12–17% strain-induced α′-martensite at the intersections of the twin-faults. Thermal treatment of 670°C for 0.1–10 h, however, results in a rapid annihilation/transformation of the strain-induced martensite and the concurrent formation of zones containing mixed thermal martensite laths and fine-grained austenite, though the thermal martensite also decreases with increasing heat treatment time. Simultaneous with these thermomechanically-induced microstructural changes, TG chromium-rich carbides form at intersections of twin-faults and on fine-austenite or thermal martensite boundaries in the SS; however, no correlation between strain-induced α′-martensite and carbides was observed in this work. The mechanisms of deformation-induced microstructure and (strain-induced and thermal) martensite effects on TG carbide precipitation in 304 SS are discussed.