Transmission electron microscopy examination of hardening and toughening phenomena in Aermet 100

The effect of tempering on the microstructure and mechanical properties of ultrahigh strength Aermet 100 steel was examined. In the as-quenched condition, the steel contained a dispersion of relatively fine, undissolved, (CrTiFeMo)C and (CrFeMo)₂₃C₆ carbides in a martensitic matrix. Upon tempering at 427 °C, the martensite decomposed to form a high density of cementite particles concomitant with a significant drop in toughness. Tempering at 454 °C resulted in peak strength (yield strength ∼ 1756 MPa) due to the precipitation of coherent zones of fine carbides. The peak in toughness (170 MPa√m), attained at a tempering temperature of 482 °C, was attributed to both the absence of cementite and the formation of reverted, stable austenite. Tempering at higher temperatures resulted in loss of both strength and toughness, which was suggested to be the result of precipitate coarsening and formation of unstable austenite, respectively. The details of the electron microscopy studies and mechanism of strengthening and toughening are discussed in light of the current understanding of this subject.     

An electron microscopy study of chip formation

Chip formation or metal cutting is a unique large strain, high strain rate plastic deformation process. Almost all the previously reported studies of chip formation have examined the problem from the point of view of the mechanics of the deformable bodies using the mathematical theory of plasticity. This study, recognizing the heterogeneous nature of chip formation as encountered in course of machining metals, examines the problem from the metal physical or metallurgical view point. Electron microscopy studies were carried out on steel as well as nonferrous metal chips produced by shop machining conditions and compared to those chips produced by ultramicrotomy. This thin film orthogonal cutting process was employed to produce chips for microscopic examinations under well controlled and repeatable experimental conditions. The experiments carried out were designed to clarify the details of the heterogeneous plastic deformation activity occurring on the microscopic level during machining. The morphological (external surface) characteristics of the chips observed with the scanning electron microscope were correlated with the internal, dislocated structure of the chips observed by transmission electron microscopy methods. The effect of a stacking fault energy (SFE) change in an Ag-Sn alloy on chip thickness ratio(ν _t) is presented for the first time, demonstrating that this deformation process is sensitive to changes in SFE. The essentially discontinuous nature of the chip formation process observed by scanning and transmission electron microscopy is analyzed with a model involving dynamic dislocation behavior in a metal in the presence of large energy dissipation arising from plastic flow to account for the observed instability. N. Y. J. T. BLACK, formerly with the Department of Mechanical Engineering, University of Vermont, Burlington Vt.     

An electron microscopy study of carbide precipitation in vanadium

The precipitation of carbon from supersaturated solid solution in vanadium has been investigated using transmission electron microscopy techniques on thin foils. High purity vanadium strip was doped to a carbon content of about 0.2 at. pct and rapidly quenched in high vacuum, followed by aging treatments for various times in the temperature range from 300° to 600°C. The strip was then thinned for transmission electron microscopy. The carbon is observed to precipitate initially as very finely dispersed carbides visible through structure factor contrast. With increasing aging the precipitate distribution coarsens, and the carbides appear as very thin coherent platelets on {310} planes, showing a pronounced displacement fringe contrast. The coherent precipitate appears to have a bcc structure closely related to that of the matrix. With further aging these platelets are observed to thicken and partially lose coherency, punching out prismatic dislocation loops and helices having axes in 〈111〉 directions. This semicoherent precipitate is found to be the hexagonal V₂C phase described by other researchers, and its orientation relationship with the matrix may be expressed: (110)v‖(00.1)v₂c, [•111]v‖[•110] v₂C. The dislocation loops are a result of the specific volume difference between the V₂C precipitation and the matrix, and have Burgers vector b = a/2〈111〉. Formerly Research Assistant, Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana Ill.     

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111} «uvw» oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.     

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111∼<uvw> oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.     

An electron microscopy study of precipitation in Cu-Ti sideband alloys

Electron transmission microscopy and diffraction have revealed directly the fine-scale precipitation processes and the mechanism of formation of the periodic microstructures which characterize the decomposition of Cu-Ti “sideband” alloys. The initial stages of decomposition involve the development of «100» compositional waves giving rise to the well-known sideband or satellite reflections along the «100» directions. This «wave-like» clustering producing a triaxially modulated structure involves the formation of two disordered phases; ordering does not accompany the initial clustering. Prolonged aging produces a periodic array of interpenetrating rods along the «100» directions of the matrix. During aging the periodic structures coarsen according to a t^1/3 law. The experimental activation energy for the coarsening of the modulated structures is approximately 48 kcal/ mole. Maximum strength is associated with the formation of the transition phase β’ which forms byin situ transformation of the titanium-rich regions; this transformation is not accompanied by a loss of coherency in the modulated structures at the aging temperatures studied in this investigation. It is suggested that these periodic structures arise from spinodal decomposition.     

Elevated temperature deformation of TD-nickel

Sensitivity of the elevated temperature (above 1/2 T_m) deformation of TD-nickel to grain size and shape was examined in both tension and creep. Elevated temperature strength increased with increasing grain diameter and increasingL/D ratio. Temperature sensitivity of the yield stress, as well as high (compared to self diffusion) apparent tensile activation enthalp-ies were the result of the internal stress not being proportional to the shear modulus. Creep activation enthalpies increased with increasingL/D ratio and, to a lesser extent, increasing grain diameter, reaching high values which may be apparent values. The thoria particle dis-persion may have been altered by elevated temperature tensile and creep deformation.     

Orientation imaging microscopy studies of recrystallization in interstitial-free steel

The origin of the γ fiber recrystallization texture in interstitial-free (IF) steel developed during continuous annealing has been investigated by scanning electron microscopy (SEM) and orientation imaging microscopy (OIM). Nucleation of {111∼<uvw> oriented crystals occurs in deformation banded γ grains and therefore a comprehensive study of microstructure of cold-rolled IF steel in the sections perpendicular to the rolling and transverse directions (TDs) and the rolling plane (RP) has been carried out to understand the formation, geometry, and microstructural features of recrystallization. The RP section gave abundant evidence of orientation gradients formed in γ oriented grains that had been subject to orientation splitting to give deformation bands. These orientation gradients across a single grain are around 5 to 30 deg and this orientation difference is sufficient to form nuclei with mobile interfaces during annealing and hence to create chains of γ oriented new grains in the original hot band γ grain envelopes. A grain impingement model requiring orientation pinning is then proposed to explain how these grains, contained in deformed γ grain envelopes, grow out into their neighbors to dominate the final recrystallization texture of IF steel. The α deformed grains contain only small lattice curvatures, and therefore in-grain nucleation is rare. These grains are mostly consumed by invading γ grains toward the end of the recrystallization process.     

An electron microscopy study of modes of intermetallic precipitation in Ti-Cu alloys

The precipitation processes that accompany the aging of supersaturatedα-titanium solid solutions containing up to 6 wt pct Cu have been studied by thin foil electron microscopy and X-ray diffraction techniques. Two mechanisms of decomposition have been identified: i) the heterogeneous nucleation and growth of Ti₂Cu at interfaces and internal substructure such as dislocations, and ii) the uniform nucleation of thin, coherent, disk-shaped precipitates which lie on . The coherent precipitates can form with densities up to 10¹⁷ per cu cm; this and many morphological features of the precipitation process in Ti-Cu are analogous to the well-known behavior of Al-Cu alloys. The coherent precipitates first lose coherency along the edge of the disk and then along the flat faces. The mechanisms by which these two processes occur are considered in detail. Formerly Predoctoral Research Associate, University of Washington, Seattle, Wash.     

Creep deformation of TD-nickel chromium

The creep behavior of thoria dispersed nickel-chromium (TD-NiCr) was examined at 1093°C (2000°F). Major emphasis was placed on 1) the effects of the material and the test related variables (grain size, temperature, stress, strain and strain rate) on the deformation characteristics, and 2) the evaluation of single crystal TD-NiCr material produced by a directional recrystallization technique. Creep activation enthalpies were found to increase with increasing grain size reaching maximum values for the single crystal TD-NiCr. Stress exponent of the steady state creep rate was also significantly higher for the single crystal material as compared to that determined for the polycrystalline TD-NiCr. The elevated temperature deformation of TD-NiCr was analyzed in terms of two parallel-concurrent processes: 1) diffusion controlled grain boundary sliding and 2) dislocation motion. The characteristics of the dislocation motion deformation mode (as observed in the single crystal TD-NiCr) suggest that strong particle-dislocation interactions are present. The relative contributions of dislocation motion and grain boundary sliding in TD-NiCr were estimated. In creep, grain boundary sliding was found to predominate for the small, equiaxed grain structures, whereas the dislocation deformation mode became significant for only the large grain TD-NiCr and the single crystal material. Formerly Graduate Assistant, Case Western Reserve University, Cleveland, Ohio 44106.     

An analytical electron microscopy study of paraequilibrium cementite precipitation in ultra-high-strength steel

To support quantitative design of ultra-high-strength (UHS) secondary-hardening steels, the precipitation of cementite prior to the precipitation of the M₂C phase is investigated using a model alloy. The microstructure of cementite is investigated by transmission electron microscopy (TEM) techniques. Consistent with earlier studies on tempering of Fe-C martensite, lattice imaging of cementite suggests microsyntactic intergrowth of M₅C₂ (Hägg carbide). The concentration of substitutional alloying elements in cementite are quantified by high-resolution analytical electron microscopy (AEM) using extraction replica specimens. Quantification of the substitutional elements in cementite confirms its paraequilibrium (PE) state with ferrite at the very early stage of tempering. The implications of these results are discussed in terms of the thermodynamic driving force for nucleation of the primary-strengthening, coherent M₂C carbide phase. The ferrite-cementite PE condition reduces the carbon concentration in the ferrite matrix with a significant reduction of M₂C driving force. The kinetics of dissolution of PE cementite and its transition to other intermediate states will also influence the kinetics of secondary hardening behavior in UHS steels.     

Diagnostic electron microscopy of neoplasms

OBJECTIVE: We describe a patient with a prolonged and severe hypercapnia occurring during an episode of status asthmaticus induced by ophthalmic instillation of carteolol. SETTING: Prehospital Emergency Medical Service and Pulmonary Intensive Care Unit in a university hospital. PATIENT: A 35-year-old female developed an acute asthma attack while at home, which required advanced life support. INTERVENTION: On hospital admission, arterial blood gases revealed a PaCO2 of 208 mmHg. Hypercapnia persisted with a PaCO2 of more than 190 mmHg for 10 h, with pH always less than 7.00. The patient was finally discharged after 26 days without sequelae. CONCLUSION: This case illustrates the cerebral and cardiovascular tolerance of severe and prolonged hypercapnia associated with major acidosis.     

New kinetic model for primary recrystallization of pure metals

In recrystallization experiments on pure metals, quite often the transformation rate observed at higher fractions transformed is distinctly smaller than predicted by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model.^[1,2,3] Stimulated by recent experimental results, two important changes to the model are suggested: a nucleation rate related to the size of the interfacial area “recrystallized-deformed” present and an exponentially decreasing growth rate for each grain. The modified model still yields a sigmoidal curve for the fraction recrystallized. Yet, plotted in order to determine the Avrami exponent, it shows a very similar deviation as that found in experimental results. Further, it will be discussed how the dependence of the recrystallization process on temperature and magnitude of deformation can be incorporated into the model and what the physical reason for the slowing down of the growth rates of the individual grains might be.