Atom probe field ion microscopy of a FeNiB glass

The embrittlement of three grades of nickel following air exposure at 1000 °C is reported. This embrittlement manifests itself in a dramatic loss in tensile ductility around the 800 °C range, and is accompanied by a transition to intergranular failure. Similar behavior has previously been reported in nickel-base superalloys, but is now shown to be a general property of nickel. Tests in a variety of atmospheres have established that the damaging species is oxygen, and that this can have a profound effect, even when present at very low partial pressures. Grain boundaries are observed to be pinned and recrystallization suppressed after oxygen exposure. This lack of boundary mobility has been used to explain the temperature dependence of the embrittlement observed, and has been related to some of the results seen in superalloys. Finally, a few implications for environment-property interactions are considered.

The embrittlement of nickel following high temperature air exposure

Shock experiments on quartz single crystals with initial temperatures −170 to +1000°C showed that ambient temperature does not affect the type of defects formed but can lower the pressure of complete amorphization. The amount of glass recovered increases with both pressure and temperature, and the shock-induced phase transformation of quartz is temperature-activated with an apparent activation energy of <60 kJ/ mol. The phase transformation is localized along three types of transformation lamellae (narrow, s-shaped, and wide) which contain fractured and/or high-pressure phases. Transformation lamellae are inferred to form by motion of linear collapse zones propagating near the shock front. Equilibrium phases, such as stishovite, were not recovered and are probably not formed at high shock pressures: the dominant transformation mechanism is inferred to be solid-state collapse to a dense, disordered phase. Melting occurs separately by friction along microfaults, but no high-pressure crystal phases are quenched in these zones. Shock of quartz thus produces two types of disordered material, quenched melt (along microfaults) and diaplectic glass (in transformation lamellae); the quenched melt expands during P-T release, leaving it with a density lower than quartz, while recovered diaplectic glass has a density closer to that of quartz. At low pressures (< 15 GPa), quartz transforms mostly by shear melting, while at higher pressures it converts mostly along transformation lamellae. We find that shock paleopiezometers using microstructures are nominally temperature-invariant, so that features observed at impact craters and the K/T boundary require in excess of 10 GPa to form, regardless of the target temperature. Shock comminution will be much more extensive for impacts on cold surfaces due to lack of cementation of fragments by melt glass; shock on hot surfaces could produce much more glass than estimated from room-temperature experiments. Because of the shock-impedance mismatch between quartz specimen and steel capsule, the incident shock wave reverberates up to a final pressure. The dynamic compression process is quasi-isentropic with high strain rates. Preheating and precooling achieves final shock pressures and temperatures representative of single-shock states of room temperature quartz and of quartz on known planetary surfaces. Stress histories were calculated by detailed 1- and 2-dimensional computer simulations. The stress history throughout the sample is relatively uniform, with minor variations during unloading. Significant differences between impact pressures calculated by the shock-impedance-match method and specimen pressures calculated by computer simulations indicate the importance of modeling shock recovery experiments computationally.

Shock metamorphism of quartz with initial temperatures −170 to + 1000° C

The redistribution of carbon atoms during the early stages of ageing and tempering of iron-carbon martensites has previously been studied only by indirect methods. The computer-controlled atom probe field ion microscope permits the direct, quantitative determination of carbon concentrations at the atomic level, and thus all the stages of the martensite decomposition process become amenable to direct study. Analyses of a low-carbon martensite, Fe-1.0 at. pct C, (Fe-0.21 wt pct C), water quenched and tempered for 10 min at 150 °C, showed a matrix carbon content of only 0.14 at. pct. Analysis of a 2 nm diam area centered on a lath boundary showed a local concentration of 2.01 at. pct C. There is some evidence that this carbon level is associated with the presence of a thin film of retained austenite at the boundary. In the case of a higher carbon martensite, Fe-0.64 at. pct Mn, 3.47 at. pct C, (Fe-0.65 wt pct Mn-0.78 wt pct C) water quenched and aged for approximately 24 h at room temperature, analysis of twinned regions showed a matrix carbon level of 2.7 at. pct and a concentration enrichment to 6.9 at. pct in a region 2 nm diam, centered on the coherent twin interface. Assuming the segregated carbon to be located in a single atomic layer at the twin interface, this result indicates that a carbon concentration of 24 at. pct exists locally at the boundary. These results appear to be the first direct demonstration of the segregation of carbon atoms to lattice defects in carbon martensites. Tempering of the higher carbon martensite for 1 h at 160 °C produced further segregation of carbon to the region of twin interfaces. The matrix carbon content fell to 1.5 at. pct and the average carbon content over a 2 nm diam region at the interface rose to 8.7 at. pct. The width of the carbon segregated regions also increased, which seems to imply that incipient carbide precipitation in the plane of the twin boundaries is occurring at this stage of the tempering process.

A study of the early stages of tempering of iron-carbon martensites by atom probe field ion microscopy

Structural characterization of some ion-nitrided steels

Field-ion microscope investigations of fine structures in as-quenched and tempered ferrous martensite

Hydrogen-ion microscopy has been used to study microstructural features of tempered martensite in a commercial FeO8% C-0·4% Mn steel, and a comparison between microstructure and measured microhardness has been made. At peak hardness in the early first stage of tempering, a high density of carbides with {100} habits and primarily in the form of very thin platelets is present. Many of these particles are coherent and some are associated with dislocations. As the hardness decreases, the density of particles decreases and their length and width increase rapidly. Their thickness, however, increases very slowly and coherency is maintained for a large number of particles throughout the first stage of tempering. In the late first stage of tempering large incoherent carbide platelets with {112} and {110} habits begin to form; and in the third stage of tempering where the hardness drops more rapidly, a comparatively low density of these large incoherent carbides is found. These results support the theory t...

Field-ion microscopy of tempered martensite

Fine structures of as-quenched martensite in an Fe-0.8% C-0.4% Mn steel were investigated using field-ion microscopy techniques. Both untwinned and twinned martensite regions were detected. Differences in image contrast and regularity of net plane edges were found in the hydrogen-ion images of pure iron and untwinned martensite. It is proposed that in the twinned martensite images the twins appear as bright bands while dark bands represent the intersection of twin boundaries with the specimen surface. The twins were found to have a {112} orientation and a width of 25–100 A.

Field-ion microscopy of ferrous martensite

SUMMARY

Hydrogen-ion microscopy has been used to study first and third stage tempered ferrous martensite. Both e-carbide and cementite particles resulting from the tempering process appeared dark in the images. The image characteristics of the matrix phases in these specimens were intermediate between those of pure iron and as-quenched ferrous martensite. The sharpness of the (111) zone lines and the ‘butterfly’ pattern in {110} regions, the development of high index planes and the regularity of low index net plane edges decreased continuously with increasing carbon content of the matrix phase. The effects of carbon content and lattice strain on specimen geometry, field evaporation, hydrogen etching and field ionization are discussed. It is suggested that carbon atoms in random solid solution lead to an irregular evaporation, similar to that previously proposed for other solid solution alloys, and that the irregular evaporation is enhanced by the associated lattice strain and hydrogen etching process. The irregular end form thus produced lacks the sharp image features found in the smoother end form of the pure metal.

B. N. Ranganathan

Papers

Field-ion microscope investigations of fine structures in as-quenched and tempered ferrous martensite

Field-ion microscopy of tempered martensite

Field-ion microscopy of ferrous martensite

Hydrogen-ion image features of tempered martensite