J. B. Vander Sande

Niobium carbonitride precipitation and austenite recrystallization in hot-rolled microalloyed steels

The response of austenites to thermomechanical treatments is studied in a series of niobium (columbium) HSLA steels. Interactions between composition, plastic deformation, strain-induced precipitation, and austenite recrystallization are described and related to previous work in the field. Niobium in solution prior to deformation leads to significant retardation of subsequent austenite recrystallization if Nb(C,N) precipitation takes place prior to or during the early stages of recrystallization. Such straininduced precipitation proceeds in two stages: initially at austenitic grain boundaries and deformation bands, and later on substructural features in the unrecrystallized austenite. The latter precipitation is accelerated only if it occurs in the unrecrystallized austenite; if recrystallization precedes Nb(C,N) precipitation, then the precipitation reaction is much slower. Thus, the Nb(C,N) precipitation and austenite recrystallization reactions are coupled phenomena. The conditions necessary for such an interaction are analyzed, and it is proposed that the level of supersaturation of Nb(C,N) in the austenite at the deformation temperature is a critical factor in determining whether or not an effective interaction will operate at that temperature.

New technique for decorating dislocations in olivine.

Oxidation of iron-rich olivine induced in the laboratory causes preferential precipitation on lattice dislocations. This simple dislocation decoration technique greatly reduces the cost and time involved in surveying the dislocation structures of deformed olivine crystals and opens the way to a more thorough understanding of the deformation of this important geologic material.

The effect of nitrogen on stacking fault energy of Fe-Ni-Cr-Mn steels

The effect of nitrogen content on stacking fault energy (SFE) has been measured in a series of Fe-21Cr-6Ni-9Mn alloys. Stacking fault energies were determined from node measurements using weak beam imaging techniques in transmission electron microscopy. Nitrogen additions lower the SFE from 53 mJ/m2 at 0.21 wt pct to 33 mJ/m2 at 0.24 wt pct. Further increases to 0.52 wt pct do not markedly change the SFE. Carbon and silicon had no effect on SFE in the ranges 0.010 to 0.060 wt pct C and 0.17 to 0.25 wt pct Si. The shift in SFE from 0.21 to 0.24 wt pct N is accompanied by a transition to a more planar plastic deformation mode. The sharp transition precludes the use of linear regression analysis for relating SFE to nitrogen content in this class of alloys.

Spinodal decomposition during aging of Fe-Ni-C martensites

A collaborative study of the aging of virgin Fe-Ni-C martensites has combined the techniques of transmission electron microscopy (TEM), atom-probe field-ion microscopy (APFIM), and electrical resistometry. Aging at room temperature leads to the rapid development of a finescale structural modulation along 〈203 〉 lattice directions. Atom-probe analysis of Fe-15Ni-lC martensite reveals the formation of carbon-rich regions whose carbon concentration increases with time and approaches 11 at. pct C on prolonged aging. The early stage kinetics of this process are composition-dependent and are consistent with carbon-diffusion control. The morphological features of the aging reaction are explained by elastic strain-energy considerations. In accordance with previous thermodynamic models, it is concluded that virgin Fe-C martensites are unstable and that phase separation occurs by a spinodal mechanism. The martensitic substructure does not appear to exert any substantial influence on this decomposition behavior.

Carbide precipitation during stage I tempering of Fe-Ni-C martensites

Microstructural changes which accompany the first stage of tempering have been studied by transmission electron microscopy (TEM) and electrical resistometry in two Fe-Ni-C alloys that form platelike martensite. The ε-carbide transition phase in these alloys adopts a platelike shape with a habit plane near {012=α. Electron diffraction data indicate that the carbide may be partially ordered, resulting in orthorhombic symmetry, and therefore, this phase is designated as ε′- carbide. The carbide particles contain a fine internal substructure which appears to represent an internal accommodation deformation (faulting) on the carbide basal plane. Detailed analysis of the kinematics of carbide precipitation suggests that the observed habit plane and accommodation deformation permit an invariant-plane strain transformation which minimizes elastic strain energy. The apparent selection of only a limited number of possible orientation variants is explained in terms of the symmetry of the parent martensitic phase, which is known to undergo spinodal decomposition prior to the nucleation of the transition carbide. The martensitic substructure is not found to exert any significant influence on this overall precipitation behavior.

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.

Refractory amorphous inter-transition metal alloys☆

NbNi and TaNi alloys were the first non-crystalline inter-transition metal alloys to be retained by splat cooling. In the meantime, several related amorphous alloys such as ZrNi and ZrCu have been prepared in a similar way. It was then not clear whether the Nb and Ta alloys were amorphous or microcrystalline, and the second interpretation was preferred. The present work suggests that these alloys are amorphous in the same way as other metallic glasses. They have high thermal stability, crystallizing rapidly only at temperatures approaching 1000 K; a high microhardness (1020 Kg/mm2) was also found. Results to be reviewed include X-ray diffraction data, high-resolution transmission electron microscopy, Mossbauer spectroscopy, differential scanning calorimetry, SEM fractography and microhardness. As discussed, these results either directly confirm the proposed amorphous structure or are at least consistent with it.

Crystallization of an amorphous Cu-Zr alloy

Abstract A Cu-40 at. % Zr alloy was splat cooled into a non-crystalline structure. The crystallization behavior of this alloy was studied by calorimetry, transmission electron microscopy, and hardness testing. Calorimetric studies indicated that crystallization of the initial non-crystalline material occurred by a single exothermic reaction. The heat release was 1260 cal/mole and occurred at 480°C. Evidence for a glass transition at approx. 450°C was also found. Samples previously isothermally annealed below the crystallization temperature exhibited two exothermic reactions when studied calorimetrically. The additional second peak increased in size and shifted to temperatures higher than the first peak with increased annealing. Transmission electron microscopy indicated that crystallization occurred by a nucleation and growth mechanism during isothermal annealing. The crystallites were found to have a fine substructure with a subgrain size of approx. 300–500 A. The crystallization product was single phase. Calorimetry results, direct observations from transmission electron microscopy, and changes in the electron diffraction pattern of annealed samples led to the conclusion that the initial non-crystalline matrix transforms to a second non-crystalline structure which, in turn, transforms to the crystalline phase. Hardness measurements were performed to follow the crystallization process. Results indicated that hardness monitors the transformation of the initial non-crystalline structure with high sensitivity.

Transmission electron microscopy investigation of the defect microstructure of four natural orthopyroxenes

The defect structure of crustally deformed orthopyroxenes from a dunite, a peridotite, and a pyroxenite are characterized and their defect structures are compared with that of an orthopyroxene of a lherzolite from a volcanic xenolith. The microstructures contained isolated unit dislocations, isolated stacking faults, and Ca-rich, clinopyroxene lamellae. The isolated dislocations have Burgers vectors, b, which were predominantly [001]. The stacking faults have a displacement vector R =1/4[001]. A lamellae consisted of a 1/4 μ wide Ca-rich region bounded by complex dislocation arrays. These lamellae are usually 100 μ or more in length and are nearly parallel to the (100) in the matrix. The dislocations in the boundary regions are spaced about 500 Å apart. The lherzolite orthopyroxenes were nearly free of isolated defects, in comparison to the other samples. Annealing at 1390° C for 1 hr produced no detectable recovery of the isolated defects in the orthopyroxene substructure.

Phase stability limits of Bi2Sr2Ca1Cu2O8+δ and Bi2Sr2Ca2Cu3O10+δ

We determined the phase stability limits of Bi2Sr2Ca1Cu2O8+δ and Bi2Sr2Ca2Cu3O10+δ in the temperature range 650–880 °C using a solid‐state electrochemical technique. These phases decompose by incongruent melting above ∼790 °C, whereas they decompose by a solid‐state reaction at lower temperatures. The solid‐state decomposition reaction is reversible for Bi2Sr2Ca1Cu2O8+δ, but not for Bi2Sr2Ca2Cu3O10+δ.