The microstructure of discontinuously precipitated lamellae in an austenitic Fe-42.4 Wt Pct Ni- 4.15 Wt Pct AI-0.45 Wt Pct C alloy

Phase decomposition of austenitic Fe-42.4 wt pct Ni-4.15 wt pct Al-0.45 wt pct C on aging at 823 K was investigated by means of electron microscopy, selected area diffraction and microdiffraction, and microprobe chemical analysis. During annealing, κ phase of L′l₂ structure fully coherent with the matrix formedvia the nucleation and growth mechanism. The matrix phase and cubic κ carbide were later gradually encroached upon by discontinuously precipitated lamellar phases. The duplex fine lamellae are composed of alternately arranged carbon-depleted Ll₂ phase and cementite. Between the two constituents in the lamellae, the Pitsch orientation relationship is fulfilled, and at the same time, the matrix phase of the grain which the discontinuously precipitated lamellar colony has left behind maintains the crystallographic cube-to-cube correspondence with the product Ll₂ phase.     

Pulsed laser induced deformation in an Fe-3 Wt Pct Si alloy

The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5×10⁸ w/cm² caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning. B. A. WILCOX, formerly affiliated with Battelle, Columbus Laboratories     

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.     

Intergranular fracture in an Al-15 Wt Pct Zn alloy

Crack propagation rates for the intergranular fracture of an Al-15 wt pct Zn alloy tested in air, distilled H₂O, and 0.5M NaCl have been studied using a double cantilever beam specimen. This technique has been applied to two different types of specimens: polycrystals with large equiaxed grains and bicrystals. The susceptibility to intergranular fracture in air and 0.5M NaCl increases as the volume fraction of G.P. zones in the matrix increases. The microstructure which is most susceptible to fracture exhibits intense planar slip traces while the more resistant microstructures have a more diffuse surface slip pattern. The dependence of cracking rates on the microstructure is explained in terms of the blocking of inhomogeneous plastic flow in the matrix by grain boundaries, resulting in locally severe stress concentrations at the boundaries. Bicrystal tests substantiate these results and show that orientations favoring partial continuity of slip bands across grain boundaries are less susceptible to stress corrosion cracking than arbitrarily oriented boundaries with no slip continuity. WILLIAM J. KOVACS, formerly Graduate Student, Carnegie-Mellon University, Pittsburgh, Pa. This paper is based upon a thesis submitted by WILLIAM J. KOVACS in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Metallurgy and Materials Science at Carnegie-Mellon University.     

Deformation behavior of an Al-3.37 Wt Pct Li alloy

Al-3.37 wt pct Li alloy was deformed by differential strain rate and constant initial strain rate test techniques to investigate deformation and failure behavior over the strain rate range of 10^-5 to 10^-2 s^-1 and the temperature range of 22 °C to 580 °C. Flow stress first increases then decreases with an increase in test temperature, whereas ductility shows a sigmoidal relationship with the test temperature. The maximum ductility of about 80 pct is obtained at intermediate strain rate and 550 °C. Failure is noted to occur by cavity interlinkage and crack formation. Strain rate sensitivity (m) and activation energy (Q) for deformation are determined to be 0.04 to 0.13 and 96.2 to 157.4 kJ/mol, respectively. Toward lower test temperatures, both them andQ are found to have lower values. Deformation at high temperature is suggested to be controlled by dislocation climb. However, under non-steady-state conditions due to cavitation,m andQ both vary with strain. Formerly B. Tech. Final Year Student, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay.     

Ferrite and carbide morphologies in and below the “bay region” of an Fe-0.19 Pct C-2.30 Pct Mo alloy

Optical and transmission electron microscopy were used to study ferrite and carbide morphology from temperatures just above to well below the bay temperature in an Fe-0.19 pct C-2.30 pct Mo alloy. Suppression of idmanstätten ferrite morphologies at the bay and their reappearance below, initially in highly degenerate form, previously reported by Boswellet al. in Fe-C-2 pct Mo alloys, were confirmed. These effects were again explained in terms of a solute drag-like effect. Just above the bay, the usual fibrous and interphase boundary Mo₂C structures appear. However, fibrous carbides disappear immediately below the bay whereas interphase boundary carbides continue to form until approximately 60 K below the bay. In a narrow temperature range below the bay, large, widely spaced Mo₂C laths are also observed. Many laths are associated with terraces and risers of ferrite superledges; some, however, nucleate on dislocations within ferrite. Nucleation of Mo₂C on such dislocations occurs throughout the temperature range investigated (898 to 748 K). At the lower temperatures studied, (Fe, Mo)₃C appears as plates approximately 10³ longer than those of Mo₂C also formed at these temperatures. Explanations for these observations on carbides are offered in terms of interactions between ferrite and carbide precipitation and the concepts of ledgewise diffusional growth Orientation relationships between ferrite and carbides in several types of microstructure are reported, but the equivocal nature of the information which these data supply about transformation mechanism is emphasized.     

Creep mechanism in Fe-1.8 At. Pct Mo alloy at high temperatures

High temperature creep of Fe-1.8Mo alloy, the stress exponent,n, of which is about 4, has been studied to check whether then value can be a good indication of the creep mechanism or not. Creep tests were carried out at 1124 K under 5.9 to 39.2 MPa. After sudden stress increments during the steady-state creep, inverse-type transient creep curves were obtained with no instantaneous permanent strain. Mean internal stresses were determined by stress-transient dip-tests using a back extrapolation technique. The mean internal stress was obviously smaller than the applied stress. These results indicate that creep deformation of Fe-1.8Mo alloy under the present conditions is controlled by the viscous glide motion of dislocations, thoughn is not close to 3. Steady-state creep rates as well as the value ofn calculated from the Orowan-type equation using experimentally obtained values for every parameter, are in reasonable agreement with the observed ones. These findings suggest that classification of creep behavior according to then value is not appropriate in some cases for discussing mechanisms of high temperature creep. Formerly was a Graduate Student, Tohoku University.     

Formation mechanism of bainitic ferrite in an Fe-2 Pct Si-0.6 Pct C alloy

The bainite transformation at 723 K in an Fe-2 pct Si-0.6 pct C alloy (mass pct) was investigated with transmission electron microscopy (TEM) and quantitative metallography to clarify the growth mechanism of the ferritic component of bainite. In early stages of transformation, the bainitic ferrite was carbide free. The laths of bainitic ferrite within a packet were parallel to one another and separated by carbon-enriched retained austenite. The average carbon concentration of the bainitic ferrite was estimated to be 0.19 mass pct at the lowest, indicating that the ferrite was highly supersaturated with respect to carbon. The laths did not thicken during the subsequent isothermal holding, although they were in contact with austenite of which the average carbon concentration was lower than the paraequilibrium value. In the later stage of transformation, large carbide plates formed in the austenite between the laths, resulting in the decrease in the carbon concentration of the austenite. Subsequently, the ferrite with a variant different from the initially formed ferrite in the packet was decomposed for the completion of transformation. The present results indicate that the bainitic ferrite develops by a displacive mechanism rather than a diffusional mechanism. Formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Modeling of primary and secondary dendrites in a Cu-6 Wt Pct Sn alloy

Microstructure of gas-atomized CuSn6 particles has been investigated using scanning electron microscopy (SEM), and it is shown that the dendrite arm spacing (DAS) is related to the particle diameter (d) so that DAS=0.19d ^0.72. Formation of microstructures in the particles are modeled using a numerical solidification model. This model concerns tips of cells and dendrites, but in the present investigation, it is, in a simple manner, extended to comprise whole cells and dendrites. Furthermore, ripening of dendrite arms is taken into consideration. It is found that for increasing growth rates there is a transition from dendrites to cells when the growth velocity approaches the diffusional velocity in the melt,i.e., when the Peclet number is equal to one. It is also shown that both primary stem spacing and dendrite spacing are related to the ratio between the volume in the liquid where there is solute diffusion and to the surface area of the cells and dendrites (D/A). The relation between spacing and D/A is the same for cells and dendrites, indicating that the spacing selection is controlled purely by solute diffusion in the melt.     

A study of the early stages of tempering in an Fe-1.2 Pct alloy

Mössbauer effect spectroscopy has been used to study changes in the microstructure of an Fe-1.22. wt pct C alloy due to tempering between 373 and 523 K. The orthorhombic transition carbide, η-Fe₂C, was identified by transmission electron microscopy and the similarity of ∈-carbide electron diffraction patterns to η-carbide diffraction patterns is noted. Systematic changes in the Mosbauer parameters of martensite and austenite are presented for the various stages of tempering. The same amount of C remains randomly dissolved in the retained austenite throughout tempering and some C is retained in the martensite throughout the range of transition carbide formation. Two sets of Mössbauer parameters corresponding to magnetic phases other than martensite and cementite have been found. These parameters may come from η-carbide, but alternative interpretations are presented.     

Room-temperature deformation and stress-induced phase transformation of laves phases in Fe-10 At. Pct Zr alloy

The microstructure within particles of the Laves phase Fe₂Zr in a two-phase Fe-10 at. pct Zr alloy after compression was investigated using X-ray diffraction and electron microscopy. Stressinduced phase transformation between C36 and C15 structures was found to be a major deformation mode for the Laves phase. Twinning and stacking faults were also found within C15 regions. Phase transformation models based on partial dislocations are discussed.     

An examination of the interparticle contact area during sintering of W-0.3 Wt Pct Co

As a powder compact sinters, its microstructure evolves. One way to quantify the scale of the microstructure is to consider the interparticle contact area. This study examines two known models for calculating the interparticle contact area: the classic two-sphere model and the Voronoi cell model. Both models have particular assumptions about the microstructure that make them not applicable for treating densification to near full density with concurrent grain growth. The classic two-sphere model assumes a regular packing of particles and a perfectly spherical particle geometry and neglects an increasing particle coordination number with sintering. The Voronoi cell model assumes that the scale of the microstructure remains constant; i.e., as long as the compact is densifying, grain growth does not occur. We propose a modified Voronoi cell that accounts for an increasing grain size, making it applicable to a general case where grain growth occurs during sintering. The three models are compared to the interparticle contact area data, obtained by stereology techniques, for W-0.3 wt pct Co sintered from green state to near full density. The original Voronoi cell model fits the data only at low temperatures, before the onset of grain growth. Below approximately 90 pct relative density, the two-sphere model with an assumed coordination number of six (coordination number in a green compact) and the modified Voronoi cell model provide a good fit to the data. At higher densities, both models overestimate the interparticle contact area.     

The morphology,crystallography, and mechanism of carbide precipitation in an Fe-0.12 Pct C-3.28 Pct Ni alloy

Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed (“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately 11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite. The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase boundary precipitate sheets was near {110}_α//{011}_c consistent with cementite precipitation on low-energy {110}_α//{111}_γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution of eutectoid microstructures.     

Determination of the absorptivity of AI-33Cu Wt Pct alloy during laser treatment by an inverse method

A. ZRYD, formerly Postdoctoral Fellow with the LMPH.     

Flow of interdendritic liquid and permeability in pb-20 Wt Pct Sn alloys

Directionally solidified Pb-20 wt pct Sn alloys of uniform microstructures were produced with various primary and secondary dendrite arm spacings. Permeabilities of these alloys were investigated with approximately 0.19 and 0.29 volume fraction liquid, for flow parallel to the direction of primary dendrite arms. The permeabilities of the samples with approximately 0.19 volume fraction liquid were also obtained for flow normal to the primary dendrite arms. It was found that for flow parallel to the primary dendrite arms, permeability varied with d ₁ ² and g _L ² (d₁ is the primary arm spacing and g_L is the volume fraction of liquid). There appears to be no relation between permeability for this parallel flow and the secondary dendrite arm spacing. For flow perpendicular to the primary dendrite arms, permeability is approximately 0.06 to 0.20 that for parallel flow, and in this case the permeability appears to be strongly dependent upon the secondary dendrite arm spacing.