Deformation substructure and strain-hardening characteristics of metastable Fe−Mn austenites

The 500° and 550°C strain-hardening characteristics of manganese austenites based on Fe-14 pct Mn-0.4 pct C with carbide forming elements (Cr, V, Mo), were determined and deformation substructures were examined by transmission electron microscopy. Of the carbide formers investigated, molybdenum is the most effective in enhancing the strain-hardening rate of manganese austenite, followed closely by vanadium. The effect of chromium is relatively small. The effectiveness of carbide formers is independent of the strain-induced precipitation reaction, though this varies greatly in extent. Examination of thin foils revealed widely extended stacking faults which persist after heavy deformation, indicating a low stacking fault energy. It is tentatively suggested that the essential role of molybdenum, vanadium, and chromium is to progresively lower the stacking fault energy of a manganese austenite, and thereafter enhance its strain-hardening rate. The lower stacking fault energy postpones cross-slip and, as a result of stacking fault-dislocation interaction, increases the dislocation density and strain-hardening rate. It is concluded that the precipitates play only a minor role in strain-hardening manganese austenites.     

Ordering kinetics of Fe3Pt austenites and reversed austenites

Fe₃Pt austenites undergo an ordering reaction belowT _c in addition to the martensite transformation at a much lower temperature. It is to be expected that theM _s temperature will be affected by the degree of austenite order and the progress of austenite ordering can be conveniently represented by the variation ofM _s temperature. In this study it was found that heat treating the annealed austenites belowT _c , a single-stage ordering reaction which followed first order kinetics was observed. The ordering rate or the rate ofM _s temperature change was found to be composition dependent, faster for alloy closer to the stoichiometry. The reversed austenite contains transformation induced defect structure. Consequently, heat treating the reversed austenites belowT _c a two-stage reaction was observed. Annealing of defect structure was found to precede the ordering reaction. Both annealing and ordering followed first order kinetics. The salient feature during annealing was that the initial rate ofM _s temperature change appeared to be independent of alloy composition.     

Thermomechanical treatment and transformation characteristics of Fe-Mn austenites

The effect of thermomechanical treatment on the tensile properties of metastable manganese austenites was determined, and deformation-induced transformation characteristics were analyzed by X-ray diffraction and by data derived from stress-strain curves. An improved combination of tensile strength and ductility is obtained through a processing treatment entailing ausforming and deformation-induced transformation. The two treatments are very strongly interrelated, the first imposing some limiting factors on the latter. After small ausforming deformation, the strain-induced austenite-to-martensite transformation at room temperature is slightly stimulated, but additional increments of austenite deformation have a strong retarding effect. It is concluded that the formation of a strong martensite in a work-hardened austenite matrix is most effective in enhancing strength and ductility. Formerly Postdoctoral Fellow, Department of Engineering Materials, University of Windsor, Windsor, Ontario, Canada     

Quantitative fractography: A modern perspective

The important aspects of quantitative fractography as a new analytical tool for understanding material fracture are discussed. The special considerations that rise in the quantification problems are examined from the purview of stereology. Two major experimental techniques for obtaining geometrical information about the fracture surface topography are critically evaluated. These methods are based on stereophotogrammetry or vertical sectioning procedures-both of which yield estimates of the true fracture surface area. The profile and surface roughness parameters which are required to transform measurements made on flat SEM fractographs to the true quantities in the fracture surface are introduced. The two roughness parameters are related by a simple parametric equation, permitting the fracture surface area to be calculated from the experimental measurement of the profile roughness parameter. Alternatively, it is shown how the fracture surface area can be obtained from the angular distribution of the profile elements by employing a transform procedure. The concept of “fractals” as it applies to quantitative fractography is introduced. Recently developed relationships which describe the true variation of the profile and surface roughness parameters with the size of the measuring unit are presented. Calculations are made of the mean area and perimeter length of dimples in the fracture surface of a 4340 steel. Three fracture surface configurations are examined: (1) an assumed flat- , (2) an assumed randomly-oriented- , and (3) the actual partially-oriented fracture surface. Significant differences are demonstrated between the true and the assumed situations, illustrating the importance of quantitative methods in fractography. By means of examples, it is shown how the quantitative methods permit detection of subtle changes in the fracture surface topography as influenced by the materials’ microstructure. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Decomposition of hypo-eutectoid Fe-C austenites; a numerical diffusion model

A numerical model is presented which describes the growth rate of ferrite during the decomposition of austenite in Fe-C alloys. The growth rate is modelled assuming carbon diffusion in austenite as the rate determining mechanism. The effect of the definition of the diffusion coefficient of carbon in austenite on the growth rate is shown. The model is used to examine the effects of initial carbon concentration, the local carbon concentration in austenite at the interface and the initial austenite grain size on the growth rate. Good agreement between theoretical results and both new and existing experimental data was observed.     

Precipitation kinetics and solute strengthening in high temperature austenites containing Al and N

Hot torsion tests were performed on 6 plain C steels containing increasing concentrations of Al and N to maxima of 0.084 and 0.028% by weight respectively. The experiments were carried out at 875, 900 and 925°C and at ten strain rates ranging from 6 × 10⁻⁴to 0.6 s⁻¹. The dependence of the peak strain on strain rate and composition was established at each of the three temperatures; in this way, the kinetics for the dynamic precipitation of A1N were determined. At 875°C, precipitation was initiated after 6–11 s and was completed in 45–130 s. The rate of precipitation decreased with increasing temperature and increased with increasing Al concentration. No effect of N concentration or of offstoichiometry was observed. The present rate of A1N precipitation is similar to that of Nb(CN) and of VN in deformed materials; the retarding influence of Al in solution on recrystallization is considerably less than that of Nb however. The presence of Al in solid solution in γ-Fe increases the flow stress by about 12 ± 5% per 0.1 at.%; by contrast, N in solid solution leads to a flow stress decrease of 9 ± 5% per 0.1 at.%. The hardening and softening effects of Al and N in γ-Fe are discussed in terms of lattice parameter, modulus and electronic effects.     

On image analysis in fractography (Methodological Notes)

As other spheres of image analysis, fractography has no universal method for information convolution. An effective characteristic of an image is found by analyzing the essence and origin of every class of objects. As follows from the geometric definition of a fractal curve, its projection onto any straight line covers a certain segment many times; therefore, neither a time series (one-valued function of time) nor an image (one-valued function of plane) can be a fractal. For applications, multidimensional multiscale characteristics of an image are necessary. “Full” wavelet series break the law of conservation of information.     

Quantitative fractography and dislocation interpretations of the cyclic cleavage crack growth process

Fractographic features of cyclic cleavage crack growth in Fe and Fe-binary alloys as a function of applied stress intensity and test temperature are documented. Cyclic striations, cleavage rivers and twist angle misorientations are measured and discussed in terms of geometrical and local stress considerations. It is shown that the river height displacement must account for both twist angle misorientation between adjacent grains and the crack-tip opening displacement of an advancing fatigue crack. With regards to the rate of an advancing cleavage crack, a dislocation dynamics model is derived which qualitatively gives the correct ordering of power-law slopes (da/dN vs ΔK) and growth rate magnitudes as a function of test temperature.     

Structure and formation of the metastable phasem-TiBe

The metastable ordered phasem-TiBe was discovered in the course of studying liquidquenched Ti-Be and Ti-Be-Zr alloys. m-TiBe forms, together with the α-Ti solid solution, on heating binary (and ternary) metallic glasses to 700°C and is also observed directly in as-quenched alloys lying outside the composition limits for glass formation. This phase has a B2 (CsCl-type) crystal structure. Its existence can be justified in terms of crystal chemical factors leading to the conclusion that TiBe most likely does not appear as an equilibrium phase in normally cooled alloys due to the greater stability of the Laves phase TiBe₂.     

The stable and metastable Ti-Nb phase diagrams

The phase transformations which occur in the Ti-Nb binary alloy system have been discussed in two recent papers. The phase relationships were investigated by varying alloy composition and thermal history. In this paper, these results are summarized in complete and thermodynamically consistent calculations of the stable and metastable phase diagrams. The calculations of the metastable equilibria are relevant to the Ti-V and Ti-Mo systems, as well as to several other titanium and zirconium-based transition metal alloy systems. Formerly with the National Bureau of Standards, Gaithersburg, MD 20899.     

Dynamic consolidation of metastable nanocrystalline powders

Nanocrystalline metal powders synthesized by mechanical alloying in a ball mill resulted in micron-sized powder particles with a nanosized (5 to 25 nm) substructure. Conventional consolidation methods resulted in considerable coarsening of the metastable nanometer crystallites, but dynamic consolidation of these powders using explosive techniques produced fully dense monoliths while retaining the 5- to 25-nm substructure. Numerical modeling, used to guide the experimental phase, revealed that the compression wave necessary for suitable consolidation was of the order of 10 GPa for a few tenths of a microsecond. The consolidation process is described, and the retention of the metastable nanostructure is illustrated. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.