Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1976"

A general mechanism of martensitic nucleation: Part I. General concepts and the FCC → HCP transformation

Abstract: Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites.

Grain-boundary sliding and its accommodation during creep and superplasticity

Abstract: The roles of grain-boundary sliding (GBS) and of other creep mechanisms in creep and fine-grain superplasticity are presented in relation to a model based on the division of grains into their central “cores“ and peripheral “mantles”; GBS and its accommodation is limited to the latter, which changes with the mode of accommodation,viz by fold formation, dislocation motion in the mantle or pure diffusion. This description is used to adapt from the literature or develop equations for creep rate based on dislocation or vacancy mechanisms, which are then combined to give plots of the various regimes of superplastic (or creep) behavior, all of which involve GBS in a quantitatively defined manner. The predictions of these equations are compared with a number of results in the literature and with those of a lead-thallium alloy of grain sizes intermediate between superplastic behavior and normal creep. Some preliminary comparisons of measured GBS are also made with the predictions of the model. Agreement is good.

Hydrogen transport by dislocations

Abstract: A kinetic model is presented for the transport of hydrogen, as Cottrell atmospheres on dislocation, at a rate appreciably in excess of that for lattice diffusion. The particular destinations for the hydrogen which are modeled here are ductile fracture initiation sites such as inclusions and microvoids. The functional predictions of the mechanism are shown to be consistent with available experimental evidence on ductile fracture behavior in the presence of hydrogen.

Dendritic growth-A test of theory

Abstract: Steady-state theories of dendritic solidification are reviewed, and three nonisothermal theories, expressed as simple power laws, are chosen for experimental verification. Specifically, the axial growth rate,V, of a freely growing dendrite can be expressed asV =βGΔθn, wheren andβ are the exponent and prefactor derived from each theory,G is a lumped material parameter, andΔθ is the supercooling. Succinonitrile, a low entropy-of-fusion plastic crystal, was prepared in several states of purity as the test system, and dendritic growth was studied both in the usual manner in long tubes, and in a novel apparatus in which the conditions for “free” dendritic growth were attained. Kinetic measurements show that only when “free” growth conditions obtain are the data reconcilable with current theory in the form discussed above. In particular, we show thatn = 2.6, in agreement with the theories of Nash and Glicksman and that of Trivedi; however, the prefactorsβ of those theories do not agree with the value determined for succinonitrile, which is the only substance for whichG is known accurately. Tip radius measurements, taken over a relatively narrow range of supercooling, when combined with the growth rate data prove that the Peclet number-supercooling relationship derived for each of the three nonisothermal steady-state theoriesall agree with experiment. This curious agreement, along with the inability to “decompose” the Peclet numbers into acceptable velocity-supercooling and tip radius-supercooling relationships is explained on the basis of the limitations imposed by the steady-state assumption itself. Directions for future theoretical and experimental investigation are discussed in the light of the findings presented.

Transformation from austenite in alloy steels

Abstract: This paper is concerned with the direct transformation of austenite at high temperatures to form ferrite and alloy carbide dispersions. The ferrite/austenite interfaces vary from high energy random boundaries to low energy planar boundaries which grow by step propagation, while the alloy carbide morphologies include a pearlitic form, fine fibers and fine banded arrays of particles. It is shown that these morphologies are closely related to the mode of growth of the ferritic matrix. The role of various alloying elements on the carbide dispersion is examined, and the effects of other metallurgical variables on the banded dispersions are discussed, including factors which influence the dispersion stability. The mechanical properties of directly transformed alloy steels are shown to depend largely on the ferrite grain size and the state of the carbide dispersion. Micro-alloyed steels subjected to controlled rolling provide an excellent example of the achievement of high strength and toughness levels by control of these variables. The paper finally attempts to show how such benefits can be achieved in low and medium alloy steels, and in particular where resistance to creep failure at elevated temperatures is an important property.

Evaluation of Toughness in AISI 4340 Alloy Steel Austenitized at Low and High Temperatures

Abstract: It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,KIc, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenKIc and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onKIc or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.

A general mechanism of martensitic nucleation: Part II. FCC → BCC and other martensitic transformations

Abstract: The general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, bcc → hcp, and related transformations, including mechanical twinning. Where thermodynamic data are available, the conditions at the observedMs temperatures are consistent with nucleation from a defect composed of four or five properly spaced lattice dislocations. Examples of nucleation by faulting on the planes predicted are found in published electron microscopy. The faults are observed at the types of sites where the required dislocation groups are expected. These include grain boundaries, incoherent twin boundaries, and inclusion particle interfaces. Having defined the function of a nucleation site, mechanisms of strain induced nucleation and autocatalysis are then considered.

The role of microstructure on the strength and toughness of fully pearlitic steels

Abstract: An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.

The Effect of Hydrostatic Pressure on the Deformation Behavior of Maraging and HY-80 Steels and Its Implications for Plasticity Theory.

Abstract: Earlier results showed that the difference between the tensile and compressive strengths of tempered martensites is primarily a manifestation of the general pressure dependence of flow stress in these materials. However, the same results also showed that the volume expansion after deformation was much smaller than that predicted by the normality flow rule of plasticity theory for materials with such pressure dependence. Additional results now obtained on maraging and HY-80 steels support these conclusions. The results for all these materials exhibit a strong, but not perfect, correlation between pressure dependence, yield stress, and volume expansion. The volume expansion, however, which is believed to result primarily from the generation of new dislocations, is very small and does not appear to be essential to the pressure dependence. Most of the pressure dependence, the portion responsible for the discrepancy with the normality flow rule, may be an effect on dislocation motion. The results suggest that an appropriate plasticity model would be one in which the octahedral shear yield stress is linearly dependent on the mean pressure, but the volume change is negligible in violation of the normality flow rule. Such a model has been proposed previously for the plastic deformation of soils. However, unlike that model, the present theory includes strain hardening.

The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy

Abstract: As-quenched and tempered martensite in an Fe-0.2 pct C alloy were subjected to tensile testing and structural characterization by light and transmission electron microscopy. The light temper, 400°C-l min, did not change packet morphology, but did reduce dislocation density, coarsen lath size and cause the precipitation of carbides of a variety of sizes. The yield strength of the as-quenched martensite was strongly dependent upon packet size according to a Hall-Petch relationship, but tempering significantly diminished the packet size dependency, a result attributed to packet boundary carbide precipitation and the attendant elimination of carbon segregation present in the as-quenched martensite because of autotempering. Examination of thin foils from strained tensile specimens showed that a well-defined cell structure developed in the as-quenched martensite, but that the random distribution of jogged dislocations and carbide particles produced by tempering persisted on deformation of the tempered specimens.

A general mechanism of martensitic nucleation: Part III. Kinetics of martensitic nucleation

Abstract: The growth of martensitic fault embryos in the fault plane, the development of their interfacial structure, and the thickening of the embryos normal to the fault plane are examined as possible rate limiting steps in the total martensitic nucleation process. Growth of the embryos in the fault plane appears the most probable rate limiting step, capable of accounting for both the observed isothermal and athermal kinetic behavior depending on the parameters (such as activation volume) which control the motion of the transformational dislocations. The thermally activated nucleation of dislocation loops responsible for lattice invariant deformations is a possible rate limiting step for some isothermal transformations, though such deformations are not required for all martensitic transformations. Embryo thickening by the nucleation of discrete loops of transformation dislocations appears improbable in bulk material; instead, a plausible pole mechanism for embryo thickening is presented which incorporates existing “forest” dislocations intersected by embryos growing in the fault plane. Lattice softening phenomena may lower the critical chemical driving force for nucleation, but are not essential for martensitic nucleation by the proposed faulting mechanism.

Temper embrittlement of Ni-Cr Steels by phosphorus

Abstract: Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann.

Plastic deformation below worn surfaces

Abstract: Large strains occur at and near the worn surface of a ductile material (e.g. ∼8 at a depth of 10 µm after trepanning, ∼2.5 at 2.5 µm below an abraded surface). The wear of such materials is thus controlled by their strain-hardening behavior. Measurements have been made of the strain and microhardness at points below the worn surface of copper-silver solder composite specimens. The results are consistent with a model of the abrasion process that suggests the strain below the surface should be proportional to the abrasive grit size and to the square root of the applied load, but the strain at the surface should be independent of these factors. A simple energy balance confirms the work done during wear is determined by the plastic deformation at and below the worn surface. The measured volume wear rate shows the linear dependence on load expected on the model of the abrasion process, but there is also some dependence on grit size at small grit sizes and possible reasons for this are discussed.

Martensite decay during rolling contact fatigue in ball bearings

Abstract: During rolling contact fatigue of ball bearings structural changes may occur below the raceway, in the region of maximum shear stress. A great number of 6309 type bearings were tested at two stress levels for varying numbers of revolutions, and the resulting structural changes in the inner rings were studied by optical and electron microscopy. The present observations have been compared to previously reported observations, which are often described in unsystematic terminologies and sometimes apparently contradictory. A unified terminology is worked out, on the basis of which, the structural changes are described. The following features occur, in chronological order. i) A ferritic phase, containing an inhomogeneously distributed excess carbon content, corresponding to that of the parent martensite. A mixture of this phase with residual martensite constitutes the well known dark etching region. ii) Disc-shaped regions of ferrite, thermodynamically stable, about 0.1 μm thick, inclined by about 30 deg to the raceway, and sandwiched between carbide rich discs. The latter are constituted by very small carbide particles, and are not necessarily compact. iii) A second set of larger disc-shaped regions about 10 μm thick, of plastically deformed ferrite in thermodynamic equilibrium, forming an angle of about 80 deg to the raceway. Transformation mechanisms are proposed. Particular attention is paid to short range carbon diffusion, induced by the cyclic stresses.

The effect of morphology on the strength of pearlite

Abstract: The effects of various morphological features on the strength of high-purity pearlite were studied. A continuous-cooling mode of transformation from different austenitizing temperatures was used to produce variations in average nodule diameter and minimum interlamellar spacing. It was found that, for a constant transformation temperature, nodule size was directly related to prior austenite grain size. On the other hand, minimum interlamellar spacing is controlled by transformation temperature, independent of prior austenite grain size and nodule size. Both the yield strength and fracture stress of pearlite was found to be inversely proportional to interlamellar spacing and independent of prior austenite grain size and nodule size.

Dendrite morphology of steady state unidirectionally solidified steel

Abstract: Steady state directional freezing experiments have been performed with two steels containing 0.59 and 1.48 pct carbon. Primary and secondary arm spacings were directly measured. In addition, average primary arm spacings were computed from the number of arms present on the observed area using the model of a hexagonal arrangement. The latter method seems to be more objective and reproducible than the line counting method. Arm spacings λ were related by the empirical equation λ =c RmGn to growth rateR and temperature gradientG. For primary arms, the exponentsm andn were different, whereas for secondary arms they were almost identical. Some consideration is given to dendrite spacings in ingot solidified steel, where under parabolic growth conditions thermal gradients and growth velocity are coupled by heat flow. Hence, a single variable may be used if the boundary condition for heat flow remains the same. Using the present results the laws describing dendrite spacings as a function of local solidification time are derived and compared with previous data available in the literature.

Accelerated Evolution of Hydrogen from Metals During Plastic Deformation

Abstract: Plastic deformation accelerates the release of hydrogen from iron, Type 304L stainless steel, nickel, Inconel 718, and 5086 aluminum. The release rate is strain dependent: it increases rapidly when plastic deformation begins, reaches a maximum, and then decreases with additional strain with a final large release at fracture. The release rate is constant during Luder’s extension for iron, and fluctuates coincidentally with the serrated flow of 5086 aluminum. The release rate during deformation also depends on temperature and strain rate. The accelerated release rate during deformation is discussed in terms of hydrogen-dislocation interactions and is interpreted as being caused by the egress of of dislocations and their associated hydrogen atmospheres during plastic deformation.

The thermoelastic martensitic transformation inβ′ Ni-Al alloys: I. Crystallography and morphology

Abstract: This investigation describes a study of the crystallographic features of the thermoelastic martensitic transformation inβ′ Ni-Al alloys. Experimental measurements of the habit plane, shape strain and orientation relationship have been made, and the results are found to be in good agreement with the predictions of the Bowles-Mackenzie phenomenological theory, assuming δ = 1.000, p2', and {110}β', and d2 = β'. The martensite habit plane normals are close to {2, 14, 15}β' and are typically clustered in self-accommodating groups of four crystallographically equivalent variants centered around {011}β' poles. The experimental shape strain is found to be exactly an invariant plane strain with the displacement direction lying ∼92 deg from the habit plane normal.

High temperature creep in a semi-coherent NiAl-Ni 2 AlTi alloy

Abstract: Creep experiments have been made on a Ni-Ti-Al alloy, which has a microstructure consisting of a distribution of semi-coherent NiAl(β) precipitates with a Ni2AlTi(β′) Heusler phase matrix. The creep strength of this bcc type structure alloy is at least comparable with that of the nickel-base superailoy MARM-200 for values ofT/T m in the range 0.68 to 0.82. Quantitative electron microscope experiments show that both undissociated α0〈110〉 dislocations, and paired α0〈100〉 dislocations coupled by a sublattice A.P.B. exist within the β′ phase;α 0 is the lattice parameter of a bcc cell of which the large Ni2AlTi unit-cell is composed. The sublattice A.P.B. is a crystallographic fault created by wrong bonds between atoms on the Al-Ti sublattice. Theoretically the energy γ of a sublattice A.P.B. is shown to be minimum on {100}, and the experimental value for γ on {100} is ~40 mJ/m2.

The Bauschinger effect in precipitation strengthened aluminum alloys

Abstract: The Bauschinger effect in precipitation strengthened Al-Cu-Mg, Al-Zn-Mg and Al-Cu polycrystals was measured as a function of applied strain. Alloys heat treated to contain easily shearable precipitates,i.e., GPB, GP and θ″ exhibited a small Bauschinger effect, on the order of that in pure aluminum. In contrast, alloys with nonshearable precipitates, S′, η and θ′ showed an anomolously large effect. A unique hysteresis loop shape, with a region of convex curvature between sharp inflection points, was observed in the nonshear-able precipitate alloys. The large Bauschinger effect and unusual hysteresis loop shape are due to internal elastic or back stresses exerted by the strong precipitates on the matrix. A nonlinear elastic hardening model is proposed in which the overall work harden-ing is partitioned into an elastic back stress component and a frictional dislocation forest hardening term. Plastic relaxation around the precipitates and inhomogeneous deformation in the polycrystal reduces the level of the internal stresses below that predicted theoretically by the Brown and Stobbs hardening theory.

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

Abstract: To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior.

Enhanced low-temperature sintering of tungsten

Abstract: The effects of various transition metal additions on the sintering of a well-characterized, fine tungsten powder were analyzed using both isothermal and constant heating rate experiments in the temperature range 900 to 1400°C. Approximately four atomic mono-layers of palladium on the tungsten powder surface were found to be the optimal enhancer, followed by nickel, cobalt, platinum, and iron. The addition of Cu to the tungsten had no appreciable effect on the sintering kinetics. Sintering enhancement by these transition metals is related to their periodic chart position (i.e., electron structure). An overall non-Arrhenius shrinkage temperature dependence is attributed to grain growth in the activator-treated specimens. The activation energy for tungsten densification was determined to be 430 to 450 kJ/mol, which is in general agreement with a grain boundary diffusion process.

A Criterion for ductile fracture in sheets under biaxial loading

Abstract: A criterion for ductile fracture is developed based on the statistical process of shear joining of voids and on the assumption that the voids responsible for fracture have experienced considerable growth prior to this stage of shearing. From the knowledge of uniaxial flow properties and fracture strain measurement, this model is capable of predicting the strain at fracture for other strain states. The predicted data are in good agreement with experiments. Although this model assumes spherical inclusions, some quantitative estimates for elongated inclusions can also be made.

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

Abstract: The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65a and a small amount of incoherent η (MgZn2). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35a. Aging the 7050 alloy to produce particles larger than 50a gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δep/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.

Effect of microstructure on notch fatigue properties of Ti-6Al-4V

Abstract: The effect of microstructure on the notch fatigue properties of Ti-6A1-4V was investigated. Specimens with five distinctly different microstructures were tested and subsequently examined in detail. It was found that the notch fatigue performance of the alloy varied significantly as the microstructure was altered by heat treatment. The best high cycle fatigue strength was found in specimens heat treated above the beta transus temperature, containing an almost totally transformed acicular alpha structure. The fatigue performance of specimens with this microstructure appeared to be controlled by the size of the nucleated crack. It is suggested that at low stress levels the nucleated crack is limited in size to the width of a single alpha needle, while at high stresses the nucleated crack may be as large as an entire colony of similarly aligned alpha needles.

Grain Size Effects in Hydrogen-Assisted Cracking

Abstract: There is conflicting evidence in the literature with respect to the effect of grain size on hydrogen embrittlement. Differences may arise because of the degree of segregation in different grain size materials, because of different structures obtained in the effort to produce varying grain sizes, or because of the grain-size dependency of diffusion and growth processes. An extremely dirty heat of 4340 steel with 0.07 S and 0.015 P was investigated so that any tramp element segregation or hydrogen recombination poison effects would be present. Measurements were obtained on cathodically-charged samples with average grain sizes of 20, 50, 90 and 140 μm. In general, tramp element effects were not controlling. For those cases where the grain diameter was significantly larger than the plastic zone, increased grain size improved resistance. This was reflected by a slight increase in threshold stress intensity and an inverse grain-size squared dependence of crack velocity. Although the data are consistent with a pressure tensor hydrogen-assisted migration model, they could also be interpreted in terms of high austenitizing temperatures promoting retained austenite.

Creep of titanium-silicon alloys

Abstract: Operative creep mechanisms in laboratory melts of Ti-5Zr-0.5Si and Ti-5Al-5Zr-0.5Si have been investigated as a function of microstructure, creep stress, and temperature. From creep rate data and transmission electron microscopy results, it has been shown that an important creep strengthening mechanism at 811 K in Si bearing Ti alloys is clustering of solute atoms on dislocations. All of the alloys investigated showed anomalously high apparent activation energies and areas for creep, and a high exponent (n) in the Dorn equation. In addition, the effect of heat treatment was investigated and it is shown that the highest creep strength was obtained by using a heat treatment which retained the maximum amount of silicon in solution. This is consistent with the proposed creep strengthening mechanism. An investigation of the creep behavior of several other Si containing alloys including two commercial alloys, Ti-11 and IMI-685 indicated similar results.

Phase transformation of stainless steel during fatigue

Abstract: Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous.

Liquid metal cooling: A new solidification technique

Abstract: This paper describes a new and highly efficient directional solidification process using liquid metal as a coolant. A laboratory version of this process is reviewed in detail. The selection of a coolant is also discussed. Process thermal characteristics such as thermal gradient, growth rate and cooling rate are measured and compared with established directional solidification processing. Microstructural refinement of primary dendrites, secondary dendrite arms and MC carbides is demonstrated in the case of Ni-base super-alloys. Special advantages of the process, such as a lack of interdependence of growth rate and thermal gradient are discussed. Liquid Metal Cooling (LMC) offers a wide range of rate-gradient combinations and i therefore the most flexible directional solidification process discovered to date. The high levels of thermal gradient which are available make LMC a natural choice for the growth of eutectics. Alternatively, growth rates of dendritic materials can be substantially increased leading to significantly finer microstructures. Thus, LMC promises to be a highly useful process.