Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1986"

Effect of Cyclic Deformation on the Pseudoelasticity Characteristics of Ti-Ni Alloys

Abstract: Change in the pseudoelasticity characteristics of Ti-Ni alloys during tension cycling was investigated. The critical stress for inducing martensites and the hysteresis of a stress-strain curve decreased with increasing number of cyclic loading, while the permanent elongation increased. The degree of the change in these values showed a strong dependence on the maximum applied stress during stress-induced martensitic transformation. However, no change was induced by cyclic elastic deformation even though high stress was applied. It was also found that the stabilization of the pseudoelasticity characteristics during cyclic loading was established using special thermomechanical treatments which are effective to raise the critical stress for slip. Based on these results it is concluded that the cause for the effect of cyclic deformation is the generation of dislocations in the martensitic phase.

An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites

Abstract: ANNEALED aluminum/silicon carbide (Al/SiC) composites exhibit a relatively high density of dislocations, which are frequently decorated with fine precipitates, in the Al matrix. This high dislocation density is the major reason for the unexpected strength of these composite materials. The large difference (10:1) between the coefficients of thermal expansion (CTE) of Al and SiC results in sufficient stress to generate dislocations at the Al/SiC interface during cooling. In thisin situ investigation, we observed this dislocation generation process during cooling from annealing temperatures using a High Voltage Electron Microscope (HVEM) equipped with a double tilt heating stage. Two types of bulk annealed composites were examined: one with SiC of discontinuous whisker morphology and one of platelet morphology. In addition, control samples with zero volume percent were examined. Both types of composites showed the generation of dislocations at the Al/SiC interface resulting in densities of at least 1013 m-2. One sample viewed end-on to the whiskers showed only a rearrangement of dislocations, whereas, the same material when sectioned so that the lengths of whiskers were in the plane of the foil, showed the generation of dislocations at the ends of the whiskers on cooling. The control samples did not show the generation of dislocations on cooling except at a few large precipitate particles. The results support the hypothesis that the high dislocation density observed in annealed composite materials is a result of differential thermal contraction of Al and SiC. The SiC particles act as dislocation sources during cooling from annealing temperatures resulting in high dislocation densities which strengthen the material.

Deformation and transition behavior associated with theR-phase in Ti-Ni alloys

Abstract: Deformation behavior associated with theR-phase (rhombohedral phase) transition and the subsequent martensitic transformation was studied systematically in Ti-Ni alloys by tensile testing over a wide temperature range covering belowM f to aboveT′ R (>Af). Since the deformation and transition characteristics showed a strong dependence on thermo-mechanical treatment and Ni-content, internal structures were examined by electron microscopy in specimens with various Ni-content and thermo-mechanical treatment. As a result precipitates and/or dislocations were revealed in the specimens in which theR-phase transition occurs. Based on the above results, the effects of thermo-mechanical treatment and Ni-content on the deformation and transition characteristics were clarified for both theR-phase transition and the martensitic transformation.

Precipitation processes in near-equiatomic TiNi shape memory alloys

Abstract: Metallographic studies have been made of precipitation processes in Ti-50 pct Ni and Ti-52 pct Ni (at. pct) shape memory alloys. The eutectoid and peritectoid reactions previously reported for near-equiatomic and Ni-rich TiNi alloys were not observed for either composition. In the Ti-52Ni alloy, diffusional transformations take place, similar to those in supersaturated alloys. The precipitation sequence can be written asβ 0 → Ti11Ni14 → Ti2Ni3 → TiNi3. The solidus line of the TiNi phase in the Ti-52Ni alloy lies at 812 ± 22 °C. Morphological characteristics of the various precipitate phases are described in detail.

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

Abstract: The modification of lamellar alpha phase in Ti-6A1-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 °C (1750 °F), followed by annealing at 925 °C (1700 °F) to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate shearing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness — 3.4 ώm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness ≃ 6 μm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.

Strain Hardening of Hadfield Manganese Steel

Abstract: The plastic flow behavior of Hadfield manganese steel in uniaxial tension and compression is shown to be greatly influenced by transformation plasticity phenomena. Changes in the stress-strain (σ−e) curves with temperature correlate with the observed extent of deformation twinning, consistent with a softening effect of twinning as a deformation mechanism and a hardening effect of the twinned microstructure. The combined effects give upward curvature to the σ−e curve over extensive ranges of plastic strain. A higher strain hardening in compression compared with tension appears to be consistent with the observed texture development. The composition dependence of stacking fault energy computed using a thermodynamic model suggests that the Hadfield composition is optimum for a maximum rate of deformation twinning. Comparisons of the Hadfield steel with a Co-33Ni alloy exhibiting similar twinning kinetics, and an Fe-21Ni-lC alloy deforming by slip indicate no unusual strain hardening at low strains where deformation is controlled by slip, but an unusual amount of structural hardening associated with the twin formation in the Hadfield steel. A possible mechanism of anomalous twin hardening is discussed in terms of modified twinning behavior (pseudotwinning) in nonrandom solid solutions.

A Relationship between Indigenous Impurity Elements and Protective Oxide Scale Adherence Characteristics

Abstract: A new and a radically different mechanism to account for the beneficial effects that small additions of elements such as yttrium have on the adherence of oxide scales is proposed. It has been long:known and has been reaffirmed here that indigenous impurities such as sulfur, known to be present at tramp levels (<100 ppm) within nickel and nickel-based alloys, can segregate to metal surfaces. However, here it has been disclosed that such sulfur segregation can also markedly affect the adherence of the protective oxide scale. In the absence of elements like yttrium, such segregation effects weaken the bond between the protective scale and the substrate metal. The role of the yttrium is to interact with such indigenous sulfur to form a refractory sulfide. This interaction lessens the amount of sulfur available to segregate to and concentrate at the critical scale-metal interface. Results reported here have focused on sulfur because sulfur has been long known to be a common tramp impurity in nickel and nickel-based alloys. However other elements, e.g., phosphorus, chlorine, etc., could very probably produce similar effects. The results of experiments involving Auger spectroscopy, optical, scanning electron microscopy, scanning electron microprobe, and scanning transmission electron microscopy techniques in conjunction with isothermal and cyclic oxidation testing which have led us to propose this mechanism are presented.

The influence of Fe and Cr on the microstructure of cast Al-Si-Mg alloys

Abstract: The detailed microstructure of cast aluminum alloys based on Al-7 pct Si-0.3 pct Mg (A356) has been investigated as a function of Fe and Cr content and solidification rate. It was found that the coarse β phase (FeSiAl5) platelets, which form during solidification of alloys containing Fe, are replaced by “Chinese script” α(bcc) [(Cr, Fe)4Si4Al13] dendritic particles when Cr is added. Coarse π (FeMg3Si6Al8) phase and fine α(bcc) precipitates were found in all of the investigated alloys. Quantitative metallography showed that the length of β phase platelets and the arm length of α(bcc) dendrites increased with decreasing solidification rate and increased Fe content. The T6 heat treatment resulted in the dissolution of π phase and the uniform fine scale precipitation of β′(Mg2Si) in the aluminum rich phase.

The welding metallurgy of HASTELLOY alloys C-4, C-22, and C-276

Abstract: The welding metallurgy (solidification and solid state transformations) of HASTELLOY* Alloys C-4, C-22, and C-276 has been determined. Varestraint hot-cracking tests performed on commercial alloys revealed a weldability ranking as follows: C-4 > C-22 > C-276. All alloys would be expected to have good weldability, with Alloy C-4 having a very low hot-cracking tendency, comparable to 304L stainless steel. Microstructures of gas-tungsten-arc welds of these alloys have been characterized by scanning electron microscopy and analytical electron microscopy. Intermetallic secondary solidification constituents have been found associated with weld metal hot cracks in Alloys C-276 and C-22. In Alloy C-276, this constituent is a combination ofP and ώ phases, and in Alloy C-22, this constituent is composed of σ,P, and ώ phases. With phase composition data obtained by AEM techniques and available ternary (Ni-Cr-Mo) phase diagrams, an equivalent chemistry model is proposed to account for the microstructures observed in each alloy's weld metal.

Microstructural dependence of Fe-high Mn tensile behavior

Abstract: The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (e) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced e →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced e→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the Ms and Md temperatures for both the e and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.

Transmission electron microscopic observations of mechanical twinning in metastable beta titanium alloys

Abstract: Microstructure and crystallography of a stress-induced plate in metastableβ titanium alloys, Ti-V, Ti-Mo, Ti-Nb, Ti-Fe, were investigated by a combination of two surface trace analysis and transmission electron microscopy. Stress-inducedω phase transformation or {332} 〈113〉 twinning appeared in circumstances whereβ phase was very unstable. It was found that there were two types of {332} 〈113〉 twinning, depending on whether one variant ofω phase was preferentially induced in a twin or not. Stress-inducedω phase not relating to {332} 〈113〉 twinning was not observed in matrix. This suggests that stress-inducedω phase transformation is accompanied with {332} 〈113〉 twinning. Preferential formation of oneω variant was influenced not only by alloy system and its composition, but also by cooling rate from solution treatment temperature. The relation between athermalω structure and stress-inducedω phase transformation or plastic deformation mode is also discussed.

The trapping and transport phenomena of hydrogen in nickel

Abstract: Hydrogen thermal analysis experiments have been employed to study the trapping and transport phenomena of hydrogen in nickel. Dislocations in nickel act as trapping sites of hydrogen, and the hydrogen trap activation energy at dislocations appears to be lower than the activation energy for the bulk diffusion of hydrogen. It is suggested that both hydrogen trapping at grain boundaries and short-circuit diffusion through grain boundaries in nickel are present. The trap binding energy at grain boundaries is estimated as 20.5 kJ ⋅ mol-1. Using the hydrogen thermal analysis experiments, the solubility and diffusivity of hydrogen in nickel have been measured. The temperature dependences of those are described by C (H atoms/Ni atom) = 1.57 × 10-3 exp(-11.76 kJ ⋅ mol-1/RT) and D (m2 s-1) = 7.5 × 10-7 exp(-39.1 kJ ⋅ mol-1/RT), respectively.

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Abstract: Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.

Development of Al-Ti-C grain refiners containing TiC

Abstract: Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A14C3 and Ti3AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB2 commercial grain refiner. TiC particles introducedvia the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals.

Inclusion Phases and the Nucleation of Acicular Ferrite in Submerged Arc Welds in High Strength Low Alloy Steels

Abstract: Series of submerged arc welds of HSLA steel made with three different fluxes and metallic additions of Ti, Mo, and Cr have been examined to study the inclusions and their role in the nucleation of acicular ferrite. Inclusion phases and compositions have been analyzed by electron diffraction and X-ray microanalysis. These analyses have shown that the inclusions contained many different compounds, the proportions of each depending upon both the flux and metallic additions. Six inclusion phases have been identified: galaxite (Al2O3 ⋅ MnO), a titanium-rich compound (probably TiO), a copper sulfide, a manganese sulfide, a silica, and an aluminum-rich phase. No correlation was found between the amount of acicular ferrite in the weld metal and either average inclusion composition or individual inclusion phases. No epitaxial relationships between inclusions and adjacent ferrite grains could be identified. It has been concluded that inclusions nucleate acicular ferrite by acting as inert substrates according to the classical theory of heterogeneous nucleation. Because most inclusions are multi-phase and are touched by several ferrite grains, it has also been concluded that each inclusion can nucleate several ferrite grains, due to local regions of high surface energy on the inclusion.

Three-dimensional convection in laser melted pools

Abstract: Computer simulation was carried out to describe three-dimensional convection in laser melted pools,i.e., for the case where the workpiece is moving with respect to the laser beam. Two different types of driving forces for flow were considered in the model,i.e., the buoyancy force and the surface tension gradient at the pool surface. Laser surface melting of 6063 aluminum alloy was carried out using a continuous-wave CO2 laser, and the power delivered to the workpiece was measured calorimetrically. The calculated and observed fusion boundaries were compared and very good agreement was obtained. Finally, the effect of the surface tension temperature coefficient δγ/δT on the convection pattern and penetration of laser melted pools was demonstrated with the model.

Roles of dislocations and grain boundaries in martensite nucleation

Abstract: In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (Ms) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress Ms in fine-grained specimens.

The effect of solidification rate on microsegregation

Abstract: An X-ray diffraction technique has been utilized to determine the second phase content and average composition of the primary phase in aluminum-copper and aluminum-silicon alloys solidified at a cooling rate in the range of 0.06 to l05 K/s. For the Al-Cu samples, the normalized Θ phase content (ratio of the 6 content to the value predicted by the Scheil model) was found to be 0.71 at 0.1 K/s (solidification rate = 0.001 cm/s) and to increase with increasing cooling rate to 0.96 at about 180 K/s (1 cm/s). Beyond this cooling rate, it decreased with increasing the cooling rate to 0.44 at about 3.7 x lO4 K/s. The same trend was observed in the Al-Si samples, except that the normalized silicon content was much lower. Also, for both systems the normalized average composition of the primary phase was found to decrease progressively with increasing the solidification rate until it reached a minimum at 1 cm/s, beyond which it increased with higher solidification rates. The results are discussed with respect to the prevailing segregation models that include back-diffusion in the solid, dendrite tip undercooling, and the eutectic temperature depression. An equation which combines these effects at all cooling rates is given.

An analysis of the microstructure of rapidly solidified Al-8 wt pct Fe powder

Abstract: Rapidly solidified powders of Al-8 wt pct Fe exhibit four distinct microstructures with increasing particle diameter in the size range of 5 μm to 45 μm: microcellular α-Al; cellular α-Al; a-Al + Al6Fe eutectic; and Al3Fe primary intermetallic structure. Small powder particles (~10 μm or less) undercool significantly prior to solidification and typically exhibit a two-zone microcellular-cellular structure in individual powder particles. In the two-zone microstructure, there is a transition from solidification dominated by internal heat flow during recalescence with high growth rates (microcellular) to solidification dominated by external heat flow and slower growth rates (cellular). The origin of the two-zone microstructure from an initially cellular or dendritic structure is interpreted on the basis of growth controlled primarily by solute redistribution. Larger particles experience little or no initial undercooling prior to solidification and do not exhibit the two-zone structure. The larger particles contain cellular, eutectic, or primary intermetallic structures that are consistent with growth rates controlled by heat extraction through the particle surface (external heat flow).

Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel

Abstract: The microstructural parameter(s) controlling the critical cleavage fracture stress, σF, of fully pearlitic eutectoid steel have been investigated. Independent variation of the pearlite interlamellar spacing,Sp, and the prior austenite grain size were accomplished through heat treatment. Critical cleavage fracture stresses were measured on bluntly-notched bend specimens tested over the temperature range -125 °C to 23 °C. The cleavage fracture stress increased with decreasingSp, and was independent of prior austenite grain size. Fine pearlitic microstructures exhibited temperature, strain-rate, and notched-bar geometry independent values for σF, consistent with propagation-controlled cleavage fracture. Coarse pearlitic specimens exhibited temperature-dependent values for σF over a similar temperature range. Inclusion-initiated fractures were generally located at or beyond the location of the peak normal stress in the bend bar, while cracking associated with pearlite colonies was observed to be closer to the notch than the predicted peak stress location. The calculated values for σF were independent of both the type and location of initiation site(e. g., inclusion, pearlite colony). Thus, although inclusions may provide potent fracture initiation sites, their presence or absence does not necessarily change σF in fully pearlitic microstructures.

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part II. Low cycle fatigue behavior

Abstract: The low cycle fatigue (LCF) properties of a single-crystal nickel-base superalloy, Rene* N4, have been examined at 760 and 980 °C in air. Specimens having crystallographic orientations near [001], [01l], [111], [023], [236], and [145] were tested in fully reversed, total-strain-controlled LCF tests at a frequency of 0.1 Hz. At 760 °C, this alloy exhibited orientation dependent tension-compression anisotropies of yielding which continued to failure. Also at 760 °C, orientations exhibiting predominately single slip exhibited serrated yielding for many cycles. At 980 °C, orientation dependencies of yielding behavior were smaller. In spite of the tension-compression anisotropies, cyclic stress range-strain range behavior was not strongly orientation dependent for either test temperature. Fatigue life on a total strain range basis was highly orientation dependent at both 760 and 980 °C and was related chiefly to elastic modulus, low modulus orientations having longer lives. Stage I crack growth on (111) planes was dominant at 760 °C, while Stage II crack growth occurred at 980 °C. Crack initiation generally occurred at near-surface micropores, but occasionally at oxidation spikes in the 980 °C tests.

A modified interaction parameter formalism for non-dilute solutions

Abstract: A simple modification of the interaction parameter formalism for the thermodynamics of dilute solutions is proposed. The modified formalism reduces exactly to the standard formalism at infinite dilution and preserves the notation as well as the numerical values of the parameters. Hence, existing compilations of interaction parameters can be used directly in the modified formalism. However, the modified formalism is thermodynamically consistent at finite concentrations, whereas the standard formalism is not. Equations for first-order and higher-order modified formalisms are presented for systems of any number of components. In the first-order modified formalism: {fx1211-01}

Effect of microstructure on strength and toughness of heat-treated low alloy structural steels

Abstract: Several low alloy structural steels with different levels of Ni, Cr, and Mo and carbon contents ranging from 0.12 to 0.42 wt pct have been studied to determine the effect of transformation structures on strength and toughness. The strength and toughness were, respectively, evaluated with the yield stress (0.2 pct proof stress) in tensile tests under ambient temperature and ductile-brittle transition temperature (DBTT) in Charpy impact tests. The significant conclusions are as follows: well-defined packets are observed in martensitic and lower bainitic structures, and in this case the packet diameter is the primary microstructural parameter controlling the yield stress and DBTT. The mechanical properties are also improved to a lesser degree with decreasing width of the lath present within the packet. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls the yield stress and DBTT.

A rationalization of stress-strain behavior of two-ductile phase alloys

Abstract: An extensive literature review indicated that the law of mixture rule can at times account for stress-strain behavior of two-ductile phase alloys in terms of the stress-strain behavior of component phases. In the present investigation, various factors which can contribute to the stress-strain behavior of two-ductile phase alloys are considered, using Ti-Mn alloys as the model system. Particular attention is focused on the effect of elastic, elasto-plastic, and plastic interactions between the phases on the stress-strain behavior. It is shown that the law of mixture cannot adequately explain the stress-strain behavior. The following equation is proposed to describe the stress-strain behavior of two-ductile phase alloys: Pα-β = fαPαc+ fβPβc+ Iα-βp, where Pα-β is a given stress-strain property, fα and f/gb are respective volume fractions of α and β-phases, Pαc and Pβc are corrected properties of α and β-phases, and Iα-βp is the interaction term. It is found that for α- β Ti-Mn alloys, for 0.2 pct yield strength, Iα-βp is positive, negative, or zero depending on the microstructure; but Iα-βp is always positive for the ultimate tensile strength and strain hardening rates and its magnitude depended on the microstructure. The reasons for the nature or sign of the interaction parameter for a given property are discussed in detail.

Precipitation and ostwald ripening in dilute AI Base-Zr-V alloys

Abstract: The coarsening rates of both cubic and tetragonal Al3Zr precipitates in Al were measured The tetragonal Al3Zr coarsened 16 times faster than the cubic modification in keeping with the fact that the latter is coherent and coplanar with the matrix while the former forms a semicoherent interface with the matrix giving a larger interfacial energy Partial substitution of V for Zr reduced the precipitate-matrix mismatch for both phases and slowed both coarsening rates as well as retarded the cubic to tetragonal transformation Reducing strain and interfacial energy no doubt is the origin of this effect Since the cubic particles are spherical, their volume fraction is small, and the coherency strains are small, this would appear to be an ideal system for testing the Lifshitz-Slyozov-Wagner theory of diffusion controlled Ostwald ripening While the theory seems to hold, the calculated diffusivity of Zr in Al is much higher than the value reported in the literature Because of the low coarsening rates of the dispersed particles, the Al-Zr-V system shows promise as the basis for a high temperature Al alloy

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

Abstract: The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered.

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part III. Tension-compression anisotropy

Abstract: Single crystal superalloy specimens with various crystallographic orientations were tested in compression at room temperature, 650, 760, 870, and 980 °C. These results are compared with the tensile behavior studied previously. The alloy, Rene* N4, was developed for gas turbine engine blades and has the nominal composition 3.7 Al, 4.2 Ti, 4 Ta, 0.5 Nb, 6 W, 1.5 Mo, 9 Cr, 7.5 Co, balance Ni, in weight percent. Slip trace analysis showed that primary cube slip had occurred even at room temperature for the [111] specimens. With increasing test temperature more orientations exhibited primary cube slip, until at 870 °C only the orientations near [001] and [011] exhibited normal octahedral slip. The yield strength for octahedral slip was numerically analyzed using a model proposed by Lall, Chin, and Pope to explain deviations from Schmid’s law in the yielding behavior of a single phase γ’ alloy, Ni3(Al,Nb). The Schmid’s law deviations in Rene N4 were found to be largely due to a tension-compression anisotropy. This is one of the sources of the Schmid’s law violations observed in Ni3(Al, Nb) which are rationalized by the model. A second effect, which increases the strength of orientations away from [001], was found to be small in Rene N4. Analysis of recently published data on the single crystal superalloy PWA 1480 yielded the same result.

Effects of the prior austenite grain size on the ductility of fully pearlitic eutectoid steel

Abstract: The role of large prior austenite grain sizes in promoting low ductility of fully pearlitic eutectoid steels has been investigated. Independent variation of the pearlite interlamellar spacing,S p, and the prior austenite grain size enabled determination of the microstructural feature controlling ductility in these steels. Tests on a variety of specimens over the temperature range —196 °C to 310 °C showed that specimens heat treated to contain a large prior austenite grain size consistently exhibited lower fracture strains (i.e., ductility), regardless of the type of fracture present, than did fine grained specimens. It appears that the prior austenite grain size controls ductility through its influence prior to catastrophic failure.

Icosahedral and decagonal phase formation in Al-Mn alloys

Abstract: The solidification conditions leading to the formation of the icosahedral phase in Al-Mn alloys have been investigated, using samples prepared by melt spinning and electron beam surface melting. It is found that the icosahedral phase can grow with a range of compositions, but that it grows in competition with another metastable phase which is decagonal. Both of these phases can displace the equilibrium intermetallic phases by nucleating ahead of them in the melt when the solidification velocity is greater than a few centimeters per second. The relative abundance of the icosahedral and decagonal phases varies with composition and solidification rate. Icosahedral crystals in electron beam melt trails are often about 25 μm in diameter, and they grow dendritically along a preferred crystallographic direction.

Computer Simulation of Convection in Moving Arc Weld Pools

Abstract: Computer simulation for three-dimensional convection in moving arc weld pools was described, with three distinct driving forces for flow considered — the electromagnetic force, the buoyancy force, and the surface tension gradient on the pool surface. Formulation of the electromagnetic force in the weld pool was presented. The calculated and experimentally observed fusion boundaries were compared. The arc efficiency and spatial distributions of the current density and power density used in the calculations were based on experimentally measured results, in order to verify the model. The effects of the electromagnetic and surface tension forces were discussed.