Microstructural and microhardness characteristics of laser-synthesized Fe-Cr-W-C Coatings

The effects of laser-processing parameters on the microstructure and microhardness of Fe-Cr-W-C quaternary alloy coatings were investigated experimentally. The coatings were developed by laser processing a powder mixture of Fe, Cr, W, and C at a weight ratio of 10:5:1:1 on a low-carbon steel substrate using a 10 kW continuous wave CO₂ laser. Depending on the processing parameters, either hypoeutectic or hypereutectic microstructures were produced. The hypoeutectic microstructures comprised primary dendrites of nonequilibrium face-centered cubic (fcc) austenite γ phase and eutectic consisting of pseudohexagonal close-packed (hcp) M₇C₃ (M = Cr, Fe, W) carbides and fcc γ phase. The hypereutectic microstructures consisted of hcp M₇C₃ primary carbides and eutectic similar to that in the hypoeutectic microstructures. The formation of hypoeutectic or hypereutectic microstructures was influenced by the alloy composition, particularly the C concentration, which depends on the amount of powder delivered into the melt pool and the extent of substrate melting. The enhancement of the lattice parameter of the γ phase is associated with the significant dissolution of alloying elements and lattice strains resulting from rapid quenching. The higher hardness of the hypereutectic microstructures is principally attributed to the formation of hcp M₇C₃ primary carbides. The relatively lower hardness of the hypoeutectic microstructures is related to the presence of y phase in the primary dendrites, excessive dilution from the base material, and relatively low concentrations of Cr and C. The results provide insight into the significance of laser-processing conditions on the composition and hardness of Fe-Cr-W-C alloy coatings and associated solidification characteristics.     

Study of the fusion zone and heat-affected zone microstructures in tungsten inert gas-welded INCONEL 738LC superalloy

The fusion zone and heat-affected zone (HAZ) microstructures obtained during tungsten inert gas (TIG) welding of a commercial superalloy IN 738LC were examined. The microsegregation observed during solidification in the fusion zone indicated that while Co, Cr, and W segregated to the γ dendrites, Nb, Ti, Ta, Mo, Al, and Zr were rejected into the interdendritic liquid. Electron diffraction and energy-dispersive X-ray microanalyses using a transmission electron microscope (TEM) of secondary phases, extracted from the fusion zone on carbon replicas, and of those in thin foils prepared from the fusion zone showed that the major secondary solidification constituents, formed from the interdendritic liquid, were cubic MC-type carbides and γ-γ’ eutectic. The terminal solidification reaction product was found to consist of M₃B₂ and Ni₇Zr₂ formed in front of the interdendritic γ-γ’ eutectic. Based on the knowledge of the Ni-Ti-C ternary system, a pseudoternary solidification diagram was adapted for IN 738 superalloy, which adequately explained the as-solidified microstructure. The HAZ microfissuring was observed in regions surrounding the fusion zone. Closer and careful microstructural examination by analytical scanning electron microscopy revealed formation of re-solidified constituents along the microfissured HAZ grain boundaries, which suggest that HAZ cracking in this alloy involves liquation cracking. Liquation of various phases present in preweld alloy as well as characteristics of the intergranular liquid film contributing to the alloy’s low resistance to HAZ cracking were identified and are discussed.     

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.     

On the nature of eutectic carbides in Cr-Ni white cast irons

The mechanical and tribological properties of white cast irons are strongly dependent on whether they contain M₇C₃ or M₃C carbides (M = Fe, Cr,etc.). In an effort to improve the wear resistance of such materials, the United States Bureau of Mines has studied the effects of adding 0.3 to 2.3 wt pct (throughout) Si to hypoeutectic irons containing approximately 8.5 pct Cr and 6.0 pct Ni. The eutectic carbides formed were identified by electron microprobe analysis, X-ray diffraction, and scanning electron (SEM) and optical microscopies. In addition, differential thermal analysis (DTA) was used to study the process of solidification. At Si contents of 0.3 and 1.2 pct, the eutectic carbides exhibited a duplex structure, consisting of cores of M₇C₃ surrounded by shells of M₃C. Additionally, the microstructure contained ledeburite (M₃C + γFe (austenite)). At the higher Si content of 1.6 pct, the eutectic carbides consisted entirely of M₇C₃, and some ledeburite remained. Last, when the Si content was raised to 2.3 pct, the eutectic carbides again consisted entirely of M₇C₃, but ledeburite was no longer formed. These observations can be explained in terms of the effects of Si and, to a lesser extent, of Ni on the shape of the liquidus surface of the metastable Fe-Cr-C phase diagram. The addition of Si reduces the roles played by the four-phase class IIp reactionL + M₇C₃ → M₃C + γFe and the ledeburitic eutectic reactionL → M₃C + γFe in the overall process of solidification. N.H. Macmillan, for-merly with the Albany Research Center.     

Microstructure evolution in tial alloys with b additions: Conventional solidification

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α₂ (DO₁₉) and γ (Ll₀) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB₂ (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB₂ for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB₂ are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α₂ and later decompose to a lath structure consisting of alternating layers of γ and α₂.     

Evolution of boride morphologies in TiAl-B alloys

The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB₂ in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB₂ particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix. Formerly Research Assistant, University of California-Santa Barbara (UCSB) Formerly Professor of Materials and Dean of the College of Engineering at UCSB     

Miscibility gap in Fe-Ni-Al and Fe-Ni-Al-Co systems

The phase equilibria in Fe-Ni-Al and Fe-Ni-Al-Co systems have been investigated by the diffusion couple technique, and the miscibility gap that separates α₁ (Fe-rich disordered bcc phase) from α₂ (NiAl-rich ordered bcc phase) has been determined. It has been ascertained that the three-dimensional shape of the miscibility gap is not simple helmetlike, but has a peculiar ridge at the order-disorder transition temperature. The miscibility gap is narrowed and shifted to the Fe-rich corner by the addition of Co, and Co atoms are distributed to the α₂ rather than the α₁ Furthermore, by alloying Co, the fcc γ phase is stabilized and the miscibility gap between α₁ and α₂ is hidden to some extent by the phase regions concerned with the γ phase.     

Correlating the microstructure of the die-chill skin and the corrosion properties for a hot-chamber die-cast AZ91D magnesium alloy

The corrosion of a hot-chamber die-cast AZ91D thin plate (1.4 mm in thickness) was investigated in terms of its microstructure, to elucidate the role of die-chill skin in corrosion. The die-chill skin was composed of a thin layer of chill zone and a thick layer of an interdendritic Al-rich α-Mg/Al₁₂Mg₁₇ β-phase particle/α-Mg grain composite microstructures. The chill zone (4±1 μm in thickness) had fine columnar and equiaxed grains and contained a distribution of submicron Mg-Al-Zn intermetallic particles. Beneath the chill zone, Al₁₂Mg₁₇ β particles were irregularly shaped but did not have an interdendritic network morphology. Furthermore, Al-rich α phase (also known as eutectic α) was in the interdendritic network, which occupied a higher volume fraction than the β phase in the die-skin layer. Corrosion characteristics were studied via constant-immersion and electrochemical tests. Although previous studies have ascribed the fine microstructure to good corrosion resistance for the AZ91D alloy, the present study showed severe corrosion of the sample with a die skin in chloride solution. Moreover, the sample without the die skin on the surface corroded more slowly. The inferior corrosion performance of the die skin was considered to be related to the high volume fraction of the interdendritic network of Al-rich α phase contained in the die skin, owing to the high cooling rate during solidification. The Al-rich α phase does not increase the corrosion resistance of the AZ91D alloy.     

A comparative study of the microstructures observed in statically cast and continuously cast Bi-In-Sn ternary eutectic alloy

A ternary eutectic alloy with a composition of 57.2 pct Bi, 24.8 pct In, and 18 pct Sn was continuously cast into wire of 2 mm diameter with casting speeds of 14 and 79 mm min⁻¹ using the Ohno Continuous Casting (OCC) process. The microstructures obtained were compared with those of statically cast specimens. Extensive segregation of massive Bi blocks, Bi complex structures, and tinrich dendrites was found in specimens that were statically cast. Decomposition of γSn by a eutectoid reaction was confirmed based on microstructural evidence. Ternary eutectic alloy with a cooling rate of approximately 1 °C min⁻¹ formed a double binary eutectic. The double binary eutectic consisted of regions of BiIn and decomposed γSn in the form of a dendrite cell structure and regions of Bi and decomposed γSn in the form of a complex-regular cell. The Bi complex-regular cells, which are a ternary eutectic constituent, existed either along the boundaries of the BiIn-decomposed γSn dendrite cells or at the front of elongated dendrite cell structures. In the continuously cast wires, primary Sn dendrites coupled with a small Bi phase were uniformly distributed within the Bi-In alloy matrix. Neither massive Bi phase, Bi complex-regular cells, nor BiIn eutectic dendrite cells were observed, resulting in a more uniform microstructure in contrast to the heavily segregated structures of the statically cast specimens.     

The behavior of silicon in the solidification of Zn-55Al-1.6Si coating on steel

Silicon is an essential element in the Zn-55Al-1.6Si coating. It is added to promote the formation of an adherent coating and prevent the excessive growth of an intermetallic alloy layer at the steel/coating interface. The addition of silicon also results in the formation of a silicon phase distributed in the interdendritic region of the overlay, having a flowery pattern on the surface, and appearing needlelike when observed inside the overlay. The behavior of silicon during the solidification process of the Zn-55Al-1.6Si coating is examined in the current study. It is found that the coating solidification proceeds in three stages. At stage I, primary α-Al dendrites develop at about 566 °C to 520 °C, forming the framework of the coating structure. This is followed by stage II at about 520 °C to 381 °C, where the binary Al-Si eutectic reaction takes place, with the majority of the silicon phase forming at about 520 °C to 480 °C. At stage III the remaining molten phase undergoes a ternary Al-Zn-Si eutectic reaction forming the interdendritic zinc-rich network. The ternary Al-Zn-Si eutectic reaction is essentially equivalent to the binary Al-Zn eutectic reaction because of the very low level of silicon at the Al-Zn-Si eutectic point.     

Carbide reinforcement in two directionally solidified alloyed nickel eutectic alloys

Two directionally solidified carbide-reinforced alloyed nicel eutectics were evaluated; an alloy consisting of monocarbide fibers in a single phase matrix and one containing monocarbide fibers in a two-phase γ-γ′ matrix. The mechanical properties and microstructures of these alloys are compared to those of two directionally solified alloys having the same nominal matrix compositions, but not containing carbide fibers. The calculated strengths of the monocarbide fibers in the γ′-containing eutectic alloy are 1,400,000 psi (9650 mn/m²) and 243,000 psi (1680 mn/m²) at room temperature and 1000°C, respectively, while those in the single phase γ matrix eutectic at the same temperatures are 590,000 psi (4060 mn/m²) and 298,000 psi (2050 mn/m²). At room temperature, the lower strength of fibers from the γ matrix alloy is believed to result from stress concentrations induced by the presence of growth facets on the fibers. The lower apparent strength at 1000°C of fibers from the γ′-containing eutectic alloy is related to nucleation of needles believed to be M₂₃C₆ on the monocarbide fibers during deformation. These needles appear to act as stress raisers and cause early failure of fibers.     

Microstructural Quantification of Rapidly Solidified Undercooled D2 Tool Steel

Rapid solidification of D2 tool steel is investigated experimentally using electromagnetic levitation (EML) under terrestrial and reduced gravity conditions and impulse atomization (IA), a drop tube type of apparatus. IA produces powders 300 to 1400 μm in size. This allows the investigation of a large range of cooling rates (~100 to 10,000 K/s) with a single experiment. On the other hand, EML allows direct measurements of the thermal history, including primary and eutectic nucleation undercoolings, for samples ~6 to 7 mm in diameter. The final microstructures at room temperature consist of retained supersaturated austenite surrounded by eutectic of austenite and M₇C₃ carbides. Rapid solidification effectively suppresses the formation of ferrite in IA, while a small amount of ferrite is detected in EML samples. High primary phase undercoolings and high cooling rates tend to refine the microstructure, which results in a better dispersion of the eutectic carbides. Evaluation of the cell spacing in EML and IA samples shows that the scale of the final microstructure is mainly governed by coarsening. Electron backscattered diffraction (EBSD) analysis of IA samples reveals that IA powders are polycrystalline, regardless of the solidification conditions. EBSD on EML samples reveals strong differences between the microstructure of droplets solidified on the ground and in microgravity conditions. While the former ones are polycrystalline with many different grains, the EML sample solidified in microgravity shows a strong texture with few much larger grains having twinning relationships. This indicates that fluid flow has a strong influence on grain refinement in this system.

     

M23C6 carbide dissolution mechanisms during heat treatment of ASTM F-75 implant alloys

The dissolution of M₂₃C₆ carbides in an ASTM F-75 alloy was experimentally followed, during a liquid-phase homogenization treatment, in as-cast and pretreated for partial carbide dissolution (PTPCD) specimens. The results revealed that before the fusion of the carbides, solid-state diffusion of the elements forming the carbides occurred. After the fusion of the carbides, a serrated interface developed. Treatment periods longer than 1000 seconds led to a liquid-carbide zone morphology showing the presence of dendrites within the liquid phase. Energy dispersion spectrometry (EDS) analysis revealed that the composition of such dendrites was very close to that of the α-phase matrix. The observed microstructure features are explained in terms of a solutal diffusion-driven mechanism leading to the growth of the matrix by consuming the liquid phase formed by the carbide fusion.     

Modeling of irregular eutectic microstructures in solidification of Al-Si alloys

A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the growth of primary α dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally.     

Processing effects on the grain-boundary character distribution of the orthorhombic phase in Ti-Al-Nb alloys

The grain-boundary character distribution of the orthorhombic (O) phase in Ti₂AlNb intermetallic alloys was investigated. The alloys were thermomechanically processed either above or below the bcc transus temperature. Using electron backscattered diffraction, the twin-related O-phase variant interfacial planes were identified and quantified. For the subtransus-processed samples, the equiaxed-O/equiaxed-O grain boundaries tended to primarily prefer 65-deg misorientations and secondarily prefer 90-deg boundaries. Of the 65-deg misoriented boundaries, which were preferentially rotated about [001], ∼40 pct contained (110) twin-related interfacial planes. The observations were rationalized by the α ₂-to-O phase transformation. It is suggested that for subtransus processing within the α ₂-containing phase regimes, the resulting heat-treated O+bcc microstructures evolve such that the O/O boundaries tend to exhibit distinct twin-related variants with misorientations between 55 and 65 deg. For a supertransus-processed alloy, it was found that approximately equal distributions of the six resolvable O variants were formed from the dominant parent bcc orientation. The resulting O/O boundaries tended to cluster at near-90-deg misorientations, which can be explained by the bcc/O orientation relationship. It is suggested that whenever the O phase primarily transforms from the bcc structure, the resulting O+bcc microstructures evolve such that the O/O boundaries tend to exhibit misorientations near 90 deg.     

Phase transformation in an Fe-10.1Al-28.6Mn-0.46C alloy

The microstructure of an (α + γ) duplex Fe-10.1Al-28.6Mn-0.46C alloy has been investigated by means of optical microscopy and transmission electron microscopy (TEM). In the as-quenched condition, extremely fine D0₃ particles could be observed within the ferrite phase. During the early stage of isothermal aging at 550 °C, the D0₃ particles grew rapidly, especially the D0₃ particles in the vicinity of the α/γ grain boundary. After prolonged aging at 550 °C, coarse K’-phase (Fe, Mn)₃AlC precipitates began to appear at the regions contiguous to the D0₃ particles, and —Mn precipitates occurred on the α/γ and α/α grain boundaries. Subsequently, the grain boundary β-Mn precipitates grew into the adjacent austenite grains accompanied by a γ→ α + β-Mn transition. When the alloy was aged at 650 °C for short times, coarse. K-phase precipitates were formed on the α/γ grain boundary. With increasing the aging time, the α/γ grain boundary migrated into the adjacent austenite grain, owing to the heterogeneous precipitation of the Mn-enrichedK phase on the grain boundary. However, the α/γ grain boundary migrated into the adjacent ferrite grain, even though coarse K-phase precipitates were also formed on the α/γ grain boundary in the specimen aged at 750 °C.     

The structures expected in a simple ternary eutectic system: Part II. The Al-Ag-Cu ternary system

Ternary alloys of various compositions from the aluminum rich corner of the Al-Ag-Cu system were directionally solidified at several different growth rates ranging from 6.4 × 10⁻¹ mm·S⁻¹ to 5.6 × 10⁻³ mm· s⁻¹. The region of two phase coupled growth between α-Al and CuAl₂ was determined at a growth rate of 6.4 × 10⁻¹ mm· s⁻¹. The composition range over which a fully ternary eutectic structure formed was investigated for several different growth rates. The results are found to be consistent with the predictions of the competitive growth model set out in Part I,¹ and it would seem that the ternary eutectic composition of the published phase diagram may be incorrect. Scanning electron microscopy, using the backscattered electron signal, was used, together with optical microscopy, to study the microstructures formed. The ternary eutectic between α-Al, Ag₂Al, and CuAl₂ was found to be semiregular, and the unusual morphology of the two phase dendrites between α-Al and Ag₂Al is explained. Formerly with Alcan Laboratories Ltd., Banbury, Oxon, United Kingdom Formerly with Alcan Laboratories Ltd.     

Effects of interstitial oxygen on microstructure and mechanical properties of Ti-48Al-2Cr-2Nb with fully lamellar and duplex microstructures

The effect of added oxygen in the range of 1000 to 4000 wt ppm on the microstructures of a Ti-48Al-2Cr-2Nb alloy has been investigated and compared to the microstructures for a high-purity alloy. For specimens cooled from theα phase, interstitial oxygen stabilizes the lamellar microstructure with respect toγ grains, with higher than equilibrium values for theα ₂ volume fraction. For specimens cooled from theα +γ phase field, the lamellar microstructure still tends to be favored by oxygen, but it is found that the phase volume fractions are not significantly different from equilibrium values. This suggests that interstitial O essentially reduces the kinetics of theα toα +γ transformation. Thus, interstitial oxygen will have a strong effect on microstructures obtained by continuous cooling fromα, but significantly less on those, such as the duplex microstructure, obtained by long treatment in theα +γ phase field. In general, increased O content is well correlated with reduced ductility. Finally, the role of interstitial oxygen on this phase transformation is discussed.