Age-hardening in the V-C and Nb-C systems

Age hardening measurements have been carried out for alloys of vanadium and niobium containing 0.3 at. pct carbon. For both systems, four distinct stages of hardening are observed as a function of time. For all stages, carbon diffusion in the metal is the rate controlling factor. The process is thought to involve formation of a sequence of subcarbides prior to formation of the stable V₂C or Nb₂C, according to the following scheme: solid solution → carbon-rich zones → series of subcarbides → Me₂C (hexagonal) Overaging occurs as the final coherent subcarbide begins to transform into the stable hexagonal phase with attendant loss of lattice coherency.     

High-temperature nitridation of Ni-Cr alloys

The nitriding behavior of nickel-chromium alloys was investigated at 1398 K over the range 1 to 6000 bar of external nitrogen pressure. The morphology of the nitrided zone depends on the concentration of chromium in the initial alloy and the N₂ pressure (fugacity) applied upon the system. The transition from CrN to Cr₂N precipitation was observed within the reaction zone after nitriding at 100 to 6000 bar of N₂ when the chromium content in the initial alloys was 28.0 at. pct or higher. It is shown that the ternary phase π (Cr₁₀Ni₇N₃) is formed in this system at 1273 K. through a peritectoid reaction between Cr₂N and nickel solid solution and becomes unstable above 1373 K. The thermodynamic evaluation of the Ni-Cr-N system was performed and phase equilibria calculated. Evidence for “up hill” diffusion of nitrogen near the reaction front during the internal nitridation of Ni-Cr alloys at 1398 K was found. It was attributed to the relative instability of chromium nitrides and strong Cr-N interaction in the matrix of the Ni-based solid solution within the nitrided zone.     

Phase transitions in reactive formation of Ti5Si3/TiAl in situ composites

A new method is developed for preparing Ti₅Si₃/TiAl in situ composites by incorporating metastable phases (called metastable precursors) into TiAl (a mixture of elemental Ti and Al) matrix powders. Metastable precursors with a starting composition of Ti-14Al-21Si are prepared by mechanical alloying (MA). They have been proven through X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) observations to be mainly consisting of mixtures of nanostructured solid solutions and milling-formed TiAl compound. Particularly, phase reactions and transitions in the precursors and the composites during heating have been investigated in detail by using diffraction thermal analysis (DTA) in conjunction with XRD. It has been found that Ti₅Si₃ is in situ formed through a phase transition chain, TiSi₂ → Ti₅Si₄ → Ti₅Si₃. When the composite powder (precursor, Ti and Al) is heated, a combustion reaction first occurs in the matrix, which results in the formation of TiAl₃ and/or TiAl followed by the completion of the previously mentioned silicide transitions in a very short time. Scanning electron microscope (SEM) observations indicated the locations of reinforcements in the reaction-formed composite, and TEM observation provided some details of the structures for the reinforcements and their neighborhood. This method is intriguing because a designed phase hierarchy is possible.     

Combustion synthesis of α-Si3N4-(MgO, Y2O3) composites

The possibility of obtaining composite powders of α-Si₃N₄-Y₂O₃ and α-Si₃N₄-MgO by self-propagating high-temperature synthesis (SHS) is studied. It is established that the α → β phase transformation starts at temperatures lower than the melting points of the corresponding eutectics. Metastable composite powders based on α-Si₃N₄ with a high rate of phase transition are obtained. The composite powders (α-Si₃N₄-Y₂O₃, α-Si₃N₄-MgO) are used in hot pressing technology. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 10–14, 2007.     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C. The transition in morphology of the carbide phase is discussed in terms of the relative volume fraction of the phases, growth rate, and orientation relationships. The carbide morphology is influenced by the growth rate and carbon content. For constant growth rate increasing the volume fraction of the carbide phase favors the lamellar morphology. At low growth rates the lamellar morphology is favored, and at high growth rates the rod-like morphology is favored. Growth crystallography has no direct influence on the transition in carbide morphology.     

The effect of additives on the eutectoid transformation of ductile iron

The eutectoid transformation of austenite in cast iron is known to proceed by both the meta-stable γ → α + Fe₃C reaction common in Fe-C alloys of near eutectoid composition, and by the direct γ → α + Graphite reaction, with the graphite phase functioning as a car-bon sink. In addition, the meta-stable cementite constituent of the pearlite can dissolve near the graphite phase (Fe₃C → α + Graphite), producing free ferrite. Isothermal trans-formation studies on a typical ductile iron (nodular cast iron) confirmed that all of these reaction mechanisms are normally operative. The addition of 1.3 pct Mn was found to substantially retard all stages of the transformation by retarding the onset of the eutectoid transformation, decreasing the diffusivity of carbon in ferrite, and stabilizing the cemen-tite. Minor additions of Sb (0.08 pct) or Sn (0.12 pct) were found to inhibit the γ →α + Graphite reaction path, as well as the Fe₃C → α + Graphite dissolution step, but did not significantly affect the meta-stable γ → α + Fe₃C reaction. Scanning Auger microprobe analysis indicated that Sn and Sb adsorb at the nodule/metal interphase boundaries during solidification. This adsorbed layer acts as a barrier to the carbon flow necessary for the direct γ → α + Graphite and Fe₃C → α + Graphite reactions. With the graphite phase dis-abled as a sink for the excess carbon, the metal transforms like a nongraphitic steel. The effects of Mn, Sn, and Sb on the eutectoid transformation of ductile iron were shown to be consistent with their behavior in malleable iron.     

Coupling phenomena in the transfer of elements between a gas phase and iron melts

The simultaneous transfer of Si and C from a gas phase containing SiO and CO to liquid Fe-C alloys has been investigated. It was found that, although the silicon content of the melt increased with time as expected, the carbon content initially decreased, in spite of the fact that the carbon potential of the gas phase was above that of the liquid alloy. These phenomena are interpreted in terms of irreversible thermodynamics which shows that the overall transfer reactions are comprised of coupled reactions: SiO(g) + CO(g) →Si + CO₂(g) andC + CO₂(g) → 2CO(g). It is also shown that the simultaneous transfer of carbon and oxygen from gas mixtures of CO and CO₂ to liquid iron occurs via the coupled reactions CO₂(g) →O + CO(g) and 2CO(g) →C + CO₂. In each case there is a predominant, or driving reaction which promotes the other.     

Phase transformation in an Fe-9.0AI-29.5Mn-1.2Si alloy

The microstructure of an (α + γ) duplex Fe-9.0Al-29.5Mn-l.2Si alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, extremely fine D0₃ particles were formed within the ferrite matrix by a continuous ordering transition during quenching. After being aged at 550 °C, the extremely fine D0₃ particles existing in the as-quenched specimen grew preferentially along (100) directions. With increasing the aging time at 550 °C, a (Si, Mn)-rich phase (designated as “L phase”) began to appear at the regions contiguous to the D0₃ particles. The L phase has never been observed in various Fe-Al-Mn, Fe-Al-Si, Fe-Mn-Si, and Mn-Al-Si alloy systems before. When the as-quenched specimen was aged at temperatures ranging from 550 °C to 950 °C, the phase transformation sequence occurring within the (α + D0₃) region as the aging temperature increases was found to be (α + D0₃ + L phase) → (α + D0₃ + A13 β-Mn)→ (B2 + D0₃ + A13 β-Mn)→ (B2 + A13β-Mn)→ (α + A13 β-Mn)→ (α +γ)→α.     

Time-temperature-precipitation characteristics of high-nitrogen austenitic Fe−18Cr−18Mn−2Mo−0.9N steel

The time-temperature-precipitation (TTP) and corresponding mechanical properties in high-nitrogen austenitic Fe−18Cr−18Mn−2Mo−0.9N steel (all in weight percent) were investigated using electron microscopy and ambient tensile testing. The precipitation reactions can be categorized into three stages: (1) high-temperature region (above 950°C)—mainly coarse grain-boundary (intergranular) Cr₂N; (2) nose-temperature region—integranular Cr₂N→cellular Cr₂N→intragranular Cr₂N+ sigma (σ); and (3) low-temperature region (below 750°C)—intergranular Cr₂N→cellular Cr₂N→ intragranular Cr₂N+σ+chi(χ)+M₇C₃ carbide. After cellular Cr₂N precipitation became dominant above 800°C, yield and tensile strength gradually decreased, whereas elongation abruptly deteriorated with aging time. On the contrary, prolonged aging in the low-temperature regime increased tensile strength, caused by the precipitation of fine χ and M₇C₃ within grains. Based on the analyses of selected area diffraction (SAD) patterns, the crystallographic features of the second phases were analyzed.     

Precipitation processes in near-equiatomic TiNi shape memory alloys

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. M. NISHIDA, formerly with the University of Illinois     

Thermodynamic assessment of the Nb-N system

The phase equilibrium and thermodynamic information of the Nb-N system was reviewed and assessed by using thermodynamic models for the Gibbs energy of individual phases. Although there was a large amount of experimental information of the system, heat capacity data of the Nb₂N and NbN were not available either in low or high temperatures. In the present study, low-temperature heat capacity and the^o S ₂₉₈ values were estimated using estimated entropy Debye temperatures. Only the Nb₂N (hcp) and NbN (fcc) nitrides were considered to be the true binary phases and were included in the present evaluation in addition to the N₂ gas, liquid, andα-solid solution (bcc). Three thermodynamic models were used: a two-sublattice model for the solid solution phases, a substitutional model for the liquid phase, and an ideal-gas model for the N₂ gas. The model parameters were evaluated by fitting to the selected data by means of a computer program. A consistent set of parameters was obtained which satisfactorily described most of the experimental and estimated data.     

Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V₂N and δ-VN_1−x develop. The morphology of the β-V₂N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V₂N and δ-VN_1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V₂N) and 2.93 eV (δ-VN_1−x ). By using δ-VN_1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V₂N is formed on the surface of the couple as a result of out-diffusion of nitrogen.     

Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution (γ′_N) based on the γ′-Fe₄N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at. pct) and 〈110〉 streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above the 20 at. pct γ′-Fe₄N-phase value and leads to a lattice parameter that is greater than that of the γ′-Fe₄N phase. The subsurface of the layer is composed of a supersaturated solid solution based on γ-austenite, which is an expanded austenite, γ _N. Its morphology shows the basketweave or “tweedlike” contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The ε martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of ε martensite in the subsurface of the layer indicate that nitrogen lowers the stacking fault energy of austenite.     

Formation and stability of a nitride with the structure of beta manganese in Ni-Cr-N ternary system

A nitride with the metal-atom arrangement of β manganese, which is designated as π phase, was confirmed to exist as an equilibrium phase in the ternary Cr-Ni-N system. A Cr-40 mass pct Ni binary alloy was nitrided under a pure nitrogen atmosphere at 1523 K to prepare a Cr-40Ni-5N alloy consisting of a nickel-rich face-centered cubic (fcc) γ phase and a dichromium nitride, Cr₂N, designated as ε phase. Upon aging the alloy at 1273 K, the π phase was formed through a peritectoid reaction between the γ and ε phases. A volume fraction of the π phase reached 95 pct after 3.6 × 10⁵ s, and no more change of the volume fraction was observed, even after 3.6 × 10⁷ s aging. The chemical composition of the π phase was determined to be Cr₁₃Ni₇N₄, whose lattice parameter wasa = 0.6323 nm. The π phase became unstable above 1473 K, which explains a previous unsuccessful attempt to produce the Cr-Ni-N ternary π phase by the replacement of molybdenum in Mo₁₂Ni₈N₄ by chromium at 1473 K.     

Microstructure and Phase Evolution in Laser Cladding of Ni/Cu/Al Multilayer on Magnesium Substrates

A multilayer coating of Ni/Cu/Al was fabricated on magnesium substrates using laser cladding. The solidification behavior and the phase evolution of the compositionally graded coating were studied. The results of the X-ray diffraction (XRD) analysis together with the metallographic study showed that a series of phase evolutions had occurred along the gradient (Mg) → (Mg) + Al₁₂Mg₁₇ → (Mg) + Q + λ ₂ → λ ₁ → λ ₁ + γ ₁ → γ ₁ + (Cu) + λ ₁ → (Cu) + λ ₁ → (Cu) → (CuNi) → (Ni). The rapid solidification condition had suppressed the invariant reactions that existed in the ternary Mg-Al-Cu alloy system. As a result, many of the predicted Al-rich brittle intermetallic compounds, which are detrimental to the performance of the coating, were not produced. The solidification path during the laser cladding of the Al and Cu layers was determined and the various phases, as predicted by the corresponding phase diagram, agreed well with the experimental results. Finally, the primary arm spacing (PAS) and the solidification morphology of the dendrites in the Cu and Ni layers were analyzed in relation to the solidification conditions.     

Phase transformation and microstructural evolution in Ti-44Al-4Nb-4Zr alloy during heat treatment

Phase transformation and microstructural evolution have been studied in Ti-44Al-4Nb-4Zr-0.2Si-0.1B alloys that were cooled from theα +β phase region with various cooling rates. It has been shown that the cooling rates have different influence on the morphology of the transformation products for the three phase transformations studied,α →α ₂, B2 →ω, andα →γ. Under slow cooling, all three transformations can be fulfilled. Under rapid cooling, B2 →ω is partially detained and a diffuseω phase forms as metastable phase, butα →γ is almost completely suppressed, which supports that theγ lamellae formation is diffusion controlled.     

The electron metallography of ordering reactions in Fe-Al alloys

The microstructural changes during the α→ FeAl, FeAl → Fe₃Al, and α→ Fe₃Al transitions were studied by transmission electron microscopy. The ordering of ferromagnetic α was observed to occur in a classical manner by the nucleation and growth of particles of the FeAl or Fe₃Al type phases. However, the ordering of paramagnetic α to FeAl and paramagnetic FeAl to Fe₃Al occurred by a mechanism which showed many of the characteristics expected of a second or higher degree transition. These included critical point fluctuations in the degree of long-range order which also appeared to be greater in the vicinity of antiphase domain boundaries. The FeAl phase was also observed to partially disorder in the initial stages of the FeAl → α + FeAl transition and this effect appears to account for the anomalous magnetic behavior of some FeAl alloys.     

Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V₂N and δ-VN_1−x develop. The morphology of the β-V₂N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V₂N and β-VN_1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V₂N) and 2.93 eV (δ-VN_1−x ). By using δ-VN_1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V₂N is formed on the surface of the couple as a result of out-diffusion of nitrogen.     

Fabrication and characteristics of AA6061/Si3N4p composite by the pressureless infiltration technique

The tensile properties and microstructures of AA6061/Si₃N₄ particle composites fabricated by pressureless infiltration under a nitrogen atmosphere were analyzed. In addition, the control AA6061 without Si₃N₄ particles fabricated by the same method was investigated to separate the effect of Si₃N₄ particle addition. It was found that AlN particle layers formed on the surface of Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the following reaction: Mg₃N₂ + 2Al → 2AlN + 3Mg. Thus, the spontaneous infiltration results from a great enhancement of wetting via the formation of Mg₃N₂ by the reaction of Mg vapor and nitrogen gas. The increased tensile strength and 0.2 pct offset yield strength in the control AA6061 were largely due to fine AlN particles formed by the aforementioned in situ reactions, as compared to commercial AA6061. In the composite reinforced with Si₃N₄ particles, of course, the AlN was also formed through the following additional reaction at the Si₃N₄ particle/Al melt interfaces: Si₃N₄ + 4Al → 4AlN + 3Si. However, this AlN may not contribute to the increase in strength because its formation is compensated by the consumption of Si₃N₄ particles. Consequently, the strength increase of the composite fabricated by the present method is attributed to the fine AlN particles formed in situ, as well as the fine reinforcing Si₃N₄ particles, as compared to commercial AA6061.