A Eutectoid Reaction for the Decomposition of Austenite into Pearlitic Lamellae of Ferrite and M<Subscript>23</Subscript>C<Subscript>6</Subscript> Carbide in a Mn-Al Steel

We discovered a eutectoid reaction in an Fe-13.4Mn-3.0Al-0.63C (wt pct) steel after solution heat treatment at 1373 K (1100 °C) and holding at temperatures below 923 K (650 °C). The steel is single austenite at temperatures from 1373 K to 923 K (1100 °C to 650 °C). A eutectoid reaction involves the replacement of the metastable austenite by a more stable mixture of ferrite and M₂₃C₆ phases at temperatures below 923 K (650 °C). The mixture of ferrite and M₂₃C₆ is in the form of pearlitic lamellae. The morphology of the lamellae of the product phases is similar to that of pearlite in steels. Thus, we found a new pearlite from the eutectoid reaction of the Mn-Al steel featuring γ  → α + M₂₃C₆. A Kurdjumov–Sachs (K-S) orientation relationship exists between the pearlitic ferrite (α) and M₂₃C₆ (C6) grains, i.e., (110)_α // (111)_C6 and [[`1] \overline{1} 11]<sub>α</sub> // [0[`1] \overline{1} 1]_C6. The upper temperature limit for the eutectoid reaction is between 923 K and 898 K (650 °C and 625 °C).     

On the decomposition of β phase in some rapidly quenched titanium-eutectoid alloys

The decomposition of the β phase in rapidly quenched Ti-2.8 at. pct Co, Ti-5.4 at. pct Ni, Ti-4.5 at. pct, and 5.5 at. pct Cu alloys has been investigated by electron microscopy. During rapid quenching, two compctitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The β phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pct Co and Ti-4.5 pct Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pct Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid α and the intermetallic compound in the nonlamellar type as well as between the proeutectoid α and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti₂Cu phase during rapid quenching involved the formation of a metastable phase closely related to an “ω-type” phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti₂Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid a crystals formed during rapid quenching.     

Fracture and the formation of sigma phase,M23C6, and austenite from delta-ferrite in an AlSl 304L stainless steel

The decomposition of delta-ferrite and its effects on tensile properties and fracture of a hot-rolled AISI 304L stainless steel plate were studied. Magnetic response measurements of annealed specimens showed that the transformation rate of delta-ferrite was highest at 720 °C. Transformation behavior was characterized by light microscopy, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy on thin foils. The initial transformation of delta-ferrite (δ) to austenite (γ) and a chromium-rich carbide (M₂₃C₆) occurred by a lamellar eutectoid reaction, δ⇄M₂₃C₆ +γ. The extent of the reaction was limited by the low carbon content of the 304L plate, and the numerous, fine M₂₃C₆ particles of the eutectoid structure provide microvoid nucleation sites in tensile specimens annealed at 720 °C for short times. Sigma phase(σ) formed as a result of a second eutectoid reaction,δσ +γ. Brittle fracture associated with the plate-shaped sigma phase of the second eutectoid structure resulted in a significant decrease in reduction of area (RA) in the transverse tensile specimens. The RA for longitudinal specimens was not affected by the formation of sigma phase. Tensile strengths were little affected by delta-ferrite decomposition products in either longitudinal or transverse orientations. Y. Shen, formerly with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is deceased.     

Alloying of a P/M titanium-nickel composite of eutectic composition

Conclusions The addition of 2% P or B has no effect on the melting point of a Ti-Ni alloy of eutectic composition, 7% Si raises it by 50°K, and 10% Cu lowers it by 100°K. Alloying with boron and phosphorus does not alter the phase composition and structure of the alloy. Boron and phosphorus dissolve in the titanium phase of the eutectoid, thereby increasing the latter's hardness. Alloying with copper suppresses the eutectoid decomposition of the titanium-nickel alloy during cooling. Copper becomes distributed between the two structural constituents, as a result of which the hardness of the intermetallic compound Ti₂Ni falls and that of the eutectic rises. Alloying with silicon leads to the formation of a new phase, Ti₃Si.Translated from Poroshkovaya Metallurgiya, No. 2(254), pp. 57–60, February, 1984.     

Determination of Solid Fraction from Cooling Curve

A new method to determine directly the solid fraction using the cooling curve was proposed for solidification of undercooled melts. Then, to construct three different baselines, a sudden function ξ _α (x) is introduced. In terms of the ξ _α (x) function, accordingly, the solid fractions during solidification of Ni-3.3 wt pct B, Al-7 wt pct Si, Al-14 wt pct Cu, and Fe-4.56 wt pct Ni alloys were predicted. The predictions of the primary, the regular lamellar eutectic, the anomalous eutectic, and the peritectic phases from cooling curves of the solidified samples coincide with the results of measurement or the available methods.     

The Coexistence of Two Different Pearlites,Lamellae of (Ferrite + M<Subscript>3</Subscript>C), and Lamellae of (Ferrite + M<Subscript>23</Subscript>C<Subscript>6</Subscript>) in a Mn-Al Steel

Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel. The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M₃C and M₂₃C₆ carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously at temperatures below 923 K (650 °C). One is lamellae of ferrite and M₃C carbide, and the other is lamellae of ferrite and M₂₃C₆ carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M₂₃C₆. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately.     

Microstructural Evolution of Brazed CP-Ti Using the Clad Ti-20Zr-20Cu-20Ni Foil

Microstructural evolution of the clad Ti-20Zr-20Cu-20Ni foil brazed CP-Ti alloy has been investigated. For the specimen furnace brazed below 1143?K (870???C), the joint is dominated by coarse eutectic and fine eutectoid structures. Increasing the brazing temperature above 1163?K (890???C) results in disappearance of coarse eutectic structure, and the joint is mainly comprised of a fine eutectoid of (Ti,Zr)₂Ni, Ti₂Cu, Ti₂Ni, and ??-Ti.     

Thermodynamic prediction of the eutectoid transformation temperatures of low-alloy steels

The experimental eutectoid transformation temperatures (A ₁) of low-alloy steels, as reported in the USS Atlas of I-T diagrams, have been compared to the thermodynamic predictions of a model proposed by Kirkaldy and Venugopalan. The analysis is consistent with the model prediction that Cr atoms are almost fully partitioned, while Ni and Mo atoms are scarcely partitioned, during the eutectoid transformation. This study also shows that Mn atoms are partitioned fully or partly in C-Mn, Cr-Mn, and Mo-Mn steels, while they are scarcely partitioned in Ni-Mn steels. The difference (ΔT) between the orthoequilibrium (OE) eutectoid temperature (A _e1) and the paraequilibrium (PE) eutectoid temperature (A _p1) has been investigated as a function of the content of each substitutional alloying element. The slope of ΔT increases with substitutions of Mo, Ni, Mn, Si, and Cr, with Mo having the least effect, Ni the next-greatest effect, and so on. Considering both Mn partitioning and the slope of ΔT, the equation for the prediction of A ₁ temperatures of low-alloy steels proposed by Kirkaldy and Venugopalan is modified. This new equation is in better agreement with the experimental A ₁ temperatures.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

Effect of Rare-Earth (La,Ce, and Y) Additions on the Microstructure and Mechanical Behavior of Sn-3.9Ag-0.7Cu Solder Alloy

In this article, we report on the microstructure and mechanical properties of Ce- and Y-containing Sn-3.9Ag-0.7Cu solders. The microstructures of both as-processed solder and solder joints containing rare-earth (RE) elements (up to 0.5 wt pct) are more refined compared to conventional Sn-3.9Ag-0.7Cu, with decreases in secondary Sn dendrite size and spacing and a thinner Cu₆Sn₅ intermetallic layer at the Cu/solder interface. These results agree well with similar observations seen in La-containing solders reported previously. The monotonic shear behavior of reflowed Sn-3.9Ag-0.7Cu-X(Ce, Y)/Cu lap shear joints was studied as well as the creep behavior at 368 K (95 °C). The data were compared with results obtained for Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-XLa alloys. All RE-containing alloys exhibited creep behavior similar to Sn-3.9Ag-0.7Cu. Alloys with Ce additions exhibited a small decrease in ultimate shear strength but higher elongations compared with Sn-Ag-Cu. Similar observations were seen in La-containing solders. The influence of the RE-containing intermetallics (CeSn₃ and YSn₃) that form in these alloys on the microstructural refinement, solidification behavior, and mechanical performance of these novel materials is discussed.     

Kinetics of austenite-pearlite transformation in eutectoid carbon steel

The kinetics of the austenite-to-pearlite transformation have been measured under isothermal and continuous-cooling conditions on a eutectoid carbon (1080) steel using a diametral dilatometric technique. The isothermal transformation kinetics have been analyzed in terms of the Avrami Equation containing the two parametersn andb; the initiation of transformation was characterized by an empirically determined transformation-start time (t_Av). The parametern was found to be nearly constant; and neithern norb was dependent on the cooling rate betweenT _A1 and the test temperature. Continuous-cooling tests were performed with cooling rates ranging from 7.5 to 108 °C per second, and the initiation of transformation was determined. Comparison of this transformation-start time for different cooling rates with the measured slow cooling of a test coupon immersed in a salt bath indicates that, particularly at lower temperatures, the transformation in the traditional T-T-T test specimen may not be isothermal. The additivity rule was found to predict accurately the time taken, relative to t_Av, to reach a given fraction of austenite transformed, even though there is some question that the isokinetic condition was met above 660 °C. However, the additivity rule does not hold for the pretransformation or incubation period, as originally proposed by Scheil, and seriously overestimates the incubation time. Application of the additivity rule to the prediction of transformation-finish time, based on transformation start at T_A1, also leads to overestimates, but these are less serious. The isothermal parameters—n (T),b (T), and t_Av (T)—have been used to predict continuous-cooling transformation kinetics which are in close agreement with measurements at four cooling rates ranging from 7.5 to 64 °C per second.     

Interfacial Microstructure and Bonding Strength of Copper Cladding Aluminum Rods Fabricated by Horizontal Core-Filling Continuous Casting

Copper cladding aluminum (CCA) rods with a diameter of 30 mm and a sheath thickness of 3 mm were fabricated by horizontal core-filling continuous casting (HCFC) technology. The microstructure and morphology, distribution of chemical components, and phase composition of the interface between Cu and Al were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersive spectrometer (EDS). The formation mechanism of the interface and the effects of key processing parameters, e.g., aluminum casting temperature, secondary cooling intensity, and mean withdrawing speed on the interfacial microstructure and bonding strength were investigated. The results show that the CCA rod has a multilayered interface, which is composed of three sublayers—sublayer I is Cu₉Al₄ layer, sublayer II is CuAl₂ layer, and sublayer III is composed of α-Al/CuAl₂ pseudo eutectic. The thickness of sublayer III, which occupies 92 to 99 pct of the total thickness of the interface, is much larger than the thicknesses of sublayers I and II. However, the interfacial bonding strength is dominated by the thicknesses of sublayers I and II; i.e., the bonding strength decreases with the rise of the thicknesses of sublayers I and II. When raising the aluminum casting temperature, the total thickness of the interface increases while the thicknesses of sublayers I and II decrease and the bonding strength increases. Either augmenting the secondary cooling intensity or increasing the mean withdrawing speed results in the decrease in both total thickness of the interface and the thicknesses of sublayers I and II, and an increase in the interfacial bonding strength. The CCA rod with the largest interfacial bonding strength of 67.9 ± 0.5 MPa was fabricated under such processing parameters as copper casting temperature 1503 K (1230 °C), aluminum casting temperature 1063 K (790 °C), primary cooling water flux 600 L/h, secondary cooling water flux 700 L/h, and mean withdrawing speed 87 mm/min. The total thickness of the interface of the CCA rod fabricated under the preceding processing parameters is about 75 μm, while the thicknesses of sublayers I and II are about 1.1 and 0.1 μm, respectively.     

Failure Mechanism of a Stellite Coating on Heat-Resistant Steel

The Stellite 21 coating on the heat-resistant steel X12CrMoWVNbN10-1-1 (so-called COSTE) used in a steam turbine valve was found to be fatigue broken after service at around 873 K (600 °C) for about 8 years. In order to investigate the failure mechanism, a fresh Stellite 21 coating was also prepared on the same COSTE steel substrate by using the similar deposition parameters for comparison. It was found that the Stellite 21 coating was significantly diluted by the steel, resulting in a thin Fe-rich layer in the coating close to the fusion line. Such high Fe concentration together with the incessant Fe diffusion from the steel substrate to the coating during the service condition (about 873 K (600 °C) for long time) induced the eutectoid decomposition of the fcc α-Co(Fe,Cr,Mo) solid solution, forming an irregular eutectoid microstructure that was composed of the primitive cubic α′-FeCo(Cr,Mo) phase and the tetragonal σ-CrCo(Fe,Mo) phase. The brittle nature of such α′/σ eutectoid microstructure contributed to the fatigue fracture of the Stellite 21 coating, resulting in an intergranular rupture mode.

     

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.     

Oxidation Behavior of CuZr-Based Glassy Alloys at 400 °C to 500 °C in Dry Air

The oxidation behavior of the Cu_47.5Zr_47.5Al₅ (Cu3) and Cu₄₇Ti₃₄Zr₁₁Ni₈ (Cu4) bulk metallic glasses (BMGs) was studied over the temperature range of 400 °C to 500 °C in dry air. The oxidation kinetics of both alloys generally followed a multistage parabolic-rate law, and the steady-state parabolic-rate constants (k _p values) fluctuated with temperature for the Cu3 BMG, but increased with increasing temperature for the Cu4 BMG. The scales formed on the BMGs were strongly dependent on the temperature and alloy composition, and were composed primarily of tetragonal-ZrO₂ (t-ZrO₂) and minor amounts of Al₂O₃, Cu₂O, and CuO at 400 °C for the Cu3 BMG, while the monoclinic-ZrO₂ (m-ZrO₂) phase is present at T ≥ 425 °C, and the Cu₂O phase is absent at 500 °C. Conversely, the scales formed on the Cu4 BMG consisted exclusively of CuO at 400 °C, while minor amounts of t-ZrO₂, TiO₂, and ZrTiO₄ formed at 425 °C to 450 °C, and TiO was also detected at higher temperatures. It was found that both amorphous Cu3 and Cu4 substrates transformed into different crystalline phases, and were strongly dependent on temperature and duration of time. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred during the TMS Annual Meeting February 25–March 1, 2007, in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Age-Induced Precipitating and Strengthening Behaviors in a Cu–Ni–Al Alloy

Microstructural response and variations in strength and electrical conductivity of a Cu−20 at. pct Ni–6.7 at. pct Al alloy during isothermal aging at temperatures from 723 K to 1023 K (450 °C to 750 °C) were investigated to discuss the age-induced precipitation behavior and strengthening mechanism. At all aging temperatures, fine spherical γ′-Ni₃Al particles were found to nucleate coherently with parent Cu grains by continuous precipitation and then grew gradually by Ostwald ripening. Domains with a high density of twins developed at grain boundaries during aging below 873 K (600 °C) followed by cellular components composed of fiber-shaped γ′-Ni₃Al and Cu solid solution phases at the domain boundaries later. Both the domains and cellular components were suppressed at aging above 923 K (650 °C). The age-induced strengthening principally resulted from fine dispersion of γ′-Ni₃Al coherent particles in the grains. The precipitation strengthening by the fine γ′-Ni₃Al coherent particles exhibited a maximum at an aging temperature of 873 K (600 °C), resulting in excellent mechanical properties such as a high hardness of 340 ± 7 HV and an ultimate tensile strength of 980 ± 14 MPa, which are comparable to those of other commercial age-hardened Cu–Be, Cu–Ni–Si, and Cu–Ti alloys.

     

The Role of Si and Cu Alloying Elements on the Dendritic Growth and Microhardness in Horizontally Solidified Binary and Multicomponent Aluminum-Based Alloys

Horizontal directional solidification (HDS) experiments were carried out with Al-3wtpctCu, Al-3wtpctSi, and Al- 3wtpctCu-5.5wtpctSi alloys in order to analyze the interrelation between the secondary dendrite arm spacing (λ ₂) and microhardness (HV). A water-cooled horizontal directional solidification device was applied. Microstructural characterization has been carried out using traditional techniques of metallography, optical, and SEM microscopy. The ThermoCalc software was used to generate the phase equilibrium diagrams as a function of Cu and Si for the analyzed alloys. The effects of Si and Cu elements on the λ ₂ and HV evolution of the hypoeutectic binary Al-Cu and Al-Si alloys have been analyzed as well as the addition of Si in the formation of ternary Al-Cu-Si alloy. The secondary dendrite arm spacing was correlated with local solidification thermal parameters such as growth rate (V _L), cooling rate (T _R), and local solidification time (t _SL). This has allowed to observe that power experimental functions given by λ ₂ = Constant (V _L)^−2/3, λ ₂ = Constant (T _R)^−1/3 and λ ₂ = Constant (t _SL)^1/3 may represent growth laws of λ ₂ with corresponding thermal parameters for investigated alloys. Hall–Petch equations have also been used to characterize the dependence of HV with λ ₂. A comparative analysis is performed between λ ₂ experimental values obtained in this study for Al-3wtpctCu-5.5wtpctSi alloy and the only theoretical model from the literature that has been proposed to predict the λ ₂ growth in multicomponent alloys. Comparisons with literature results for upward directional solidification were also performed.

     

Discussion of “effects of tensile stress on microstructural change of eutectoid Zn-AI alloy”

In a recent contribution,[1] Zhu and Orozco presented a phase transformation of the ternary alloy Zn-20.2 wt pct Al-1.8 wt pct Cu, studied under tensile stress by using X-ray diffraction and scanning electron microscopy techniques. The authors report the existence of three phases in the alloy at room temperature after furnace cooling,α,ε, and a newη _′ ^T instead of the zinc-rich solid solutionη, as appears in the phase diagrams. The reported parameters for this hcp metastable phase are^[1,2] a = 0.2663 andc = 0.4873 nm; these values are close to the parameters of pure zinc,^[3] witha = 0.2664 nm andc = 0.4946 nm. The difference betweenη _′ ^T and zinc in thea parameter is around 0.03 pct, and it is 1.47 pet for thec parameter. When zinc is saturated with aluminum in the Zn-AI alloys, thea parameter shrinks^[3] to 0.2660 nm. It is possible to see that the value ofa of theη _′ ^T phase lies in-between the values of pure zinc and zinc-aluminum solid solution. The solubility of Al and Cu in Zn^[4] at 100 °C is 0.3 wt pct Cu and 0.06 wt pct Al. The covalent radius of Cu (0.117 nm) is smaller than the covalent radius of Al (0.118 nm) and Zn (0.125 nm), so the introduction of Cu in the zinc structure can result in a reduction of thec parameter. These values suggest that the metastable phaseη _′ ^T could be the hcp zincrich solid solution with low aluminum and copper contents. The article of Zhu and Goodwin,^[5] cited by Zhu and Orozco in their Reference 14, is related not to the eutectoid alloy, as they argue, but to an alloy with 27 wt pct Al, and no reports about the transformation ofε intoT′ were found. The presence of the metastable e phase (CuZn₄, sometimes called CuZn₅) at room temperature and its transformation to the stable phaseT′ (rhombohedral intermetallic phase, Al₄Cu₃Zn) have been observed by other authors.^[6,7] Y.H. ZHU and E. OROZCO:Metall Mater. Trans. A, 1995, vol. 26A, pp. 2611-15.