Eutectic-type Reaction Research Articles

Diffusion-limited Nonequilibrium Phase Transformation of Nickel/Aluminum Dissimilar Materials during Laser Welding Part I: Precipitation-strengthened Polycrystalline Nickel-based Superalloy

Polycrystalline heterogeneities of grain growth, phase transformation and tensile properties are discussed by metallography, spectroscopy and fractography when choosing laser welding with butt-joint configuration to satisfactorily minimize effect of inappropriate weld pool shape on microstructure and mechanical properties of γʹʹ precipitation hardable nickel-based superalloy in aerospace industry. There is parabolic relationship between weld penetration and diffusion-limited nucleation, growth and coarseness of secondary dendrite arm spacing alongside fusion boundary through nonequilibrium solidification process, which is dendritically susceptible to grain growth variation and metallurgical discontinuities. The amount, size, morphology and distribution of nonequilibrium intermetallic phase near the dendrite boundaries are kinetically and thermodynamically rely on weld pool shape whose crystal structure is incoherent with γ solid solution austenite phase and increases preference for crack initiation and propagation of brittle intergranular and ductile dimple fracture failures to enormously contribute to losses of strength and ductility. Unsymmetrical keyhole weld is thermometallurgically inconvenient for amelioration of microstructure and mechanical properties on either side, and adversely mitigates resistance to hypereutectic-aided dendrite embrittlement. Beneficial grain refinement and suppression of detrimental nonequilibrium intermetallic phase at the same time are challenging. This problem is an integral part of inhomogeneous microstructure and mechanical properties. The finer grain size occurs, the larger grain boundary area is available for Laves/γ eutectic-type reaction and vice versa. Contributions of coarse grain kinetics and metallurgical reaction thermodynamics to weld disintegration and fracture failure mechanism of tensile properties are explained by microstructure characterization of multicomponent and multiphase weld. Finally, it is imperative to dendritically balance these important factors to minimize inevitable interface solidification products and anomalous substructure growth, and reasonably advance superior mechanical properties of reliable weld.

Phase Composition of a CrMo0.5NbTa0.5TiZr High Entropy Alloy: Comparison of Experimental and Simulated Data

Microstructure and phase composition of a CrMo0.5NbTa0.5TiZr high entropy alloy were studied in the as-solidified and heat treated conditions. In the as-solidified condition, the alloy consisted of two disordered BCC phases and an ordered cubic Laves phase. The BCC1 phase solidified in the form of dendrites enriched with Mo, Ta and Nb, and its volume fraction was 42%. The BCC2 and Laves phases solidified by the eutectic-type reaction, and their volume fractions were 27% and 31%, respectively. The BCC2 phase was enriched with Ti and Zr and the Laves phase was heavily enriched with Cr. After hot isostatic pressing at 1450 °C for 3 h, the BCC1 dendrites coagulated into round-shaped particles and their volume fraction increased to 67%. The volume fractions of the BCC2 and Laves phases decreased to 16% and 17%, respectively. After subsequent annealing at 1000 °C for 100 h, submicron-sized Laves particles precipitated inside the BCC1 phase, and the alloy consisted of 52% BCC1, 16% BCC2 and 32% Laves phases. Solidification and phase equilibrium simulations were conducted for the CrMo0.5NbTa0.5TiZr alloy using a thermodynamic database developed by CompuTherm LLC. Some discrepancies were found between the calculated and experimental results and the reasons for these discrepancies were discussed.

Investigation of high temperature phase stability, thermal properties and evaluation of metallurgical compatibility with 304L stainless steel, of indigenously developed ferroboron alternate shielding material for fast reactor applications

Towards the cause of serving economic power production through fast reactors, it is necessary to bring in functionally more efficient and innovative design options, which also includes exploration of cheaper material alternatives, wherever possible. In this regard, the feasibility of using a commercial grade ferroboron alloy as potential alternate shielding material in the outer subassemblies of future Indian fast reactors has been recently investigated from shielding physics point of view. The present study explores in detail the high temperature thermal stability and the metallurgical compatibility of Fe–15.4B–0.3C–0.89Si–0.17Al–0.006S–0.004P–0.003O (wt.%) alloy with SS 304L material. In addition, the high temperature specific heat and lattice thermal expansion characteristics of this alloy have also been investigated as a part of the present comprehensive characterisation program. The Fe–15 wt.%B alloy is constituted of principally of two boride phases, namely tetragonal Fe 2B and orthorhombic FeB phases, which in addition to boron also contains some amount of C and Si dissolved in solid solution form. This Fe–B alloy undergoes a series of phase transformation as a function of increasing temperature; the major ones among them are the dissolution of Fe 2B-lower boride in the matrix through a eutectic type reaction, which results in the formation of the first traces of liquid at 1500 K/1227 °C. This is then followed by the dissolution of the major FeB boride phase in liquid and the melting process is completed at 1723 K/1450 °C. In a similar manner, the thermal stability studies performed on combined Fe–B + 304L steel reaction couples revealed that a pronounced pre-melting or liquid phase formation occurs at a temperature of 1471 K/1198 °C, which is lower than the melting onset of both Fe–B and SS 304L. It is found that within the limits of experimental uncertainty, this pre-melting phenomenon occurred at the same fixed temperature of 1471 K/1198 °C, irrespective of the mass ratios of Fe–B and 304L steel. Further, it is also found that SS 304L is completely soluble in Fe–B alloy and the fused product upon solidification formed a mixture of complex intermetallic borides, such as (Fe,Cr)(B,C), (Fe,Cr) 2(B,C) and (Fe,Ni) 3B. In the temperature range 823–1073 K (550–800 °C), the SS 304L clad is found to interact strongly with the Fe–B alloy. The diffusion layer thickness or the attack layer depth ( x) is found to vary with time ( t) up to about 5000 h, according to the empirical rate law, x 2 = k( T) t. The temperature sensitivity of the rate constant, k( T) is found to obey the Arrhenius law, k( T) = k o exp(− Q/ RT), with Q = 57 kJ mol −1, being the effective activation energy for the overall diffusional interaction of Fe–B and SS 304L. The room temperature specific heat capacity of Fe–B alloy is found to be 538 kJ kg −1 K −1. The C P values measured over 300–1350 K, is found vary smoothly with temperature according to the expression, C P/kJ kg −1 K −1 = 0.62094 + 0.00012 T + 10685.81 T −2. The lattice thermal expansion of both FeB and Fe 2B phases are found to be anisotropic in that the c-axis expansion is found to be more than that along a and b axes. The room temperature volume thermal expansivity of FeB and Fe 2B phases are found to be of the order of 48 × 10 −6 K −1 and 28 × 10 −6 K −1, respectively. The thermal expansion of FeB is found to be more temperature sensitive than that of Fe 2B.

TEM analysis of diffusion brazement microstructure in a Ni3Al-based intermetallic alloy

Transmission electron microscopy study of brazed joint microstructure in a Ni3Al-based intermetallic alloy IC 6 was performed. A continuously distributed microconstituents consisting of γ solid-solution, M3B2-type and M23B6-type borides, which were likely formed by eutectic-type reaction(s) from residual liquated insert during cooling from brazing temperature, were observed along the joint centreline. Consideration of possible incipient melting due to eutectic-type transformation reaction of these phases is pertinent to development of post braze heat treatment for modifying the brazement microstructure.

Liquid-Solid Equilibrium and Intermediate Phase Formation during Solidification in Mg-1.3 at%Zn-1.7 at%Y Alloy

Mg-Y-Zn alloys have attracted much attention in recent years due to their excellent mechanical properties, which are said to be the effect of the intermediate phase (X phase) in the Mg solid solution (α phase). The formation behavior of the X phase has been reported to be a eutectic type reaction in the Mg-Y-Zn pseudo-binary system. However, the phase diagram of this system still requires clarification. In the present study, the 97.0Mg-1.3Zn-1.7Y (at%) alloy was prepared to clarify the liquidus and solidus in the liquid-solid coexistence region, and the solubility limit in the α phase. The chemical compositions of the alloy were analyzed by electron probe X-ray microanalysis after isothermal heat treatment. The results indicate that the Mg-Y-Zn ternary system can be represented as a pseudo-binary system between the α and X phases. In addition, the solidus line for the α phase has been clarified. The solubility limit for the α phase is not much different from that of the previous report, whereas the formation behavior of the X phase is not in the manner of a eutectic reaction, but rather that of a peritectic one.

On the role of liquated γ′ precipitates in weld heat affected zone microfissuring of a nickel-based superalloy

The role of γ′ precipitate particles, which is the principal strengthening phase of most of the precipitation hardened nickel base superalloys, on weld heat affected zone (HAZ) cracking was investigated. The HAZ microstructures around welds in a commercial nickel-based superalloy IN 738LC were simulated using Gleeble thermomechanical simulation system. Microstructural examination of the simulated HAZs and those present in actual tungsten inert gas (TIG) welded specimens showed that γ′ particles persisted during heating to the welding simulation temperatures, where they reacted with the surrounding γ matrix producing liquid film by a eutectic-type reaction, which subsequently infiltrated the grain boundary regions. The on-cooling ductility of the alloy was significantly reduced as shown by the comparatively low ductility recovery temperature. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that HAZ cracking resistance was damaged by liquation reaction involving the γ′ precipitate particles. The results support previously reported observations which indicated that γ′ precipitate particles could contribute to HAZ microfissuring in superalloys through liquation reaction besides the generally reported rapid solid-state re-precipitation of γ′ phase.

New AlDyNi(Co) glasses – decomposition behaviour

The decomposition of Al 84Dy 6Ni 10 and Al 84Dy 8Ni 6Co 2 glasses prepared by melt spinning was investigated by means of X-ray diffraction (XRD), electrical resistivity (ER), calorimetric and microhardness measurements during isothermal, isochronal and continuous heating. Irreversible and reversible relaxation phenomena as well as glass transition occur before a primary crystallisation reaction in the ternary and a eutectic-type reaction in the quaternary alloy followed by the precipitation and transformation of intermetallic phases. Short- (SRO) and medium-range order (MRO) changes are responsible for pre-peaks in the structure factor observable even at room temperature. Glass transition (GT) is accompanied by resistivity drops. The first crystallisation products have a very fine morphology which raises the hardness of the alloys.

Deformation-induced microstructural changes in Fe40Ni40P14B6 metallic glass

The effect of mechanical deformation via high-energy ball milling on the structure of the Fe40Ni40P14B6 metallic glass was studied by means of x-ray diffactometry, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). After 5 h of milling, TEM observations indicated that some nanocrystallites with a diameter of about 6 nm precipitated from surface layers of the amorphous ribbons, whereas the bulk remained amorphous. When milling time was increased to 11 h, bulk crystallization occurred. The amorphous Fe40Ni40P14B6 alloy crystallized into a mixture of γ–(Fe, Ni) and (Fe, Ni)3(P, B). To understand the microstructural changes occurring in the amorphous ribbons before the onset of bulk crystallization, the isothermal crystallization behavior of as-deformed amorphous ribbons was studied. Compared with as-quenched amorphous ribbons, the local value of the Avrami exponent, derived from isothermal DSC data, increased from 3.5 to 4.1 for bulk crystallization. The thermal crystallization mechanism of deformed amorphous Fe40Ni40P14B6 ribbons changed from an eutectic-type reaction with simultaneous precipitation of γ–(Fe, Ni) and (Fe, Ni)3(P,B) from the amorphous matrix to a primary-type reaction with precipitation of α–Fe(P,B) preceding the formation of γ–(Fe,Ni) and (Fe,Ni)3(P,B). Our results suggest that several hours of mechanical milling cause surface crystallization and some atomic rearrangements in the amorphous alloy. The latter effect may be responsible for the observed primary-type reaction for crystallization of the deformed amorphous alloy

Solidification of an alloy 625 weld overlay

The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy 625 weld overlay deposited on 2.25Cr-1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy, electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification at {approx}1,216 C by a {gamma}/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification temperature range ({approx}170 C) and formation of interdendritic ({gamma} + Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent ({gamma} + Laves) constituent with the Scheil equation by treating the overlay system as a simple {gamma}-Nb binary and using an experimentally determined k{sub Nb} value from electron microprobe data.

An analytical electron microscope investigation of the phase transformations in a simulated heat-affected zone in alloy 800

The heat-affected zone (HAZ) hot-cracking behavior of Alloy 800 was investigated with hot ductility (Gleeble) testing during a simulated HAZ thermal cycle. Microstructural analyses were performed by optical metallography, fractography, electron microprobe analysis, and analytical electron microscopy on specimens that were water quenched from selected temperatures during this thermal cycle. Analysis of analytical electron microscopy (AEM) and Auger electron spectroscopy (AES) data suggests that incipient melting of the grain boundaries occurs at temperatures of 1300 °C and above. The HAZ hot-cracking mechanism was consistent with aspects of a constitutional liquation phenomenon involving a nonequilibrium eutectic-type reaction between grain boundary Ti(C, N) and the austenitic matrix. The extent of expected HAZ cracking would be low as the liquation of Ti(C, N) was localized and no lower melting intermetallic solidification products(e.g., Laves) were observed. The mechanistic observations were consistent with classical thermodynamic and solid state diffusion models.