As-cast Microstructure Research Articles

Solidification Kinetics of an Al-Ce Alloy with Additions of Ni and Mn

Heat-treated aluminum–silicon (Al-Si)-based alloys have dominated the cast lightweight alloy industry for several decades. However, in the last decade, Al-Ce-based alloys have shown promise in replacing Al-Si alloys as they remove the need for costly heat treatments. As the properties of Al-Ce alloys depend on the as-cast microstructure, it is important to characterize the solidification kinetics of these alloys. Therefore, this study focused on characterizing the solidification of an Al-Ce alloy with additions of Ni and Mn (nominal composition Al-12.37Ce-3.26Ni-0.94Mn-0.12Fe in weight percent). The alloy was cast in a wedge mold configuration, resulting in cooling rates between 0.18 and 14.27 °C/s. Scanning electron microscopy (SEM) coupled with the energy dispersive x-ray spectroscopy (EDS) and differential scanning calorimetry (DSC) techniques characterized the evolution rate of solid phases. The SEM/EDS data revealed that an Al10CeMn2 phase was present at higher cooling rates. At lower cooling rates, near the center of the casting, a primary Al23Ce4Ni6 phase was more present. It was observed that up to 2.6 atomic percent (at.%) of Mn was dissolved in this primary Al23Ce4Ni6 phase, thereby removing a large portion of the available Mn for forming the Al10CeMn2 phase. DSC analysis showed differences in the samples’ liquidus temperatures, which indicated compositional variations. Inductively coupled plasma–atomic emission spectroscopy (ICP-OES) and Scheil solidification simulations correlated the compositional differences with phase formation, which agreed with the SEM and DSC results. This experiment provides insight into novel Al-Ce-Ni-Mn alloys and where their potential lies in industrial applications.

Cerium refinement of grains and primary carbides during solidification of Cr4Mo4V bearing steel

The effect of cerium (Ce) on the solidification microstructure of Cr4Mo4V bearing steel was investigated via a combined experimental and theoretical method. With a trace amount (0.056 wt%) of Ce addition, the coarse columnar grains in as-cast microstructure transform into equiaxed ones, and the average diameter is reduced from 56 to 27 μm. The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined, and their volume percentage decreases from 4.15 vol% to 2.1 vol%. Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries, both contribute to the refinement of grains. Thermodynamic calculations reveal that primary carbides are precipitated after γ-austenite forms near the end of the solidification process. The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted, while the growth is restrained owing to the less segregation of alloying elements.

Effect of Minor Fe, Si Contents on Microstructure and Property of 7085 Alloy during Fabrication Processes

Fe and Si elements can form non-dissolvable phases in Al-Zn-Mg-Cu alloys and furtherly affect the property of finished products while the influence of minor contents lacks sufficient investigations. In present work, 7085 alloy with different Fe and Si contents (named by LFS alloy, MFS alloy and HFS alloy as Fe and Si contents progressively enhanced) was investigated and corresponding microstructure and property during the whole fabrication were analyzed via OM, SEM and DSC observation and tensile property, fracture toughness and salty-spray corrosion property. The results showed that Fe-containing phase existed in as-cast microstructure of three alloys while part of Fe-containing phases of HFS alloy has Si element. After homogenization treatment, coarse eutectic phases had dissolved into the matrix while Fe-containing phase remained in the alloys. Similarly, Si element existed in some Fe-containing phases of HFS alloy. The order of area fraction of remaining Fe-containing phases for the three alloys was LFS alloy<MFS alloy<HFS alloy. After extrusion, Mg(Zn,Cu,Al)2 phase and Fe-containing phase possessed the majority, the former one was eliminated by solution treatment while the latter one had no obvious change. Under an unified aging treatment, the strength, elongation and fracture toughness were degraded with the increase of Fe and Si contents. Besides, the salt-spray corrosion property of HFS alloy was significantly inferior to LFS alloy and MFS alloy. This gave rise to a reference for controlling Fe and Si impurity of 7085 alloy for considering overall performance.

Microstructure and mechanical properties of hypoeutectic Al–6Ce–3Ni-0.7Fe (wt.%) alloy

A hypoeutectic, Fe-modified Al–Ce–Ni alloy (Al–6Ce–3Ni-0.7Fe, wt.%) is studied in terms of microstructure, thermal stability, ambient temperature strengthening, and creep resistance. The as-cast microstructure consists of primary Al dendrites and interdendritic binary eutectic regions (Al–Al11Ce3 and/or Al–Al9(Fe,Ni)2), with micron/submicron lamellar spacing, depending on the location along the height of the ingot. The cast alloy exhibits excellent coarsening resistance at 400 °C, with mostly unchanged microstructure and microhardness after 6 weeks of aging, indicating good thermal stability of Al11Ce3 and Al9(Fe,Ni)2. Orowan strengthening and load transfer are identified as strengthening mechanisms at ambient and elevated temperature. A high volume fraction of the intermetallic phases (providing load transfer) and relatively coarse eutectic spacing (for modest Orowan strengthening) result in a moderate as-cast microhardness of 566 ± 32 MPa. Creep resistance at 300 and 350 °C is similar to a binary Al-12.5Ce eutectic alloy (with twice the Ce content) because of two countering effects: Al–6Ce–3Ni-0.7Fe shows a higher volume fraction of strengthening intermetallic phases, but it also exhibits a large fraction of primary Al dendrites which weaken the alloy. By contrast, the alloy, when laser-remelted at the surface, has a fully eutectic microstructure without primary aluminum dendrites achieved by high undercooling on solidification, with a refined network of eutectic phases that doubles the microhardness as compared to the cast alloy. Whereas coarsening is faster due to the shorter diffusion distances between the eutectic phases, hardness remains ∼30% higher than the as-cast alloy after ∼6 weeks aging at 400 °C.

Effect of Withdrawal Rate on Solidification Microstructures of DD9 Single Crystal Turbine Blade.

Single crystal superalloys are widely used in the manufacturing of turbine blades for aero-engines due to their superior performance at high temperatures. The directional solidification process is a key technology for producing single crystal turbine blades with excellent properties. In the directional solidification process, withdrawal rate is one of the critical parameters for microstructure formation and will ultimately determine the blade's properties. In this paper, the as-cast microstructures in the typical sections of a DD9 single crystal (SX) superalloy turbine blade were investigated with 3 mm/min and 5 mm/min withdrawal rates during the directional solidification process. With increased withdrawal rate, the dendrite morphologies tended to become more refined, and the secondary dendritic arms tended to be highly developed. The dendrite in the blade aerofoil section was more refined than that in the tenon section, given the same withdrawal rate. Additionally, with increasing withdrawal rates, the size and dispersity of the γ' precipitates in the inter-dendritic (ID) regions and dendritic core (DC) tended to decrease; furthermore, the size distributions of the γ' precipitates followed a normal distribution law. Compared with the ID regions, an almost 62% reduction in the average γ' sizes was measured in the DC. Meanwhile, given the same withdrawal rate, at the blade's leading edge closest to the heater, the γ' sizes in the aerofoil section (AS) were more refined than those in the tenon section (TS). As compared with the decreasing cross-sectional areas, the increased withdrawal rates clearly brought down the γ' sizes. The sizes of the γ-γ' eutectics decreased with increasing withdrawal rates, with the γ-γ' eutectics showing both lamellar and rosette shapes.

Thermomechanical treatments for a dual phase cast high entropy alloy

Thermomechanical treatments modify the cast microstructure of a dual phase hypoeutectic high entropy alloy. The as-cast microstructure consists of an FCC matrix and a BCC eutectic phase forming a semi-continuous network. The as-cast material is subjected to various thermomechanical treatments: I) hot compression; II) hot compression with strain rate jumps; III) multi-stage hot compression with intermediate stages of isothermal soaking; IV) hot compression followed by annealing. The material recovers rapidly after the strain rate jump and no dependence of the yield stress on the previous deformation step is observed. Much of the stress relaxation occurs within seconds during the hold for the multi-stage tests. A steady-state stress is reached for the first holding cycle, while progressively slower softening is observed for later holding cycles. The eutectic BCC phase fragments during hot deformation, while discontinuous dynamic recrystallisation associated with the formation of Σ3 occurs in the FCC phase. Precipitates are formed at high temperatures and pin the high-angle grain boundary motion. The rotation of the eutectic BCC phase and the precipitates during plastic deformation also promotes local lattice rotation of the FCC phase. As a result, a fine and intricate substructure is formed in the interdendritic spaces. Complete recrystallisation is not achieved during either hot deformation or annealing, suggesting that practical grain refinement of the investigated alloy would only be achieved at larger strains or a combination of larger deformations and intermediate annealing treatments.

Solidification behavior and microstructural features of the cast and HIPed N-bearing Ti–48Al–2Cr–2Nb intermetallic alloys

Effect of N addition (0.5, 1, and 2 at. %) on solidification behavior and microstructural features of the cast and hot isostatically pressed (HIPed) Ti–48Al–2Cr–2Nb (4822) intermetallic alloy was investigated. Alloys were fabricated using vacuum arc re-melting followed by HIPing. The results showed that N addition changes the primary β phase dendrites to α phase. The morphological uniformity of the as-cast microstructures was notably increased by N addition. In addition, N addition beyond its solubility limit led to the formation of primary Ti2AlN precipitates located within lamellar colonies. A possible mechanism for nucleation of the Ti2AlN precipitates is proposed. Furthermore, N addition caused much more microstructure stability during HIPing. Although a duplex structure was formed after HIPing in the 4822 alloy, N addition decreased the formation of this structure. Moreover, highly extended network of secondary Ti2AlN precipitates were formed in almost all of (α2+γ) lamellas interfaces in the N-bearing alloys.

Co-strengthening of the multi-phase precipitation in high-strength and toughness cast Al–Cu–Zn–Mg alloy via changing Zn/Mg ratios

The effects of the Zn/Mg ratio on the as-cast microstructure, aging-precipitation behavior, and mechanical properties of cast Al–Cu–Zn–Mg alloys, an interesting precipitation-hardened Al alloy different from the 2XXX and 7XXX, were investigated. The primary phase of the as-cast alloy changed from lamellar θ and S phases to a block S phase with an increased Zn/Mg ratio. The number density of the solely S phases in the peak-aged alloy increased with the Zn/Mg ratio from 0.4 to 1.5, because the Zn promoted the formation of GPB zones. At a Zn/Mg ratio of 2.5, the major precipitates are the S, η, and T phases, due to the preferential interaction of Mg atoms with Zn, which promotes the formation of Mg–Zn clusters and GP zones in the initial stages of aging. The yield strength, ultimate tensile strength, and elongation reached maximum values of 372 MPa, 502 MPa, and 12.3%, respectively. This illustrates the cooperative strengthening of the S, T, and η phases, which pin the moving dislocations and inhibit crack initiation and propagation. A large amount of the T phase in the high Mg content alloy suppresses the formation of the S phase. This multi-phase precipitation addressed the “strength ductility trade-off” in cast Al alloys.

On the Aging Kinetics of a Flame-Resistant AZ91D-1.5%Ca Magnesium Alloy Processed with Ultrasonic Vibration.

The Mg-Al-Zn-Ca system has demonstrated excellent flame resistance and mechanical properties in the as-cast condition. However, the potential of these alloys to be heat-treated, e.g., by aging, as well as the influence of the initial microstructure on the precipitation kinetics, is yet to be comprehensively explored. Ultrasound treatment was applied during the solidification of an AZ91D-1.5%Ca alloy to promote microstructure refinement. Samples from treated and non-treated ingots were subjected to solution treatment at 415 °C for 480 min, followed by aging at 175 °C for up to 4920 min. The results showed that the ultrasound-treated material could reach the peak-age condition in a shorter period than the non-treated one, suggesting accelerated precipitation kinetics and, thus, enhanced aging response. However, the tensile properties showed a decrease in the peak age compared to the as-cast condition, probably due to the formation of precipitates at the grain boundaries that promote the formation of microcracks and intergranular early fracture. This research shows that tailoring the material's as-cast microstructure may positively affect its aging response, shortening the heat treatment duration, thereby making the process less expensive and more sustainable.

Effect of carbon addition on microstructure and mechanical properties of a typical hard-to-deform Ni-base superalloy

In this paper, the as-cast microstructure, microsegregation, and mechanical properties of GH4151 superalloys with a carbon addition were studied by scanner electron microscopy (SEM), transmission electron microscope (TEM), electron probe microanalysis (EPMA), differential thermal analyzer (DSC), and electron backscattered diffraction (EBSD). The results show that the solid solution effect of carbon in alloys is limited, the addition of C from 0.01 to 0.08 ​wt% significantly refines the secondary dendrite spacing(SDAS) from 61.7 to 57.4 and 51.8 ​μm. Due to the influence of thermal solutal convection, the most severe segregation is found in the C-0.04 alloy, and the solidification ranges of C-0.01, C-0.04, and C-0.08 alloys are 87 ​°C, 96 ​°C, and 83 ​°C, respectively. Owing to the reduced solidification pores and formed finely dispersed carbides, C-0.04 shows excellent ultimate tensile strength (442 ​MPa) and superior total elongation (1.368%) at 1100 ​°C much better than that of C-0.01(130.7 ​MPa, 0.085%) and C-0.08(248 ​MPa, 1.026%).

Influence of gadolinium on solidification behavior of Ni–Mo-Cr-Gd alloy

The influence of Gd content on the solidification behaviors and as-cast microstructures of Ni–Mo-Cr-Gd alloy have been investigated systematically. It was found that the γ/Ni5Gd eutectic phases were formed in the interdendritic region, which exhibits the divorced character. With the Gd adding, the γ/M6C eutectic phases were replaced by the γ/M2C eutectic ones. Most M2C carbides locate at the interfaces of γ/Ni5Gd eutectic phases or are enclosed by Ni5Gd phases. The segregation of alloying elements was mitigated by Gd adding. Based on the thermal analysis, composition analysis and thermodynamic calculation, the solidification behaviors, and the affecting mechanism of Gd adding were briefly discussed. During the solidification process, the γ/Ni5Gd eutectic reaction occurs first, which interrupts the solute redistribution process and thus mitigates the segregation of some alloying elements. The Ni5Gd phases accommodate a mass of Si and keep the Si content in the residual liquid phase low, which decreases the nucleation rate of M6C carbides. Then, the γ/M2C eutectic phases become the dominant ones at the final stage of the solidification process. Our results are useful in future optimization and processing research of Ni-Mo-Cr-Gd alloy for applications in molten salt reactors.

Study on the As-Cast Microstructure and Homogenization Mechanism of GH3128 Superalloy

Study on the As-Cast Microstructure and Homogenization Mechanism of GH3128 Superalloy

Phase relationships investigation of the Al-Ti-La ternary system

Phase relationships of Al-Ti-RE (rare earth elements) systems play an important role in the development of grain refiners for aluminum alloys. In the present work, the liquidus projection (>60 at% Al) and the isothermal section at 773 K (>20 at% Al) of the Al-Ti-La system have been determined. Ten primary precipitation fields were identified by the analysis of the as-cast microstructures: Al2La, γ-TiAl, Al7La3, ζ-Ti2+xAl5−x, α-Al11La3, β-Al11La3, ε(ht)-TiAl3, ε(rt)-TiAl3, τ (Al20Ti2La) and (Al). Seven type II and two type III ternary invariant reactions are proposed. The stability of five three-phase regions and seven binary compounds, α2-Ti3Al, γ-TiAl, ε(rt)-TiAl3, AlLa, Al2La, α-Al11La3, (Al) and one ternary compound τ at 773 K has been confirmed by a combination of scanning electron microscopy, energy dispersive spectroscopy, electron probe microanalysis and X-ray diffraction.

Experimental and Simulation-Based Investigations on the As-cast Microstructure of Inconel 657 Superalloy Produced by Investment Casting

Experimental and Simulation-Based Investigations on the As-cast Microstructure of Inconel 657 Superalloy Produced by Investment Casting

Effect of Melt Holding Time on the As-cast Microstructure and Hardness of Aluminum Alloy 2618

The effects of the melt holding time on the type, quantity and morphological distribution of aluminum alloy 2618 were investigated by means of OM, XRD and SEM. And the influence of the melt holding time on the hardness of alloy was explored. The results show that the heredity of the microstructure of the master alloy is not completely eliminated when the melt holding time is less than 10 min at 740 ℃. And it will make Al9FeNi phase coarse needle-like, and segregate with Al7Cu2Fe phase at the grain boundary or inside the grain. Properly prolonging the melt holding time can change the shape of Al9FeNi phase from coarse needle-like to fine needle-like and make the distribution of Al9FeNi phase and Al7Cu2Fe phase more uniform. Aluminum alloy 2618 was melted at 740 ℃ for 30 min and then poured into graphite molds at room temperature, which can make Al9FeNi phase fine and uniformly distributed in the matrix. With the extension of the melt holding time, the size of the nonequilibrium crystalline phase is basically unchanged, but the grain size will be slightly coarsened. The hardness of the alloy showed a trend of first increasing and then decreasing with the prolongation of the melt holing time within 10~60min, and when the melt was held for 30min, the hardness of the alloy reached the maximum HRB value of Rockwell hardness within the range of this test. The maximum value is 48.62HRB, and the hardness distribution of the alloy is relatively uniform. Therefore, it is beneficial to refine the precipitated phase, reduce the macro segregation, and refine the grain and improve the hardness of the alloy, while alloy 2618 was poured into the graphite casting at room temperature with melting at 740℃ and held for 30min.

Effects of Melt Hydrogenation on the Microstructure Evolution and Hot Deformation Behavior of TiBw/Ti-6Al-4V Composites

In this study, Ti-6Al-4V matrix composites reinforced with TiB ceramic whiskers were in situ synthesized and hydrogenated using the melt hydrogenation technique (MHT). The effects of MHT on the microstructure evolution and hot compression behavior of the composites were investigated by optical microscopy (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Hot compression tests were performed at strain rates of 0.1/s, 0.01/s, and 0.001/s and temperatures of 800 °C, 850 °C, and 900 °C; the hot workability of composites significantly improved after hydrogenation, for example, the 900 °C peak flow stress of hydrogenated composites (43 MPa) decreased by 53.76% compared with that of unhydrogenated ones (93 MPa) at a strain rate of 0.01/s. Microstructural observations show that MHT can effectively facilitate the dispersion of TiB whiskers and induce the α/β lath refinement of the matrix in our as-cast hydrogenated composite. During hot compression, MHT effectively promoted the as-cast composite microstructure refinement, accelerated the dynamic recrystallization (DRX) generation, and reduced the stress concentration at the interface between the reinforcement and matrix; in turn, the hydrogenated composites presented low peak stress during hot compression.

Microstructure refinement of a cast high entropy alloy by thermomechanical treatments

This work investigates the hot compressive behaviour of a cast high entropy alloy. The alloy is subjected to four different thermomechanical treatments to refine the as-cast microstructure: hot compression tests at constant strain rates (0.001s−1 and 1s−1), hot compression tests with strain rate jumps (between 0.001s−1 and 1s−1), multi-stage hot deformation with holding intervals of 1 and 5 min, and hot compression at 0.1s−1 followed by annealing. The three first tests were carried out at 1100 °C and the deformation step of the last type was carried out at 1050 °C followed by annealing at 1150 °C. The deformation at 0.001s−1 promotes dynamic recrystallisation. Dynamic recrystallisation occurs when jumping from 1s−1 to 0.001s−1 and does not occur if the jump is from 0.001s−1 to 1s−1. Multi-stage interrupted tests show that 1 min holding between the stages promote dynamic recrystallisation, while static recrystallisation occurs more pronouncedly for 5 min inter-stage holding. Due to the large initial grain size, deformation does not occur homogeneously within the grains, and heterogeneous static recrystallisation occurs within the deformed specimen. Whether the alloy is solution heat treated or deformed in the as-cast condition does not affect the recrystallisation behaviour. Overall, refining the coarse cast microstructure of the investigated alloy is possible, although more deformation steps are needed to produce a homogeneous microstructure.

Effect of B addition on microstructures and properties of Cu-15Ni-8Sn alloy

The effect of B addition on microstructures and properties of as-cast, solution-treated and aged Cu-15Ni-8Sn alloy was systematically investigated in the present study. It is found that the micron-scale Ni6.67SnB2 phase is identified in as-cast microstructure for B-containing Cu-15Ni-8Sn-xB alloys. Moreover, it is suggested the segregation of Sn element at grain boundary could be alleviated to a certain degree with B addition. After being solution-treated at 850 °C for 4 h, the Ni6.67SnB2 phase is still existed in the matrix, thereby resulting in a significant influence on the microstructures and properties during subsequent aging process. Results demonstrate that, during aging treatment at 400 °C, the transformation of DO22 ordered phase into L12 ordered phase can be increasingly inhibited with increase of B content and completely prevented for 0.3 and 0.5 wt% B addition. Additionally, the appropriate B addition (0.04, 0.08, 0.1 wt%) could remarkably suppress the formation and growth of the discontinuous precipitation. Nevertheless, with excess B addition (0.3, 0.5 wt%), the discontinuous precipitation is instead promoted. More importantly, alloys with appropriate B addition (0.04, 0.08, 0.1 wt%) could also maintain the peak hardness without deterioration as long as 18 h during the aging process, which suggests the tremendous enhancement of softening resistance. It is believed that the suppression of discontinuous precipitation by Ni6.67SnB2 phase should be responsible for such enhancement of softening resistance. It is anticipated that the current work could provide guidance for the multi-component design of high-performance Cu-Ni-Sn alloy.

Effects of the superheating degree and cooling intensity on the as-cast microstructure of CP780 thin slab

ABSTRACT This paper studied the effects of the superheating degree and cooling intensity on the solidification structure of CP 780 slab. The results indicated that the equiaxed crystal ratio decreases with the increase of superheating degree and the cooling intensity. When the superheating degree increases from 15 to 25°C, the equiaxed crystal ratio drops sharply. While the equiaxed crystal ratio decreases slightly with an increasing superheating degree from 25°C to 55°C. It is further revealed that the superheating degree has a greater effect on the equiaxed crystal ratio compared with the influence of cooling intensity. Quantitative analysed the relationship among the equiaxed crystal ratio, the superheating degree and cooling intensity is in line with the Extreme 2D model. The accuracy of the model performance can reach 97.6% when predicting the equiaxed crystal ratio of CSP-CP 780 during CSP process.

Microstructures and properties of as-cast AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV multi-principal element alloys

This study reports on the as-cast microstructures of AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV equimolar multi-principal element alloys. Thermodynamic modeling predicts that these alloys solidify as BCC solid solutions, with additional phases forming at lower temperatures. We present the measured densities, the elastic modulus values measured by nanoindentation, and the Vickers microhardness of the alloys. These properties are correlated to the underlying microstructures, as measured by x-ray diffraction, scanning electron microscopy, and atom-probe tomography. The densities of the alloys range from 5.53 to 6.42 ×103 kgm−3. The elastic modulus of AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV are 211, 194, and 175 GPa, respectively, and their Vickers microhardness values are 3.9, 6.1, and 5.7 GPa, respectively. After arc-melting, AlCrFeMnV and AlCrFeTiV are BCC solid solutions while AlCrMnTiV has two phases: a BCC phase and a minor hexagonal C14 Laves phase that is enriched in V and Cr.