Constitutional Supercooling Research Articles

Formation mechanism of Cu/Cu3Sn–Cu/Cu interconnections based on solder-filled microporous copper as interlayer via a current-assisted thermal compression bonding

In this study, a Cu/Cu3Sn–Cu/Cu interconnection can be achieved based on solder-filled microporous copper (MPC) as interlayer via a current-assisted thermal compression bonding. The high-temperature soldering connection materials can be used in the third-generation semiconductor packaging, so as to meet the promising application in high-temperature power device packaging. The influence of auxiliary current on the microstructure evolution of Cu–Sn IMCs and its formation mechanism were studied. Experiments show that the action of Joule heat coupled with the electron wind significantly enhanced the interfacial reaction at the Cu/Sn metallization interface. The microstructure of Cu6Sn5 changed from scallop-like into columnar due to constitutional supercooling, and the dispersion distribution of Cu6Sn5 also changed. The growth constitutive equations of Cu3Sn in bondlines were established under current-assisted thermal compression bonding process.

Microstructural evolution and mechanisms in additively manufactured AlCrCuFeNix complex concentrated alloys via selective laser melting

This article discusses the influence of the Ni content on the microstructures of AlCrCuFeNix complex concentrated alloys prepared by selective laser melting. The microstructural evolution mechanisms were also uncovered. It is found that the as-built Ni1.0 alloy possesses equiaxed-columnar bimodal grain structures and basket-weave microstructure consisting of striped body-centered cubic (bcc) nano-precipitates and ordered bcc (B2) phase. Adding Ni triggers the lattice transition from B2 to face-centered cubic (fcc), leading to the change of the primary solidification phase from B2 to fcc beyond the critical content of Ni. Meanwhile, the striped bcc nano-precipitates inside the B2 lamellae gradually transform into spherical morphologies due to the consumption of Fe and Cr solutes in eutectic reaction. Furthermore, the microstructures change from hypo-eutectic (Ni2.0, Ni2.5) to near-eutectic (Ni3.0) to hyper-eutectic (Ni3.5), accompanied by a columnar-to-equiaxed transformation due to the large constitutional supercooling induced by the eutectic reaction.

Colloidal mushy layers

Abstract

Effect of Ce on solute redistribution in liquid ahead of solid–liquid interface during solidification of Fe–4 wt.%Si alloy

The high efficiency of Ce addition in grain refinement of δ-ferrite in a cast Fe–4 wt.%Si alloy was verified. In order to further understand the solute effect of Ce on the grain refinement of δ-ferrite, the conventional directional solidification technique, which enabled to freeze the solid–liquid interface to room temperature, was used to investigate the interfacial morphology and solute redistribution in the liquid at the front of the interface, together with thermodynamic calculation of the equilibrium partition coefficients of Ce and Si in Fe–4 wt.%Si–Ce system using the Equilib module and the FsStel database in FactSage software system. Metallographic examination using a laser scanning confocal microscope showed a transition of the solid–liquid interface from planar to cellular in the Fe–4 wt.%Si alloy after adding 0.0260 wt.% Ce during the directional solidification experiment. Further, electron probe microanalysis revealed an enhanced segregation of Si solute in the liquid at the front of the solid–liquid interface due to the Ce addition. This solute segregation is considered as the cause of planar to cellular interface transition, which resulted from the creation of constitutional supercooling zone. Thermodynamic calculation indicated that Ce also segregated at the solid–liquid interface and the Ce addition had negligible effect on the equilibrium partition coefficient of Si. It is reasonable to consider that the contribution of Ce to the grain refinement of δ-ferrite in the cast Fe–4 wt.%Si alloy as a solute was marginal.

Embedding boron into Ti powder for direct laser deposited titanium matrix composite: Microstructure evolution and the role of nano-TiB network structure

The titanium powder embedded with 2.5 vol% TiB was in-situ fabricated by gas atomization and applied for direct laser deposited (DLD) titanium matrix composites. Consistent with the network distributed boron-rich zone in composite powder, a three-dimensional (3D) in-situ ultrafine network structure, consisting of nano-TiB whiskers (TiBw), was characterized throughout the as-fabricated composite (DLDed TiB–Ti). Both the dendritic and equiaxed network, attributed to the boron-induced constitutional supercooling and subsequent nucleation and growth of primary β-Ti, existed in the composite. The addition of boron (B) had a positive effect on the equiaxial transition and grain refinement of both primary β-Ti grains and α grains. Tensile tests showed an enhancement of strength compared with conventionally fabricated homogeneous TiB–Ti composites and selective laser melted CP-Ti. Analyzing the strength mechanism of the DLDed TiB–Ti, apart from the fine-grain strengthening and load-bearing effect of TiBw, the TiB network was found to have an additional contribution to the improvement of strength. Fracture morphology and in situ tensile observation showed the role of network structure in plastic deformation limitation, crack deflection and blunting, which was mainly attributed to the ultrafine cell size and revealed the significance of network structure.

Evolution of crystallographic orientation, columnar to equiaxed transformation and mechanical properties realized by adding TiCps in wire and arc additive manufacturing 2219 aluminum alloy

2219Al/TiCps composites were deposited by the wire and arc additive manufacturing method. The effects of TiCps content on the phase transformation, average grain size, crystal orientation, and mechanical properties of the deposited 2219 aluminium alloys were investigated as the variation of TiCps fraction is from 0.5 wt% to 2.0 wt%. The results show that the addition of 5 µm TiCps can effectively reduce the nucleation free energy of the system, which provided an opportunity for nuclei to grow on the surface of TiCps. The average grain size first decreased, then increased with the increase of TiCps weight fraction. Also, the EBSD testing results show the texture intensity both the Al-matrix and the θ-CuAl2 phase gradually decreased with increasing TiCps fraction. TiCps played an important role in generating nucleation undercooling (ΔTnu, 0.38 °C), which could be used to counteract the constitutional supercooling (ΔTcs) caused by solute segregation, and further promoted the transformation of granular θ-CuAl2 phase to a fine spot-like one within the grains. The fine spot-like phase possessed a strong interfacial bonding with the Al-matrix, thereby the mechanical properties of the deposited 2219 aluminium alloy were strengthened. Eventually, the deposited alloy exhibited a tensile strength around 405 MPa, and the elongation was up to 15.6%.

Effect of cooling rate on solidification microstructure and mechanical properties of TiB2-containing TiAl alloy

Effects of cooling rate and 0.25 at.% TiB 2 addition on solidification microstructure and mechanical properties of Ti–48Al–2Cr–2Nb alloys fabricated by the investment casting with different thicknesses were studied. The results show that with the cooling rate increasing from 37 to 2×10 2 K/s, the solidification path of the studied alloys is unchanged. The grain size of the matrix alloy is refined from 650 to 300 μm, while the grain size of Ti–48Al– 2Cr–2Nb–TiB 2 is reduced from 550 to 80 μm. The lamellar spacing of matrix alloy is reduced from 360 to 30 nm with increasing the cooling rate from 37 to 2×10 2 K/s, while TiB 2 addition shows little refinement effect on the lamellar spacing. Ti–48Al–2Cr–2Nb–TiB 2 sample under medium cooling rate (69 K/s) exhibits superior microhardness (HV 550) and ultimate tensile strength (570 MPa) among the studied alloys. The refined grain size, lamellar spacing and fine TiB 2 particles could account for the favorable mechanical properties of the studied TiB 2 -containing alloy. The microstructure evolution was discussed in light of cooling rate, constitutional supercooling and borides addition.

Microstructure and mechanical properties of repair welds of low-pressure sand-cast Mg–Y–RE–Zr alloy by tungsten inert gas welding

The sand castings of Mg–Y–RE–Zr series alloys are widely utilized in the large scales and complex shapes in the aerospace industry, as a result of which there are always some cast defects in the products. In this study, the feasibility of repair welding of sand-cast Mg–4Y–3RE–0.5Zr alloy by tungsten inert gas (TIG) welding was scrutinized with different welding currents from 150 to 210 A. The results indicated that defect-free repaired joints with good appearance could be acquired at 170 and 190 A. Interestingly, the grain size of the fusion zone (FZ) was refined initially and then increased with the linear increment of welding current. Because at the higher heat inputs, although the cooling rate of the molten pool was reduced, substantial constitutional supercooling for the grain refinement was attained after the Zr particles were transformed into Zr solutes. The tensile strength of the repaired joint at 170 A was 195 MPa with the maximum joint efficiency of 87.8%, and the elongation reached to 124.4% of the sand-cast base material (BM). However, serious grain coarsening and continuous eutectic structures generated in heat-affected zone (HAZ) above 190 A resulted in the weakening of the joint due to the brittle intergranular fracture.

Achieving low-temperature Zr alloying for microstructural refinement of sand-cast Mg-Gd-Y alloy by employing zirconium tetrachloride

The large size and uneven distribution of Zr particles always lead to a high alloying temperature and low Zr yield when MgZr master alloy is used for grain refinement of sand-cast Mg-RE alloys. In this research, the feasibility of Zr alloying by applying ZrCl4 in sand-cast GW83 alloy at a low temperature (720 °C) was investigated. The interaction procedure between ZrCl4 and Mg melt, and corresponding grain refinement mechanisms were systematically studied by thermodynamic calculation, thermal analysis, and microstructure characterization. The results showed that the yield of Zr by applying ZrCl4 was higher than that of the MgZr master alloy. Zr can be generated via the in-suit reaction between ZrCl4 and Mg. Both the nucleation temperature and nucleation undercooling increased with the increasing addition of ZrCl4. The average grain size decreased from 832 μm to 229 μm with 2.606 wt% ZrCl4 addition due to the heterogeneous nuclei of Zr particles and the constitutional supercooling provided by solute Zr. A corresponding physical model depicting the solidification behavior and grain refinement mechanism was proposed. However, the ZrCl4 addition of 4.343 wt% led to a large amount of RECl3 inclusions generated during the Zr alloying process, deteriorating the mechanical properties of the alloy.

Mechanical properties of carbon steel by compound arc and vibration shock forging-rolling

Abstract To solve problems of coarse grains and low mechanical performance in low carbon steel parts fabricated by wire and arc additive manufacturing (WAAM), a novel compound arc and vibration shock forging-rolling (CAVSFR) method was proposed in this study. Carbon steel components were fabricated by the proposed method. And results showed that the total deformation of 28.3 % was obtained and the average grain size was reduced from 9.6 μm of WAAM to 6.4 μm. Grain refining was attributed to the static-force rolling, vibration-shock forging and micro self-forging of the molten pool. The large deformation in the medium-high-temperature zone, which provided more nucleation sites, accounted for grain refining in both static-force rolling and vibration-shock forging. Vibration shock greatly promoted dendrite fragmentation and more dendrite fragments were obtained as nuclei for new grains. Besides, the larger constitutional supercooling helped these nuclei survive and induced more nucleation, which could refine grains. These two factors were responsible for the grain refining in micro self-forging of the molten pool. What’s more, the refined grains enhanced the strength and plasticity of the material. The ultimate tensile strength (UTS), yield strength (YS) and elongation of the optimized component manufactured using the proposed method were increased to 555 MPa, 391 MPa and 38.9 %, which were enhanced by 14.2 %, 8.6 % and 1.8 % respectively and exceeded that of the wrought low alloy steel with the UTS of 550 MPa. Compared with other methods, small load and simple equipment could obtain components with the high performance through the proposed method.

Microstructures and mechanical properties of ultralight cast Al-3Li-XMg-0.1Zr alloys

Ultralight cast Al-3Li-XMg-0.1Zr alloys (X = 1, 2, 3, 4) were studied by microscopy methods and tensile tests. Increasing the addition of Mg, grains in as-cast alloys were refined as a result of the intensified constitutional supercooling. After the low-to-high two-stage solution heat treatment, coarse δ′-Al3Li, Al12Mg17, δ-AlLi and S1-Al2MgLi second phases were dissolved into the α-Al matrix. Meanwhile, plenty of fine δ′-Al3Li phases and some core-shell Al3(Zr, Li) composite particles were precipitated. During the following ageing procedure, δ′ phases were coarsened following LSW theory while the size of composite particles kept almost unchanged due to the low diffusion rate of zirconium in aluminum. Magnesium was found to be able to promote not only the precipitation but also the coarsening of δ′-Al3Li phases in AlLi alloys. After ageing heat treatment at 175 °C for 32 h, Al-3Li-2 Mg-0.1Zr alloy presented optimum comprehensive mechanical performance with the ultimate tensile strength, yield strength and elongation equal to 368 MPa, 247 MPa and 4.7% respectively under the synthetic effect of grain refinement, promoted solution strengthening and the relatively narrow δ′-Al3Li precipitate free zones.

Grain refinement of stainless steel in ultrasound-assisted additive manufacturing

Metals and alloys fabricated by fusion-based additive manufacturing (AM), or 3D printing, undergo complex dynamics of melting and solidification, presenting challenges to the effective control of grain structure. Herein, we report on the use of high-intensity ultrasound that controls the process of solidification during AM of 316L stainless steel. We find that the use of ultrasound favours the columnar-to-equiaxed transition, promoting the formation of fine equiaxed grains with random crystallographic texture. Moreover, the use of ultrasound increases the number density of grains from 305 mm-2 to 2748 mm-2 despite an associated decrease in cooling rate and temperature gradient in the melt pool during AM. Our assessment of the relationship between grain size and cooling rate indicates that the formation of crystallites during AM is enhanced by ultrasound. Furthermore, the use of ultrasound increases the amount of constitutional supercooling during solidification by lowering the temperature gradient in the bulk of the melt pool, thus creating an environment that favours nucleation, growth, and survival of grains. This new understanding provides opportunities to better exploit ultrasound to control grain structure in AM-fabricated metal products.

Epitaxial growth behavior and stray grains formation mechanism during laser surface re-melting of directionally solidified nickel-based superalloys

The epitaxial growth behavior and stray grains formation mechanism at substrate during laser surface re-melting of directionally solidified superalloys were investigated in this paper. It is found that the columnar dendritic structures can be obtained by epitaxial growth from the substrate under laser surface re-melting. Meanwhile, the epitaxial dendrites have the same orientation with that of the substrate. The epitaxial behavior of dendrites is controlled by the constitutional supercooling, which is affected by the solidification conditions, i.e. temperature gradient and crystal growth velocity. Phase field method and Rosenthal’s solution were employed to describe the epitaxial behaviors of microstructure under laser surface re-melting. The disoriented stray grains appeared around the carbides and eutectic phases along the fusion line. The occurrence of stray grains can be ascribed to the liquation of carbides and eutectic phases, which results from the elemental segregation. The occurrence of stray grains is controlled by reducing the heat input to 50 J/mm. These findings could provide technical support in achieving successful repair and additive manufacturing of non-weldable directionally solidified superalloys.

Analysis of morphological stability in ternary two-phase diffusion couples

It has been established that, under certain conditions, the flat interphase boundary separating two ternary phases can be become morphologically unstable during the interdiffusion process. By a direct analogy with the constitutional supercooling criterion in alloy solidification, it has been proposed that the instability in ternary two-phase diffusion couples occurs when either or both diffusion paths enter into the two-phase region of the phase diagram. In this work a linear stability analysis of the diffusion problem is performed and the conditions for stability are expressed in terms of the diffusion coefficients, the slopes of the concentration profiles at the interface, and the system free energy as a function of composition. Through the use of a model ternary system, various examples of unstable interfaces are illustrated. It is found that the crossing of a diffusion path into the two-phase coexistence region is a necessary, but not sufficient condition for instability.

Columnar structures in pyrometamorphic rocks associated with coal-bearing spoil-heaps burned by self-ignition, La Ricamarie, Loire, France

This paper reports the discovery of columnar structures within paralavas from an active burning coal heap located in the formerly exploited coal basin of Saint-Etienne, Loire, France. The word “paralava” in this work refers to a rock that looks similar on the field to some volcanic rocks (e.g. basalts), and has been partially molten (with up to 50–60 vol% liquid). Here, the comparison between paralavas and volcanic rocks is mainly made from the point of view of the solidification process. The columnar structures occur in a decametric “sill”, located in the inner parts of the heap, exposed by quarrying operations. They are irregular; they form prisms of ca. 50 cm in diameter and are up to 2–4 m high. We report on geological, mineralogical, petrographical, geochemical, petrophysical (anisotropy of magnetic susceptibility, AMS) studies, focusing on the columns. Heterogeneities with cylindrical symmetry from the center to the rim of the columns are observed at the decimeter scale, both for geochemistry (variation of Zn is conspicuous) and AMS studies. These contribute to the understanding of the genesis of the columns, favoring the fingering hypothesis: formation of digitations within a phenomenon of constitutional supercooling (fingers of the solid grow at the expense of the liquid). Among the volatiles playing a role in this phenomenon, the role of Cl as a ligand to Zn in the melt deserves attention (the situation is different in standard basaltic rocks, where the role of H2O with respect to other volatiles is more important; Cl may be abundant as a companion to coal).

Heterogeneous microstructure and voids dependence of tensile deformation in a selective laser melted AlSi10Mg alloy

Microstructure and voids evolution of a selective laser melted (SLM) AlSi10Mg alloy during tension were systematically investigated. The SLM AlSi10Mg sample is featured with a multi-level heterogeneous microstructure that is composed of melt pools (MPs), columnar Al grains and sub-cells. According to the Si morphologies and thermal history, the MP could be divided into three regions, including the fine structure zone (FSZ), remelted zone (RMZ) and heat-affected zone (HAZ), respectively. The Si segregation phenomenon on sub-cells boundaries is observed and may attributed to the constitutional supercooling. The tensile deformation behaviors of the vertical sample (V-sample) vary from the horizontal sample (H-sample) for that heterogeneous microstructure arises strain localization in V-sample. In addition, the voids have no obvious effect on the uniform plastic deformation, while they may affect the non-uniform deformation during tension. The combined effects of strain localization and void contribute to the earlier break in V-samples than in H-samples.

Additive manufacturing of titanium-base alloys with equiaxed microstructures using powder blends

The microstructures and tensile properties of additively manufactured Ti-base alloys were investigated. Ti-6Al-4V and Co-Cr-Mo alloy powders were blended and used for directed energy deposition of bulk samples containing 5 and 10 wt.% Co. The Ti-6Al-4V and 5 wt.% Co samples exhibited an α phase at β grain boundaries, which caused a significant reduction and anisotropy of the tensile ductility. A single-phase β microstructure exhibiting equiaxed grains with a grain size of 36 μm, random texture, and isotropic tensile properties was obtained for the as-deposited 10 wt.% Co sample. These phenomena could be ascribed to the high Co content and the resulting enhanced stability of the β phase and the constitutional supercooling of the melt-pool.

Grain Refinement of Alloys in Fusion-Based Additive Manufacturing Processes

One of the less desirable aspects of fusion-based additive manufacturing is the propensity for coarse columnar grain structures crossing build layers to form. This paper initially attempts to explain the reason for the formation of columnar grain structures in terms of the high thermal gradients typically observed during solidification and the alloy compositions that are typically used which promote epitaxial growth. Successful approaches to the grain refinement of titanium alloys using alloying elements that produce constitutional supercooling are discussed along with the difficulty with nucleant additions. Much of the grain-refining technology already used in aluminium casting is shown to also be applicable to additive manufacturing, although the novelty of the effective use of nanoparticles as nucleants is highlighted. It is also shown that for other alloy systems for which there is a lack of grain-refining technology using chemical means, mechanical means, such as ultrasonic treatment, can be effective across a wide range of alloys. Finally, consideration is given to the difficulties and the possible solutions of producing parts layer by layer. In particular, the importance of understanding nucleation in solidification conditions characterized by high cooling rates and thermal gradients; the importance of melt dynamics; and how previous layers could provide possibilities for refinement of the subsequent layer are highlighted.

Investigation on the Dynamic Behavior of Weld Pool and Weld Microstructure during DP-GMAW for Austenitic Stainless Steel

The influence of heat and droplet transfer into weld pool dynamic behavior and weld metal microstructure in double-pulsed gas metal arc welding (DP-GMAW) was investigated by the self-designed high-speed welding photography system. The heat input, the arc pressure, the droplet momentum and impingement pressure were measured and calculated. It was found that the arc pressure is far less than the droplet impingement pressure. The heat input and droplet impingement pressure per unit time acting on weld pool were proportional to the current pulse frequency, which fluctuated with thermal pulse. The size and oscillation amplitude of the weld pool had noticeable periodic changes synchronized with the process of heat input and droplet impingement. Compared to the microstructure of pulsed gas metal arc welding (P-GMAW) weld metal, that of DP-GMAW weld metal was significantly refined. High oscillation amplitude assisted the enhancement of weld pool convection, which leads to more constitutional supercooling. The heat input and shear force during the peak of thermal pulse causing dendrite fragmentation which provided sufficient crystal nucleus for the growth of equiaxed grains and the possibility of grain refinement. The effects of current parameters on welding behavior and weld metal grain size are investigated for further understanding of DP-GMAW.

Refining prior-β grains of Ti–6Al–4V alloy through yttrium addition

Abstract Significant prior-β grain refinement of as-cast Ti–6Al–4V alloy was achieved through additions of yttrium. Microstructural analysis revealed three mechanisms by which yttrium refines the prior-β grains during solidification: solute yttrium provides constitutional supercooling; yttrium scavenges oxygen and forms Y2O3 particles, which pin grain boundaries suppressing grain growth; Y2O3 particles also act as active nucleation sites. Even finer equiaxed grains were achieved through laser surface remelting suggesting that yttrium additions could be an efficient β-Ti grain refiner for the Ti–6Al–4V alloy processed by additive manufacturing.