•The peritectic solidification is firstly in situ observed by real-time imaging.•The DC field changes the phase constitution of peritectic alloy in this case.•Only peritectic η phase without primaryεphase precipitates under DC field.•Secondary dendrite arm dissolving behavior of Cu6Sn5 is in situ observed.•The size and shape of intermetallics Cu6Sn5 is closely related to the DC field.The structural evolution of peritectic solidification under the direct current (DC) field action, mainly on the morphological evolution of primary phase ε (Cu3Sn) and peritectic phase η (Cu6Sn5), is in situ observed by high resolution X-ray real-time imaging with synchrotron radiation. The ultimate solidification microstructures after experiencing the imaging experiment are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). It is found that the η phase precipitates directly from parent liquid phase without experiencing corresponding peritectic reaction when the DC field of 20 A/cm2 or 200 A/cm2 is imposed during the solidification process of the Sn-10%Cu alloy. It means that the DC field has altered the phase transformation sequence and suppressed the nucleation of primary ε phase. Meanwhile, the growing and dissolving behaviors of secondary dendrite arm for intermetallic compound Cu6Sn5 under the DC field action are also in situ observed and discussed.

•The pre-eutectic Si particles nucleate ahead of the solid/liquid interface.•The nodular pre-eutectic Si is observed with the addition of 0.3% Eu.•The eutectic Si leads the irregular eutectic interface in the unmodified alloy.•The eutectic grains can nucleate on the nodular Si after modification.•The nodular Si can be attributed to IIT and poisoning of TPRE mechanism.A series of Al-40Zn-5Si alloys with different additions of Eu were prepared by permanent mold casting. The nucleation phenomenon incorporating the modification of silicon phase was investigated by using the synchrotron radiation imaging technology, scanning electron microscopy, electron probe microanalysis and high resolution high-angle annular dark-field scanning transmission electron microscopy. It was found that the primary phase was α-Al dendrites and the pre-eutectic Si particles were precipitated continuously at the front of the solid/liquid interface in Al-40Zn-5Si alloys. The nodular pre-eutectic Si particles were observed when the Eu addition was 0.3%. Meanwhile, a flake-to-fibrous transition of eutectic Si was also observed. The needle-like eutectic Si crystals were seen to shoot out from the vicinity of primary α-Al dendrites into melt with high velocities leading the irregular eutectic interface in the unmodified Al-40Zn-5Si alloy. However, the eutectic grains with smooth interfaces nucleated mainly near primary or secondary α-Al branches in 0.3% Eu-modified Al-40Zn-5Si alloy. The eutectic grains began to nucleate on the nodular pre-eutectic Si particles when the α-Al surface reached so close to them. The formation of nodular pre-eutectic Si particles was attributed to the adsorption of Eu atoms along the <112> growth direction of Si and at the intersection of Si twins. This is fully consistent with the well-known poisoning of twin plane re-entrant edge (TPRE) and the impurity induced twinning (IIT) modification mechanisms proposed for interpreting the modification of eutectic Si.

•3003/4004 clad aluminum was firstly modified by various Sr addition levels.•The optimum adding amount of Sr on the Al1.2Mn/Al10SixSr clad is 0.08 wt%.•Sr can refine primary α-Al and eutectic silicon phase of the clad simultaneously.•The Sr-modified rolled clad has smaller diffusion layer than that of unmodified.This paper examines the effects of Sr addition on the microstructure, compositon distribution and vickers hardness in the interfacial region of the as-cast and rolled 3003/4004 clad aluminum. The results reveal that the optimum adding amount of Sr on the as-cast Al-1.2Mn/Al10Si-xSr clad is 0.08 wt%. With Sr content increasing from 0 to 0.08 wt%, the average length and number of the primary α-Al phase growing from the diffusion layer significantly decreased and whose morphology appears in columar dendritic crystals, the celluar dendrite crystals, deep celluar crystals, fine celluar crystals and planar crystals, while the dendritic-crystal primary α-Al phase nucleating and growing from inner AlSi alloy side also show obvious decease in secondary dendrite spacing; meanwhile, eutectic Si phases were gradually modified from coarse plates, coralloid-plates mixed structure to fine branchy coralloid structure in three-dimensional morphology. After rolling, the diffusion layer thickness of the Al-1.2Mn/Al10Si0.08Sr clad is decreased by 66.7%, compared to that of unmodified clad alloy. This decreased diffusion layer thickness may be determined by augmented plastic strain and restraining diffusion of Si atoms in diffusion layer. Morever, average vickers hardness on interface and AlSi side of the Al-1.2Mn/Al10Si0.08Sr clad showed slight increase and more uniform distribution than that of unmodified clad alloy. This uniform distribution and improved hardness primarily attribute to presence of fine branchy coralloid silicon phase and its stronger dispersion strengthening as well as solution strengthening caused by interdiffusion of Si, Mn and Sr elements.

•The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys exhibit excellent thermal stability.•AQ-700 sample exhibits the highest hardness of HV 727.52.•AQ-800 sample shows the best comprehensive mechanical properties.The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys (HEAs) were heated at 500, 600, 700, 800 and 1000 °C, respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi2V0.5Nb0.75 HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe2Nb-type Laves phase. The NbNi4-type intermetallic phase appeared when the heat-treated temperature was higher than 600 °C. With increasing quenching temperature, the volume fraction of the NbNi4-type intermetallic phase increased while that of the eutectic regions decreased. The sample quenched at 800 °C showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as high as 2586.76 MPa, 2075.18 MPa and 16.73%, respectively. Moreover, the eutectic CoFeNi2V0.5Nb0.75 HEAs exhibited apparent age hardening, especially quenched at 700 °C, the hardness reached up to the maximum value of HV 727.52.

Homogenization annealing of the 7050/6009 bimetal slab prepared by direct-chill casting was investigated and its effects on microstructural evolution, composition distribution and mechanical properties in the interfacial region of the bimetal were studied. The results show that the optimized homogenization annealing process was 460 °C for 24 h. After homogenization annealing, the Zn-rich phases and Al15(FeMn)3Si2 phases were precipitated at the interface of the bimetal. The diffusion layer thickness of homogenized bimetal increased by 30 µm from 440 to 480 °C for 24 h, while it increased by 280 µm from 12 to 36 h at 460 °C. The Vickers hardnesses at 6009 alloy side and interface of the bimetal decreased after homogenized annealing and grain coarsening was considered as the dominating softening mechanism. The hardness variation at 7050 alloy side was complicated due to the combined action of solution strengthening, dispersion strengthening and dissolution of reinforced phases.

6009/7050 alloy bimetal slab was prepared by a direct-chill (DC) casting process. Homogenizing annealing, hot rolling and T6 treatment were successively performed and their effects on microstructure and properties of the slab were studied. The results reveal that the average diffusion layer thickness of as-cast slab, determined by interdiffusion of elements Zn, Cu, Mg and Si, was about 400 µm. Excellent metallurgical bonding was achieved because all tensile samples fractured on the softer 6009 alloy side after homogenizing annealing. After homogenizing annealing plus rolling, the average diffusion layer thickness decreased to 100 µm, while the network structure of 7050 alloy side transformed to dispersive nubby structure. Furthermore, subsequent T6 treatment resulted in diffusion layer thickness up to 200 µm and an obvious increase of the Vickers hardness for both 7050 and 6009 sides. The layered structure of the as-cast 6009/7050 bimetal is retained after hot rolling and T6 treatment.

High entropy alloy has attracted increasing attentions. However, to enhance the alloy strength often leads to impairment of the ductility, or vice versa. Here we reported a heat treatment approach on AlCrFeNi2Ti0.5 high entropy alloy, which can elevate the strength and ductility simultaneously. An ingot of AlCrFeNi2Ti0.5 weighing 2.5 kg was firstly fabricated by medium frequency induction melting. Then samples from the same height of the bulk ingot were annealed for 6 h at 600, 700, 800 and 1000 °C, respectively. After 1000 °C annealing, an optimal microstructure was obtained by using our approach which can make some precipitation particles distribute homogeneously in the dendrite interior while keep the interdendrite structure as a single solid solution phase. The mechanical test on this AlCrFeNi2Ti0.5 alloy sample showed that, the compressive fracture strength σbc was increased by about 600 MPa and the plastic strain εp was doubled, compared with those of the as-cast sample. Our approach can be readily adapted to large-scale industrial production of high entropy alloys with high strength and ductility by proper annealing treatment.

CrFeNiV0.5Wx and CrFeNi2V0.5Wx (x = 0.25, 0.5, 0.75, and 1.0) high-entropy alloys were prepared by vacuum arc melting. The effects of W element on the microstructures and mechanical properties of these alloys were investigated. The experimental results indicated that the CrFeNiV0.5Wx alloys were composed of σ, FCC, and BCC phases. Although the microstructures of the CrFeNi2V0.5Wx alloys were still constituted by FCC, BCC, and σ phases, the volume fraction of the FCC phase increased significantly. Dendrite morphology was also observed in the CrFeNi2V0.5Wx alloys. With the addition of W element, the hardness of the CrFeNiV0.5Wx alloys declined from 869 to 633 HV, while the hardness of the CrFeNi2V0.5Wx alloys increased from 226 to 305 HV. Moreover, the CrFeNi2V0.5Wx alloys exhibited better compressive ductility than the CrFeNiV0.5Wx alloys. This study was the first known incidence in which the FCC phase increased in the HEAs with a decrease of the valence electron concentration (VEC) value (i.e., the FCC phase of the CrFeNiV0.5Wx alloys increased with the addition of the BCC-structured W elements).

•The real time solidification of strontium-modified Zn–Al–Si alloys are studied.•The primary silicon has an increased growth rate with the addition of strontium.•Some eutectics are observed to nucleate away from the primary silicon.•The solidification sequence of different eutectics are described.The effect of strontium on the solidification of zinc–aluminum–silicon alloys has been studied. The microstructures of Zn–27%Al–3%Si alloys with different concentrations of strontium were investigated by optical microscope. The solidification processes of unmodified and strontium-modified Zn–27%Al–3%Si alloys were directly observed through the third generation Shanghai Synchrotron Radiation Facility. It was found that the nucleation of primary silicon reduced by additions of strontium, while the precipitation of eutectic silicon was promoted. Imaging experiment results showed that the precipitation temperature of primary silicon was greatly depressed (about 20 °C) while the precipitation temperature of α-Al increased (about 7 °C) by the addition of strontium. The suppressed nucleation resulted in an increased growth rate of primary silicon. The morphology of primary silicon crystals changed from a highly faceted to less faceted and more dendritic after modification, which might be caused by the higher growth rate, poisoning the layer growth mechanism and the Vertex-Linked of octahedral growth unit. Meanwhile, with the increase of cooling rate, the primary silicon became much more branched in the strontium-modified alloys. The needle-shaped eutectic silicon did not nucleate at the surface of the primary silicon in the unmodified alloys, but the coral-like eutectic silicon was found readily nucleating at the surface of primary silicon in the modified alloys due to the addition of strontium. After image processing, some eutectics were observed to nucleate overlapping with pre-existing α-Al or form in the remaining melt in the front of growth interface of α-Al after eutectic nucleating at the surface of the primary silicon.

•The large sized as-cast CoCrFeNiTi0.5 high entropy alloy ingot (Φ70 × 150 mm) was fabricated.•Slight volume effect was discovered in the as-cast CoCrFeNiTi0.5 alloy.•CoCrFeNiTi0.5 HEA ingot exhibited a high microstructure stability and excellent resistance to temper softening.Most previous researches focused on small casting ingots prepared by arc melting, when studying high-entropy alloys. Large sized ingots were also necessary in exploring the existence of volume effects in the multi-principal element alloys. During the experiments, a large sized CoCrFeNiTi0.5 alloy casting ingot was prepared by a medium frequency induction melting furnace. A slight volume effect occurred, reflecting mainly in the growth of crystalline grains and the increase of alloy hardness in the ingot. To investigate the effect of annealing temperature on microstructure and properties of CoCrFeNiTi0.5 alloy, several samples taken from the ingot were annealed at 600 °C, 700 °C, 800 °C and 1000 °C respectively for 6 h. Almost no effects were found to the crystalline structure and elemental distribution when the samples were annealed below 1000 °C. The crystalline structure of CoCrFeNiTi0.5 alloy was composed of one principal face-centered cubic (FCC) solid-solution matrix and a few intermetallic phases in the form of interdentrite. Dendrite contained approximately equivalent amount of Co, Cr, Fe, Ni and a smaller amount of Ti. When annealed below 1000 °C, the interdendrite stayed in (Ni, Ti)-rich phase, (Fe, Cr)-rich phase and (Co, Ti)-rich phase. After 1000 °C annealing, (Co, Ti)-rich phase disappeared, while (Ni, Ti)-rich phase and (Fe, Cr)-rich phase grew. The microhardness of the as-cast CoCrFeNiTi0.5 alloy was 616.80 HV and the macrohardness was 52 HRC. The hardness of the samples stayed generally unchanged after annealing. This indicated a high microstructure stability and excellent resistance to temper softening that the CoCrFeNiTi0.5 alloy exhibited.

The effects of joint addition of minor elements Cr, Mn, Zr, Ti and B on grain refinement of Al-Zn-Mg-Cu alloys were investigated by optical microscopy and scanning electron microscopy(SEM)as well as EDS. Results show that grains can be refined from 256 μm to 102 μm via adding (mass fraction) 0.04%Ti, 0.18%Zr and 0.008%B to the cast alloy. Joint addition of 0.20%Cr, 0.20%Mn, 0.04%Ti and 0.008%B may attain better grain refinement, and the average grain size is as fine as 55 μm. It is because atom clusters containing minor Cr and Mn help Al3Ti nucleation to become crystallization substrate. Joint adding of 0.20%Cr, 0.20%Mn, 0.03%Ti, 0.14%Zr and 0.006%B can result in a remarkable refinement with average grain size about 22 μm. This is because a part of Zr atoms enters the crystal nuclei of Al3Ti based on the atom clusters containing minor elements Cr and Mn to replace the Ti to form new crystal nuclei Al3(Tix, Zr1-x). While minor elements Cr and Mn decrease the surface tension between liquid aluminum and solid Al3Ti, Al3Zr and Al3(Tix, Zr1-x) particles, and impede the nucleation of particles to grow up.