Rod-like Eutectic Structure Research Articles

Microstructure evolution and mechanical property of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics prepared by laser directed energy deposition

Highly dense Al2O3/GdAlO3/ZrO2 eutectic ceramics are one-step additively manufactured by laser directed energy deposition technique under different scanning speed to investigate the inherent response relationship between processing parameter, microstructure, and mechanical property. During the layer-wise deposition process, planar-cellular transition occurs near the bottom of the molten pool, leading to the transformation of irregular eutectic structure into eutectic colony structure lengthened along the building direction, accompanied by the refinement of microstructure dimension. With the increase of scanning speed, the eutectic spacing decreases, and the initially irregular “Chinese script” eutectic ultimately transforms into hexagonally arranged rod-like eutectic structure induced by lamellar instability. The rod spacing is 1.136 times larger than the lamellar spacing. The mechanical property of the laser 3D-printed Al2O3/GdAlO3/ZrO2 eutectic ceramic is manifested as isotropic. The hardness increases with the refinement of the microstructure, while the fracture toughness decreases as the eutectic morphology transforms into rod-like structure. The averaged hardness and fracture toughness are 16.27 GPa and 3.39 MPa‧m1/2, respectively.

Plasma electrolytic oxidation of as-cast and heat-treated binary Al-Ni alloys

In this study, the effect of nickel addition on the plasma electrolytic oxidation (PEO) behavior of pure Al was investigated. Binary Al-Ni alloys containing 1, 3, 5 and 7 at. % Ni were produced by two step vacuum arc and induction melting methods. Produced alloys and pure Al as a reference were PEO coated in a silicate-based alkaline electrolyte through a bipolar power source under constant voltage mode for 45 min. The samples were characterized by XRD, SEM-EDS and Vickers microhardness tester before and after PEO. All alloys were consisted of Al and Al3Ni phases. Fine rod-like eutectic structure was evident for 1 (hypoeutectic) and 3 (eutectic) at. % Ni containing alloys while further addition of Ni resulted in the coarsening of eutectic Al3Ni precipitates. All coatings contained γ-Al2O3 phase while additionally, α-Al2O3 phase formed on pure Al and Al-1Ni alloy. Despite the presence of Ni in the coatings, a Ni-based oxide phase was not evident. Ni, as an alloying element, is found to decrease microhardness and thickness of the coatings, induced a thickness variation between the regions close to the edge and center of the samples. This edge effect was maximum on eutectic alloy, which consisted of homogeneously distributed fine Al3Ni precipitates. To investigate the relationship between eutectic precipitate size and the edge effect, Al-3Ni alloy was heat-treated at 620 °C for 168 h and subsequently coated by PEO. Heat-treatment almost eliminated the edge effect, increased the average coating thickness and promoted the α-Al2O3 formation. Fine eutectic morphology in Al-Ni alloys is found to be detrimental for the coatings produced by PEO.

Rod-like Eutectic Structure in Biodegradable Zn-Al-Sn Alloy Exhibiting Enhanced Mechanical Strength.

Zn alloy is recognized as a promising biodegradable metal for bone implant applications because of its good biocompatibility and moderate degradation rate. Nevertheless, the insufficient strength limits its applications. In this study, a rod-like eutectic structure was fabricated in Zn-Al-Sn alloy with the addition of Sn via selective laser melting. It was found that the Al-enriched phase nucleated primarily during cooling and caused the rapid precipitation of Zn. This inevitably consumed the liquid Zn and increased the ratio of Sn to Zn in the liquid phase, resulting in the formation of the eutectic, which was composed of the Sn-enriched phase and the Zn-enriched phase. More importantly, the coupled growth of the Sn-enriched and Zn-enriched phases and their volume differences together led to a rod-like morphology of the eutectic according to the volume fraction theory. Consequently, the yield and ultimate compressive strengths were enhanced to 180 ± 18.8 and 325 ± 29.6 MPa for the Zn-Al-2Sn alloy, respectively. This could be attributed to the pinning effect of the rod-like eutectic, which could block dislocation motion and result in dislocation pile-up, thereby conducing to the mechanical reinforcement. In addition, the Zn-Al-Sn alloy also exhibited good biocompatibility and increased degradation rate because of the enhanced galvanic corrosion. This study showed the potential of rod-like eutectic for the mechanical enhancement of the biodegradable Zn alloy.

Eutectic Structures Competition in the Stripes Strengthening the (Zn) – Single Crystal

Abstract Some eutectic stripes have been generated in a hexagonal (Zn) - single crystal. The stripes are situated periodically with the constant interstripes spacing. The eutectic structure in the stripes consists of strengthening inter-metallic compound, Zn16Ti, and (Zn) - solid solution. The rod-like irregular eutectic structure (with branches) appears at low growth rates. The regular lamellar eutectic structure is observed at middle growth rates. The regular rod-like eutectic structure exists exclusively in the stripes at some elevated growth rates. A new thermodynamic criterion is recommended. It suggests that this eutectic regular structure is the winner in a morphological competition for which the minimum entropy production is lower. A competition between the regular rod-like and the regular lamellar eutectic growth is described by means of the proposed criterion. The formation of branches within irregular eutectic structure is referred to the state of marginal stability. A continuous transitions from the marginal stability to the stationary state are confirmed by the continuous transformations of the irregular eutectic structure into the regular one.

Rod-like eutectic structure of arc-melted TiB2–TiC N1− composite

Abstract TiB 2 –TiC x N 1− x composites were synthesized by arc-melting mixtures of TiB 2 , TiC and TiN powders in a N 2 atmosphere at 60 kPa. TiB 2 –TiC x N 1− x composites were obtained at TiN contents below 20 mol%, whereas TiB formed in the composites at higher TiN contents. TiB 2 –TiC x N 1− x composites with a nominal composition of 36TiB 2 –44TiC–20TiN (mol%) had a rod-like eutectic structure, where the TiC x N 1− x single-crystal rods dispersed in the TiB 2 single-crystal matrix. The crystal orientation relationship between the TiB 2 matrix and the TiC x N 1− x rods was TiB 2 (0 0 0 1)//TiC x N 1− x (1 1 1). Vickers hardness ( H v ) of the rod-like eutectic composite was 22.5 GPa.

Thermodynamic description and unidirectional solidification of eutectic organic alloys: IV. Binary systems neopentylglycol–succinonitrile and amino-methyl-propanediol–succinonitrile

The temperature and enthalpy of transformations of organic alloys from the binary systems neopentylglycol–succinonitrile (NPG–SCN) and 2-amino-2-methyl-1,3-propanediol–succinonitrile (AMPD–SCN) were measured by means of differential scanning calorimetry (DSC). The phase diagrams of these binary systems were assessed via the CALPHAD approach using Thermo-Calc by simultaneously optimizing the thermodynamic and phase equilibrium data measured in the present work. Proper agreements between the experimental and calculated data for the phase diagrams as well as for the thermochemical properties were achieved. Experiments and calculations show that both the NPG–SCN and the AMPD–SCN systems exhibit a non-variant eutectic reaction with the eutectic point at 90.45 mol% SCN (318.0 K) and at 97.39 mol% SCN (325.7 K), respectively. In the NPG–SCN system the temperature of the eutectic reaction is about 3 K higher than the temperature of the transformation from the ordered crystals (OCs) to the orientationally disordered crystals (ODICs), whereas the eutectic reaction in the AMPD–SCN involves the OCs of AMPD and the ODICs of SCN. Unidirectional solidification experiments were performed with selected NPG–SCN and AMPD–SCN alloys in order to verify phases involved in solid–liquid equilibria and the nature of eutectic growth in these systems. We find that eutectic growth in NPG–SCN eutectic alloy occurs with both solid phases being non-facetted and with a rod-like eutectic structure. The eutectic as well as some hypo-eutectic alloys from the AMPD–SCN system show irregular eutectic growth with a non-facetted BCC_A2 phase of SCN and a facetted monoclinic phase of AMPD.

Thermodynamic description and unidirectional solidification of eutectic organic alloys: III. Binary systems neopentylglycol-( d)camphor and amino-methyl-propanediol-( d)camphor

The temperature and enthalpy of transformation of organic alloys from the binary systems neopentylglycol-( d)camphor (NPG-DC) and 2-amino-2-methyl-1,3-propanediol-( d)camphor (AMPD-DC) were measured by means of differential scanning calorimetry (DSC). The phase diagrams of these binary systems were assessed via the CALPHAD approach using Thermo-Calc by simultaneously optimizing the thermodynamic and phase equilibrium data measured in the present work. Proper agreements between the experimental and calculated data for the phase diagrams as well as for the thermochemical properties were achieved. Experiments and calculations show that both the NPG-DC and the AMPD-DC system exhibit a nonvariant eutectic reaction with the eutectic point at 36.2 mol% DC and 326.0 K and at 9.3 mol% DC and 362.0 K, respectively. In each system the temperature of the eutectic reaction is higher than the temperature of the transformation from the ordered crystals to the orientationally disordered (plastic) crystals. Unidirectional solidification experiments were performed with several alloys in order to verify the nature of eutectic growth: We find that in both systems eutectic growth occurs with both solid phases being non-facetted and with a lamellar or rod-like eutectic structure. Due to the optical activity of DC its distribution in the solid samples is well detectible in polarised light.

Thermodynamic description and unidirectional solidification of eutectic organic alloys: I. Succinonitrile-( d)camphor system

The temperature and enthalpy of transformations of organic alloys from the binary system succinonitrile-( d)camphor were measured by means of differential scanning calorimetry (DSC) within the entire composition range. The analytical description of the Gibbs energies of pure succinonitrile (SCN) and pure ( d)camphor (DC) were derived utilizing the data on temperature and enthalpy of transformations, and temperature dependencies of heat capacity available in the literature. The phase diagram for the binary SCN-DC system was assessed via the CALPHAD approach using Thermo-Calc by simultaneously optimizing the thermodynamic and phase equilibrium data available in the literature and measured in the present work. A good agreement between the experimental and calculated data for the phase diagram as well as for the thermochemical properties was achieved. Experiments and calculations show that the binary system SCN-DC has an eutectic reaction with the eutectic point at 311.5 K and 13.9 mol% DC. The enthalpy of mixing derived in the optimisation proves weak attractive interaction between dissimilar molecules. Unidirectional solidification of the eutectic alloy was performed in order to verify the nature of the eutectic: we find that eutectic growth occurs with both solid phases being nonfacetted and with a rod-like eutectic structure consisting of 23 vol% (DC) and 77 vol% (SCN). Due to the optical activity of DC its distribution in the solid sample is well detectible in polarised light.

Directional crystallization of B 4C–NbB 2 and B 4C–MoB 2 eutectic compositions

We studied the directional crystallization of different compositions in B 4C–NbB 2 and B 4C–MoB 2 systems. The eutectic compositions for both systems are evaluated. It is shown that in the first system the rod-like eutectic structure is formed, in second, the “Chinese hieroglyphics”. In both cases high hardness and high microplasticity are observed, which are much more than for individual component phases. These compositions may be considered as a new kind of self-strengthening composite materials.

Joining of NiAl to iron-based alloys by reactive casting

Nickel monoaluminide, NiAl, is exothermically synthesized from elemental liquids of aluminum and nickel and is simultaneously joined to iron-based alloys, such as carbon steels and stainless steels, by the reactive casting method, which is based on the pouring of the elemental liquids onto the base material. The exothermic reaction between the aluminum and nickel liquids produces extremely superheated NiAl liquid. Heat of the NiAl liquid is transferred to the base material, and the contact surface is melted to a depth depending on both the preheating temperature of the base material and the thickness of the NiAl produced. After solification on the base material, NiAl is strongly joined to the base material, showing a joint strength exceeding the inherent strength of NiAl. The high joint strength is attributed to a very fine, rodlike eutectic structure at the joint interface that consists of a mixture of β-NiAl and γ-iron with an interrod spacing of approximately 100 nm.

Rapid Crystal Growth in Undercooled Hypoeutectic Cu-Sb Alloys

High undercoolings of up to 141K(0.176T L ) were achieved for a bulk hypoeutectic Cu 37.5 Sb 62.5 alloy using the glass fluxing method. The solidified microstructures were then investigated for the purpose of understanding rapid crystal growth from undercooled melts of a binary eutectic system containing one intermetallic compound. It was found that the microstructures of this alloy depend strongly on the undercooling level prior to solidification. At small undercoolings, coarse plates of the primary intermetallic Cu 2 Sb compound coexist with rod-like eutectic structure. Moreover, the larger the undercooling level, the finer both the primary phase and the eutectic structure. As the undercooling level is increased to above 84K, however, the primary phase is changed from faceted Cu 2 Sb plates to Sb dendrites, and the eutectic structure changes from the above regular eutectic structure to an anomalous one. In order to understand such changes in the microstructure, the growth velocities of two primary phases and the eutectic growth velocity versus the undercooling level were calculated in terms of the current theories on rapid crystal growth from undercooled melts.

The Effect of Gravitation Forced Convection on Structure of Al-Ni Eutectic

The effect of different kinds of experimental parameters such as gravitational acceleration (microgravity, highgravity), temperature gradient, solidification rate, primary phase, (aluminium dendrite, Al 3 Ni compound), mechanical instability of the liquid (micro gravity, high gravity, vertically stabilized and unstabilized, horizontal arrangement) on the melt motion and on the Al-Al 3 Ni rodlike eutectic structure was investigated.

ステンレス鋼のレーザ表面改質に関する研究 （第６報） 耐食性に及ぼすレーザ急冷凝固処理の影響

In the previous report, it was shown that the massively solidified structure and the rod-like eutectic structure formed when stainless steels of a certain composition range have been rapidly solidified using laser surface melting technique. It was also found by the formation of these structures a homogenization of element distribution and refinement of structure could be obtained. In the present study, the corrosion resistance of stainless steels with the massively solidified structure and the rod-like eutectic structure which was formed by laser surface melting was investigated. Corrosion resistance was evaluated by the passivation current density (Icrit) and Piitting potential (V'C100). It was found that the corrosion resistance of stainless steels was markedly improved by formation of massively solidified structure or the rod-like eutectic structure. SEM observation revealed that such improvement was considered to be attributed to the alleviation of the solidification segregation and refinement of structure in the massively solidified structure, and to be mainly attributed to the increase of interface area due to formation of fine two-phase structure in the rod-like eutectic structure.

ステンレス鋼のレーザ表面改質に関する研究 （第４報） 室温組織並びにδフェライト形態に対するレーザ急冷凝固処理の影響

Microstructures of stainless steels rapidly solidified by laser surface melting (LSM) were investigated. In addition to the massively solidified structure and the rod-like eutectic structure which have been reported, characteristic structures, named as film-like ferrite, and rod-like ferrite were observed in LSM treated specimen. TEM and STEM analysis have revealed that there was not specific orientation relationship like K-S relationship between rod-like ferrite and austenite matrix, while there was K-S orientation relationship between film-like ferrite and austenitee matrix. Between ferrite and austenite matrix in vermicular ferrite structuer 4 or 5 degree deviation from K-S orientation direction had been observed. Segregation of elements due to solid transformation from ferrite to austenite was observed in vermicular ferrite and film-like ferrite structures, but no segregation due to solid transformation was observed in rod-like ferrite structure. From these results it was deduced that: (1) as for vermicular ferrite structure, delta ferrite is the primary phase of solidification, and austenite solidifies at the terminal stage on solidification and a diffusional solid transformation from ferrite to austenite takes place. (2) as for film-like ferrite, delta ferrite is dominant phase of solidification and a diffusional solid transformation follows with drop temperature than that of vermicular ferrite structure. (3) as for rod-like ferrite, LSM treated specimen solidifies completely to ferrite and a massive (patitionless) solid transformation from ferrite to austenite takes place during cooling.

ステンレス鋼のレーザ表面改質に関する研究 （第２報） レーザ急冷凝固処理部における棒状共晶凝固組織の生成機構

ステンレス鋼のレーザ表面改質に関する研究 （第２報） レーザ急冷凝固処理部における棒状共晶凝固組織の生成機構

Bridgman and Czochralski growth of Ge-TiGe 2 eutectic composites

Eutectic composites of Ge-TiGe 2 grown from the melt by Bridgman and Czochralski growth methods are compared. It is found that while Czochralski growth results in a slight reduction in the uniformity of the rod-like eutectic structure, it also yields an improvement in the electronic properties of the semiconducting matrix phase.