Recalescence Effect Research Articles

Investigation on solidification structure evolution of deeply undercooled single-phase Ni-Cu-Co alloys

In this work, the main focus was on exploring the effect of increasing the gradient content of Co element on the solidification process of Ni-Cu-Co alloy under deep undercooling conditions. The effects on its solidification rate, recalescence effect, and critical undercooling range were also explored. In the experiment, a combination of molten glass purification and cyclic superheating technology was mainly used to obtain gradient undercooling of Ni88Cu10Co2, Ni88Cu8Co4, and Ni88Cu6Co6 alloy samples at intervals of 10 K. By analyzing the microstructure evolution under different undercooling degrees through metallographic diagrams, and combining it with high-speed cameras to record the phenomenon of recalescence during the solidification process, the influence of the addition of Co element on the solidification rate, recalescence effect, and the critical undercooling range appearing in the microstructure during the overall solidification process was explored. Conduct EBSD analysis on the alloy samples under the maximum undercooling of Ni88CuCo6, and analyze the representative significance behind their characterization data. At the same time, TEM transmission analysis was conducted on the Ni88Cu6Co6 alloy sample with the highest Co content, and it was found that there are high-density dislocation and stacking fault networks as well as obvious twinning structures in the substructure area. The standard FCC diffraction pattern also represents that it is still a single-phase structure. Based on the above metallographic diagram, EBSD, and TEM data analysis, it is proven that the occurrence of grain refinement under high undercooling is mainly due to stress induced recrystallization.

Study on the microstructure evolution mechanism of copper single phase alloys under deep undercooling conditions

This article mainly used the deep undercooling technology to treat Cu60Ni40, Cu60Ni38Co2, and Cu60Ni35Co5 alloys to obtain different undercoolings. By using high speed cameras, the rapid solidification process was recorded to analyze the relationship between its evolution law and the magnitude of undercooling. When observing its microstructure, it was also found that there was a critical undercooling degree throughout its entire solidification process. The influence of Co element addition on solidification rate, recalescence effect, and critical undercooling of copper based single-phase alloys under undercooling conditions was explored. EBSD analysis was conducted on the refined microstructure of the alloy that achieved the maximum undercooling, and the significance of grain orientation and orientation difference distribution was found. TEM tests were conducted on a Cu60Ni38Co2 alloy with a maximum undercooling of 259 K, and it was found that there are high-density dislocation networks in some areas within them. Finally, the Vickers hardness of the above alloys was measured and recorded using a microhardness tester (HVS-1000Z), and the evolution between microhardness and undercooling, as well as the changes in microhardness under critical undercooling, were analyzed. Based on the analysis of EBSD, TEM, and microhardness data above, it is found that grain refinement at high undercooling is dominated by recrystallization.

Microstructure refinement mechanisms in undercooled solidification of binary and ternary nickel based alloys

Molten glass purification and cycle superheating technologies were used to make Ni65Cu35, Ni65Cu33Co2 and Ni65Cu31Co4 alloys obtain maximum undercoolings of 320 K, 292 K and 300 K respectively. In order to analyze the relationship between morphological characteristics of solidification front and undercooling change during migration of solid–liquid interface, a high-speed camera was used to capture pictures of the recalescence process. Observing the microstructure of the undercooled alloys using metallographic microscope, the characteristics and evolution of microstructure during rapid solidification process of undercooled liquids were analyzed. It was found that grain refinement mechanisms of highly undercooled Ni–Cu–Co alloys was the same as those of the Ni–Cu alloys. Dendrite remelting leads to the grain refinement at low undercooling, while the dominant factor of grain refinement at high undercooling is recrystallization process induced by stress. The internal driving force can be divided into two parts: one is the thermal stress generated by the releasing of solidification latent heat during recalescence process, and the other is the stress and strain accumulated by interaction of liquid flow and primary dendrite during rapid solidification. We also found that addition of third element Co not only played an important role in solidification rate and recalescence effect, but also significantly improved the average hardness of grain refined microstructure, which was about 80% higher than that of as cast alloy. The addition of trace Co was also conducive to the formation of non-segregation solidification microstructure.

Effect of Co addition on solidification velocity, hardness and refined structure transformation mechanisms of undercooled Ni-Cu-Co alloys

In this work, the effect of Co addition on the solidification velocity, hardness and refined structure transformation of the undercooled Ni-Cu-Co alloys have been systematically investigated. The results show that the grain refinement of Ni-Cu-Co ternary alloys at low undercooling is caused by the intense dendrite remelting. However, the grain refinement at high undercooling is attributed to recrystallization, and the driving force of recrystallization comes from the stresses and plastic strains accumulated by the interaction between liquid flow induced by rapid solidification and primary dendrites. The Co addition can significantly increase the solidification velocity and recalescence effect of Ni-Cu-Co alloys by increasing the number of solidified crystal nucleus and solute diffusion velocity. Under the same undercooling level, the increase of solidification velocity and recalescence effect have strengthened the remelting effect and stress accumulation, which can enlarge the critical range for grain refinement gradually. Meanwhile, due to the significant improvement of solidification velocity and stress-induced deformation effect by Co addition, the low angle GBs in the refined structure of Ni-Cu-Co alloys increase and the grain size decreases significantly, which have significantly improved the average hardness of the refined structure. When Co addition reaches 6 %, the refined structures can appear in almost the whole undercooling range and the hardness are improved by more than 100 % compared with the as-cast structure.

Grain refinement of 24 karat gold (99.99 wt.% pure) and 22 karat gold (Au-5.8wt.%Cu-2.5wt.%Ag) by Au-6wt.%Ti grain refiner

In this paper, the 24 karat (99.99 wt.% pure) and 22 karat gold (Au-5.8wt.%Cu-2.5wt.%Ag) were grain refined with Au-6wt.%Ti master alloy and the mechanism of grain refinement was studied. The Au-6wt.%Ti master alloy containing Au2Ti and Au4Ti intermetallic particles was added into 24 karat and 22 karat gold at various addition levels of Ti (0.1, 0.2 and 0.3 wt.%) for grain refinement. The intermetallic particles undergo peritectic reaction with liquid Au in sequence to nucleate α-Au solid. It is observed that both the 24 karat gold and 22 karat gold showed efficient grain refinement at the lowest addition level of Ti (0.1 wt.%). However, 22 karat showed better grain-refining efficiency than 24 karat at all addition level of Ti studied. The mechanism of grain refinement of gold in this study matches with the “Peritectic theory” and “Solute paradigm” proposed for the grain refinement of aluminium. It is also observed that increasing the master alloy addition level to 0.2 and 0.3 wt.% Ti coarsening in the grain size in both 24 karat and 22 karat, but significant in the case of 24 karat gold due to recalescence effect. The grain-refined 24 karat and 22 karat gold at 0.1 wt.% Ti showed improvement in the hardness in comparison to the un-grain refined one.

Investigation of nickel aluminium bronze castings properties by degassing agent technique

Nickel Aluminium Bronze (NAB) is an excellent engineering material for maritime application due to its selective mechanical properties and relatively excellent corrosion resistant. However, NAB is susceptible to porosities due to dissolved gasses absorbed during its casting processes and adversely affects the mechanical properties. So, a degassing agent is added to the melt of NAB before the pouring process to reduce the dissolved gasses. A few parameters were initiated to investigate the effect of the degassing agent. Firstly, the effect of the degassing agent on the solidification behavior of the NAB alloy is studied using thermal analysis. Cooling curves can provide information on liquidus, eutectic, undercooling, recalescence and solidus temperature of the NAB alloy. Secondly, Differential thermal analysis (DTA) technique is employed to determine this thermal arrest temperature of NAB alloy from the cooling curves. The increasing in degassing agent addition will increase the average cooling rate of the NAB during solidification process but also slightly increasing the recalescence effect at the same time. It is discovered the higher cooling rate will reduce the dendritic growth and lead to better mechanical properties. Higher degassing addition also produced a smaller dendritic structure which promotes the higher mechanical strength. It has been proven through the tensile test, hardness test and microstructure analysis using FESEM. The relationship between, DAS average length, SDAS average length, tensile stress and hardness have been studied.

Parameter identification in non-isothermal nucleation and growth processes

We study non-isothermal nucleation and growth phase transformations, which are described by a generalized Avrami model for the phase transition coupled with an energy balance to account for recalescence effects. The main novelty of our work is the identification of temperature dependent nucleation rates. We prove that such rates can be uniquely identified from measurements in a subdomain and apply an optimal control approach to develop a numerical strategy for its computation.

Analysis of thermal stability after occurrence of absolute solute trapping in undercooled Co–Cu alloy

Thermal stability after recalescence of the single-phase supersaturated granular grains prepared in Co-15 at.%Cu immiscible alloy melt was investigated. As for undercooled Co–Cu alloy, a transition from dendritic to granular crystals occurred when ΔT ≥ ΔT∗, which was induced by an abrupt change in the dendritic growth mechanism from solute-controlled to purely thermally-controlled growth. However, the recalescence effect led to grain growth and solute segregation of trapped Cu to grain boundaries. On this basis, a thermo-kinetic model applicable for nano-scale materials was extended to the system of micro-scale undercooled Co–Cu alloy. The model showed close agreement with the experimental results. It is concluded that Cu atoms captured by solute trapping re-segregated to grain boundaries, which was responsible for the reduction in grain boundary energy and improvement in thermal stability in undercooled Co–Cu alloy.

The Tensile Properties of Pearlite, Bainite, and Spheroidite

The tensile properties of four steels have been determined as a quantitative function of the measured dimensions of the aggregate structures pearlite and spheroidite, and of the austenite decomposition temperature for the structures pearlite and bainite. Studies of the recalescence effect have been performed in connection with the measurement of the reaction temperature. The strength indices (stress at corresponding strains, tensile strength, hardness) vary linearly with the reaction temperature and the logarithm of the dimensions of the aggregate. Mixtures of pearlite and bainite are intermediate in strength. The ductility indices are low for mixed structures, coarse pearlite and low temperature bainite; higher for bainite and pearlite in the middle of the reaction temperature range for each. It has been observed that spheroidized eutectoid specimens have a typical mild steel yield point; pearlitic specimens of the same tensile strength do not. The spacing of pearlite is shown to be proportional to the carbon diffusion coefficient in austenite, the logarithm of the spacing plotting as a straight line against the reciprocal of the absolute reaction temperature, with the same slope as a similar plot for the diffusion coefficient. Because of this it is concluded that a measurement of the spacing at one temperature permits its calculation at another, using the measured energy of activation for the diffusion of carbon in the steel. A rule of strength for aggregates is proposed, based on these studies, as follows: The resistance to deformation of a metallic aggregate consisting of a hard phase dispersed in a softer one is proportional to the logarithm of the mean straight path through the continuous phase. The rule works for a comparison of the properties of pearlite and spheroidite, as well as for pearlite alone over a wide range of spacings, and extrapolates to reasonable particle sizes for the finest spheroidites (tempered martensite). A simple explanation of the semilogarithmic character of the relationship is advanced.

Dynamics of solidification in Al thin films measured using a nanocalorimeter

We demonstrate how a nanocalorimeter can be used to measure the dynamics of thin film melting and solidification using aluminum as a model system. Recalescence, a rise in temperature due to enthalpy release on solidification, is observed when the rate of heat release is faster than the rate of heat extraction. For thin films, with a large density of nucleation centers at the interface, recalescence effects are observed at high cooling rates where direct measurements of heat capacities and enthalpy changes are challenging. Temperature rates of 104 K/s were applied using nanocalorimeter chips; the nanocalorimeter allows direct measurement of the under cooling, temperature rise during solidification, and the associated changes in enthalpy. This work highlights some of the challenges recalescence causes in physical measurements and provides a numerical strategy to evaluate enthalpy changes during rapid solidification.

Cast iron melting and solidification studies by advanced thermal analysis

Thermal analysis in foundry is performed by measuring the melt temperature during the free cooling in cups. Information on liquidus, eutectic, solidus temperatures and recalescence effects is collected. Advanced analyses in the laboratory use differential devices where a temperature program is imposed to both the sample and an inert reference. The sensitivity in the detection of temperature points is enhanced and quantitative determination of the heat release becomes feasible.In this work we have employed a high temperature differential scanning calorimeter to follow melting, solidification and eutectoid transformations of master alloys for the production of lamellar, compact graphite and spheroidal cast irons under protective helium atmosphere.A high silicon ductile iron alloyed with 2% wt. Al, devised for application in engine components, was analysed as well.The correlation between thermophysical data obtained by calorimetry, free cooling in cups and microstructure is shown.

Crystallization mechanisms in laser irradiated thin amorphous silicon films

Structural properties of thin polycrystalline silicon films, crystallized by single shot excimer laser annealing at different laser energy densities, have been investigated. Formation of disk structures has been observed in a wide range of energy densities, from complete melting down to 180 mJ/cm 2. These structures have been correlated to the lateral growth of grains starting from the small grains present in the central regions of the disks. We propose a new crystallization scenario for energy densities below the complete melting. In this framework, the recalescence effect plays an important role while the super lateral growth-regime is no longer a particular crystallization condition but simply represents the upper energy density limit of partial melt crystallization regime.

Dynamical evolution of microstructure in finely atomized droplets of Al-Si alloys

The complex time-dependent evolution of microstructure in rapidly solidified droplets of Al-Si eutectic alloys is quantitatively studied. Fine droplets of the alloy were obtained by using a high pressure gas atomization technique. The recalescence effect within each droplet gives rise to non-steady state growth conditions that lead to a systematic variation in microstructure across the droplet. This variation in microstructure is quantitatively characterized first by determining the nucleation undercooling and then by measuring the spatial variation in microstructural scales within the droplet. The results are compared with theoretical models to obtain the interface velocity and interface undercooling as a function of distance from the apparent nucleation site in different sized droplets. Eutectic, dendritic and cellular microstructures have been observed in droplets and the presence of these different morphologies is explained by constructing a microstructure map for the Al-Si system. The volume fractions of these three microstructures, as well as the length scale of each microstructure, are shown to determine the properties of the material. Specifically, microhardness measurements have been carried out for different diameter droplets, and a minimum in the microhardness is observed that is related to the presence of different microstructures. A microstructure map is developed for the Al-Si system to establish regimes of alloy composition and undercooling (as related to droplet diameter) required for the design of materials with optimum mechanical properties.

Containerless rapid solidification of undercooled Cu-Co peritectic alloys

Droplets of Co-16%Cu and Co-71 .6%Cu peritectic alloys were solidified during containerless processing in a 3-m drop tube. The microstructures of Co-16% Cu alloy droplets were characterized by dendritic or equiaxed a-Co phase with a small amount of Cu-rich solid solution distributed on a- Co phase boundaries. Two thresholds of droplet diameter were observed for Co-16%Cu alloy at which "equiaxed-dendritic-equiaxed" morphological transitions occur to primary a-Co phase. This conspicuous refinement of primary a-Co grains results from the fragmentation of a-Co dendrites caused by recalescence effect. For Co-71.6% Cu alloy, the primary a-Co phase forms as coarse columnar dendrites in large droplets and equiaxed dendrites in small droplets. Theoretical calculations indicate that Marangoni migration contributes more to the growth of disperse Co-rich spheres by stimulating collision and coalescence than Stokes motion caused by the residual gravity in the falling Co-71.6% Cu alloy droplets.

Numerical modelling of rapid solidification

A mathematical model of the melt spinning process has been developed based on the control-volume finite-difference method. The model avoids some of the limitations of the previous models, for example including the effect of the wheel in the heat flow calculations and the temperature dependence of the thermophysical parameters of the material. The nucleation temperature was calculated based on the heterogeneous nucleation theory. The effect of various parameters, such as the heat transfer coefficient, the nucleation temperature and the heating and type of the wheel on the rapid solidification behaviour, for example the recalescence characteristic, was investigated. The results of the model showed that the effect of heating of the wheel is difficult to prevent even for a wheel material of high conductivity. The recalescence effect was found to be influenced by the wheel surface temperature and to decrease with increasing thermal conductivity of the wheel. The observed increase in the wheel surface temperature suggests the importance of including the wheel in the numerical calculations, especially for a wheel made of a low-conductive material.

Undercooling of Ni-B and Fe-B alloys and their metastable phase diagrams

Ni-B and Fe- Balloy have been undercooled in the cell of a high temperature differential scanning calorimeter (DSC) obtaining thermal data for their melting and solidification. Primary solidification was suppressed up to an undercooling ΔT larger than 200 K: eutectic solidification was suppressed up to ΔT larger than 100 K. It is demonstrated, using the Lipton-Kurz-Trivedi model, that at low undercooling the dendrite growth process is described by the calorimeter trace owing to limited recalescence, whereas at high undercooling the signal is dominated by recalescence effects. The solidification microstructures are discussed in relation to the dendrite growth rate and the mechanism of eutectic freezing.For both Ni-B and Fe-B a metastable phase was found to solidify preferentially with respect to the stable eutectic. The metastable phase is Ni2Bh in Ni-B and very likely Fe3B in Fe-B. For each phase the enthalpy and entropy of formation are estimated and metastable phase diagrams are drawn. An evaluation of the excess specific heat of the liquid eutectics is made and the results conform to the value expected for glass-forming alloys.

Rapid solidification characteristics in melt spinning

The recalescence characteristics in rapid solidification of copper was studied using a thermokinetic model. The effects of the heat transfer coefficient, the melt thickness and the nucleation temperature were investigated. Results showed that lower nucleation temperature and thinner melt lead to a longer recalescence effect while larger heat transfer coefficient results in a weaker recalescence effect. A dimensionless number was derived to measure the extent of recalescence.

Temperature evolution of plasma-sprayed niobium particles impacting on a substrate

A fast two-color pyrometer is used to monitor the temperature evolution of individual plasma- sprayed niobium particles after their impact on a substrate. The experimental setup permits rejection of thermal events corresponding to in-flight particles intersecting the pyrometer field of view, ensuring that only particles impacting on the substrate surface are examined. Cooling rates as high as 10 8Ks -1 are measured when particles are sprayed on steel and alumina substrates. An interruption of the rapid cooling process and a transient temperature increase are observed near the niobium melting point. Experimental results are interpreted with the help of a numerical thermal flow model taking into account undercooling and the recalescence effects.

Solidification of undercooled cadmium-zinc eutectic melts

Molten specimens of Cd-Zn eutectic alloy (26.50 at.% Zn) were undercooled by the glass-slag technique using glass-forming ZnCl2 as the slag. The microstructure of the solidified samples, weighing about 20 g each, was found to be a function of the undercooling produced. At lower undercoolings the microstructure consisted of supersaturated primary cadmium dendrites co-existing with the eutectic, while at higher coolings duplex structures were observed. Possible causes for the differences in microstructure are discussed with due importance given to the recorded recalescence effects.