High Undercooling Research Articles

Microstructure evolution and grain refinement mechanism in undercooled (CoFe)50Si50 multi-principal element intermetallics

Multi-principal element alloys (MPEAs), as a novel alloy design idea, have been proved to be promising materials and gained increasing attention recently. In this study, the single-phase (CoFe)50Si50 multi-principal element intermetallics (MPEIs) was synthesized and subject to rapid solidification via the melt fluxing technique. A maximum undercooling of 314 K was achieved and the microstructure of as-solidified samples with different undercooling was characterized. An ordered B2 phase was found to dominate the phase constitution despite the formation of minor Fe-rich BCC metastable phase in highly undercooled samples. The microstructure of undercooled (CoFe)50Si50 MPEIs shows a clear morphological evolution from dendrites to equiaxed grains and then to refined dendritic seaweed-like morphology with undercooling. At low undercooling, spontaneous grain refinement can be described by the dendrite fragmentation and dendrite remelting mechanisms. At high undercooling, detailed EBSD analysis reveals that the formation of subgrains and subsequent recrystallization play a significant role, but the inherent characteristics of (CoFe)50Si50 MPEIs (such as high stacking fault energy) prevent the occurrence of complete recrystallization and hence complete grain refinement. This study could provide a reference for controlling the non-equilibrium microstructure of MPEIs and understanding the microstructure evolution from binary intermetallic compounds to MPEIs.

Effect of composition on nucleation and growth of equiaxed dendrites in the transparent alloy Neopentylglycol-(d)Camphor during solidification in microgravity conditions

We investigate the effect of composition of hypoeutectic Neopentylglycol-(D)Camphor (NPG-DC) alloy on nucleation and microstructure kinetics during alloy solidification in low gravity conditions without convection and buoyancy effects. Two alloys with compositions of 0.2 and 0.3 wt-frac. DC have been solidified in a small thermal gradient to obtain equiaxed dendritic microstructures with different crystallographic growth orientation. In-situ optical and thermal characterization in the transparent material provided evolution of equiaxed dendrite density, nucleation undercooling, dendrite tip velocity and kinetic law. Significant differences in the results for the two compositions were found, which are discussed in relation to temperature and concentration dependent solid–liquid interfacial properties. For NPG-0.2 wt-frac. DC equiaxed dendrites with 〈100〉 crystallographic growth orientation exist which tend to grow faster and fill the liquid volume quicker compared to the NPG-0.3 wt-frac. DC alloy. This behavior hinders nucleation of further dendrites and decreases their final number. In contrast, for NPG-0.3 wt-frac. DC equiaxed dendrites with 〈111〉 crystallographic growth orientation nucleate at higher undercooling and the kinetic law indicates smaller dendrite tip velocities correlated to a larger number of dendrites.

Formation mechanism of heterogeneous microstructures and shape memory effect in NiTi shape memory alloy fabricated via laser powder bed fusion

Additive manufacturing involves the process of track-by-track melt pools accompanied by the localized rapid melting/solidification, which can determine unique nonequilibrium microstructures. In this study, we report formation of heterogeneous nonequilibrium microstructures in near-equiatomic NiTi fabricated via laser powder bed fusion (LPBF) additive manufacturing, and further discuss their underlying formation mechanisms and influences on shape memory effect of the LPBF NiTi. Specifically, the heterogeneous microstructures include the core (HCP α-Ti)-shell (Ti2Ni) structural nano-sized precipitation phases in columnar grains, which were resulted from high undercooling and cooling rate during LPBF solidification, the intermediate R phase in cellular and columnar grains, which was stemmed from residual thermal stress during LPBF, the nano-sized cellular substructure in columnar grains with boundaries decorated with Ti2Ni precipitates, which was originated from enriched Ti atoms at the solidification front during directional and orderly solidification of melt pools, together with the abundant dislocations. Interestingly, the two-way shape memory strain of 0.8% in LPBF NiTi was obtained by cycle loading–unloading-heating–cooling training process. These findings achieved in this work enrich the knowledge on formation mechanism of heterogeneous microstructures in LPBF NiTi SMAs, and further pave the way for engineering applications of two-way shape memory effect of LPBF NiTi shape memory alloys.

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.

Sluggish dendrite growth in undercooled Fe-Co-Ni-Si multi-principal element intermetallics

The (CoFe)50Si50 and (CoFeNi)50Si50 multi-principal element intermetallics (MPEIs) with a B2 ordering structure were synthesized by arc melting, undercooled using the glass fluxing technique and observed in-situ by a high-speed high-resolution camera. A sharp increase of the dendrite growth velocity was found for (CoFe)50Si50 and (CoFeNi)50Si50 at the undercooling of ΔT = 250 K and ΔT = 225 K, respectively, related to the disorder trapping that occurs at high undercooling. As a result, low-ordered regions were found by HR-TEM in highly undercooled (CoFe)50Si50 and (CoFeNi)50Si50 alloys. Compared with the previous reported binary CoSi, the maximum dendrite growth velocity is decreased by nearly one order for the (CoFeNi)50Si50 MPEIs. Such sluggish growth phenomenon does exist in present MPEIs for both cases of the absolute and the homologous temperature scale. It is suggested that the number of components might dominate sluggish dendrite growth of the B2 crystal and the effect of specific elements cannot be omitted. This study reported the dendrite growth behavior of MPEIs, which is helpful for understanding the non-equilibrium solidification phenomena, thereby providing a reference for controlling the non-equilibrium solidification process.

Dendritic Solidification and Physical Properties of Co-4.54%Sn Alloy with Broad Mushy Zone

The high undercooling of binary Co-4.54%Sn alloy has a significant influence on its microstructural characteristics and physical properties. Here, we report that the dendritic growth and physical properties of broad-temperature-range Co-4.54%Sn alloy remarkably depends on the undercooling during the rapid solidification. The maximum undercooling attains 208 K at molten state, and the dendritic growth velocity is quite sluggish in highly undercooled liquid Co-4.54%Sn alloy because it has a broad solidification range of 375 K (0.21 TL); the maximum value is only 0.95 m/s at the undercooling of 175 K, which then decreases with undercooling. The microstructure refines visibly and the volume fraction of the interdendritic βCo3Sn2 phase obviously decreases with undercooling. The microhardness and electrical resistivity increase with undercooling owing to the enhancement of solute content of the primary αCo phase and refinement of the microstructure where the increased crystal boundary hinders the electronic transmission. Meanwhile, the saturation magnetization also reduces with undercooling due to the crystal particle and boundary increasing significantly, and the dendritic growth velocity and solute content increase in the primary αCo phase under rapid solidification.

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.

Pattern Formation under Deep Supercooling by Classical Density Functional-Based Approach.

Solidification patterns during nonequilibrium crystallization are among the most important microstructures in the natural and technical realms. In this work, we investigate the crystal growth in deeply supercooled liquid using the classical density functional-based approaches. Our result shows that the complex amplitude expanded phase-field crystal (APFC) model containing the vacancy nonequilibrium effects proposed by us could naturally reproduce the growth front nucleation (GFN) and various nonequilibrium patterns, including the faceted growth, spherulite, symmetric and nonsymmetric dendrites among others, at the atom level. Moreover, an extraordinary microscopic columnar-to-equiaxed transition is uncovered, which is found to depend on the seed spacing and distribution. Such a phenomenon could be attributed to the combined effects of the long-wave and short-wave elastic interactions. Particularly, the columnar growth could also be predicted by an APFC model containing inertia effects, but the lattice defect type in the growing crystal is different due to the different types of short-wave interactions. Two stages are identified during the crystal growth under different undercooling, corresponding to diffusion-controlled growth and GFN-dominated growth, respectively. However, compared with the second stage, the first stage becomes too short to be noticed under the high undercooling. The distinct feature of the second stage is the dramatic increments of lattice defects, which explains the amorphous nucleation precursor in the supercooled liquid. The transition time between the two stages at different undercooling is investigated. Crystal growth of BCC structure further confirms our conclusions.

Microstructure Transformation of Undercooled Cu-Ni-Co Alloy and Refinement Mechanism under High Undercooling

Microstructure Transformation of Undercooled Cu-Ni-Co Alloy and Refinement Mechanism under High Undercooling

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.

An Improved Cellular Automata Solidification Model Considering Kinetic Undercooling

A cellular automata (CA) model has been developed for solidification simulation considering the kinetic undercooling at the interface. The state-of-the-art model incorporates a decentered growth algorithm to suppress the grid anisotropy and a generalized height function method to calculate the curvature accurately. To develop a CA model which is independent of the mesh size, a new diffusion term is proposed to handle the diffusion between the interface cells and liquid cells. The developed CA model is employed to simulate the single-dendritic solidification of an Al–3Cu (wt pct) alloy. The simulated tip velocities agree with the prediction of the Kurz–Giovanola–Trivedi (KGT) model. Further studies show that the developed CA model converges to an equilibrium model with increasing kinetic mobility values. Moreover, it is found that the virtual liquid cell assumption which is commonly used in existing CA models may lead to a deviation in the mass balance. The mass balance error has been resolved by redistributing solutes from neighboring liquid cells in each time step. The developed CA model could be potentially used in solidification simulations with a high undercooling, which is common in welding and additive manufacturing.

Crystallographic evidences for twin-assisted eutectic growth in undercooled Ni-18.7 at.%Sn eutectic melts

(1) For the first time, the exact crystallographic orientation relationships between HCP-Ni3Sn (α-Ni) subsets were determined for the undercooled Ni-Sn eutectic alloy. (2) For the first time, the formation mechanism and path of HCP-Ni3Sn twins were revealed to show the roles of twinning and allotropy transformation. (3) Twin-assisted eutectic growth indicates that the single nucleation mode of Herlach, which has been queried in undercooled eutectic melts for several decades, is actually the case for the undercooled eutectic alloy. (4) It was showed that primary and secondary coupled eutectic dendrite growth and un-coupled growth of α-Ni and FCC-Ni3Sn might all be the origins of anomalous eutectics. Rapid solidification in undercooled Ni-18.7 at.%Sn eutectic melts was observed in-situ by a high-speed high-resolution camera and the microstructures were characterized in detail by electron backscattering diffraction. For the first time, the exact crystallographic orientation relations (ORs) between HCP-Ni 3 Sn ( α- Ni) subsets were analyzed. For HCP-Ni 3 Sn, the { 11 2 ¯ 1 } < 1 ¯ 1 ¯ 26 > and/or { 11 2 ¯ 2 } < 11 2 ¯ 3 ¯ > twin ORs (i.e., HCP-Ni 3 Sn twins) hold independently on undercooling, whereas for α- Ni, the { 111 } < 11 2 ¯ > twin OR is the case at low undercooling and would hold initially at intermediate and high undercooling. The roles of twinning and allotropy transformation (i.e., FCC-Ni 3 Sn → HCP-Ni 3 Sn) were integrated to reveal the formation mechanism of HCP-Ni 3 Sn twins, and a reversed OR transition analysis was carried out for representative samples from low to high undercooling. Consequently, novel twin-assisted eutectic growth was found to occur all along. On this basis, we showed that the single nucleation mode of Herlach is followed, and speculated that primary and secondary coupled eutectic dendrite growth and un-coupled growth of α- Ni and FCC-Ni 3 Sn might all be the origins of anomalous eutectics. This work would shed some lights on the long-time controversies about the nucleation mode and the formation mechanism of anomalous eutectics in undercooled eutectic alloys.

Investigation of eco-friendly and economic shape-stabilized composites for building walls and thermal comfort

The objective of this study is to investigate the potential of natural fibers as support materials for PCMs for buildings application using shape-stabilization. The material selection software Ansys Granta has allowed a preliminary selection of four natural fibers (fir fibers, hemp fibers, hemp shives and flax mulch) based on physical, thermal, geographical and economic criteria. The candidates fibers are impregnated with capric acid and lauric acid and their performances are compared, allowing the selection of the fiber with the highest impregnation rate. Hemp shives with maximum impregnation rate of about 50 wt% with LA and a good thermal stability under 150 °C has shown the best performances and has been selected as support material. Afterwards, it has been impregnated with 5 different pure fatty acids and 7 eutectic mixtures in order to identify the best composite. Most of the composites developed have latent heat higher than 50 J g−1, which is very promising for energy storage in buildings application. Taking into account the thermal requirements, which are high latent heat, low undercooling and temperature of fusion ranging from 15 to 45 °C, lauric acid hemp shives composites has been selected as potential materials for buildings application with promising performances (enthalpy of fusion of 79.31 J g−1 at 50 wt% of impregnation rate).

Critical undercooling of growth mode transition in undercooled Al-80%Si-1.0%P alloy

Herein, the critical undercooling of growth mode transitions in highly undercooled Al-80%Si-1.0%P alloys were investigated by high-speed video to observe and record the growth process. Under low undercooling, the dendrites with special edges of primary silicon solidify from the undercooled melt and show lateral growth characteristics. As undercooling increases, the primary silicon grows as blocky and dendritic shapes, indicating mixed growth characteristics. With sufficiently high undercooling, the sample’s surface exhibits the equiaxed primary silicon and shows non-lateral growth characteristics. The experimentally determined critical undercooling for the growth mode transition of primary silicon in undercooled Al-80%Si-1.0%P alloys are 125 and 225 K, and are consistent with the theoretically predicted values, 112 and 208 K, respectively.

Rapid dendritic solidification and structure hardening mechanism of substantially undercooled quaternary nickel alloys

The high undercooling and rapid solidification of liquid quaternary Ni-5%Cu-5%Sn-5%Si and Ni-5%Cu-5%Sn-5%Ge alloys were achieved by glass-fluxing technique to explore their dendritic growth kinetics, microstructural evolution and hardening mechanisms. The dendrite growth velocity of α-Ni phase was found to increase with the rise of bulk undercooling, which could be described by power law functions for both alloys. With the increase of liquid alloy undercooling, the morphology of α-Ni phase transformed from developed dendrites into equiaxed grains. Both the fine grain hardening and solute hardening effects resulted in the elevated Vickers hardness for these two alloys. Nano-sized Ni3Si precipitations were detected within the first alloy, which distributed homogeneously in the α-Ni phase matrix and transformed from faceted to nonfaceted morphology at sufficiently large undercoolings. The precipitate hardening effect dominated the Vickers hardness enhancement for this Si containing alloy. Rapid solidification could modulate the formation of ordered precipitate phase and thus improve the hardening effect by changing dislocation motion conditions.

Microstructure evolution and grain refinement mechanism of rapidly solidified single-phase copper based alloys

• The maximum undercoolings were achieved respectively by using the deep undercooling technology. • The recalescence behavior and microstructure refinement were systematically studied. • The relationship between solute composition and solidification process was established. • High-angle grain boundaries and twin boundaries exist in the microstructure at high undercooling. The Cu65Ni35, Cu60Ni40 and Cu55Ni45 alloys were undercooled by fluxing method, and the rapid solidification structure with different undercoolings were also obtained. At the same time, the interface migration process during rapid solidification was photographed by high-speed photography, and the relationship between the morphological characteristics of solidification front and undercooling was analyzed. The microstructures of the three alloys were observed by metallographic microscope, and the microstructure characteristics and evolution law were systematically studied. It was found that two grain refinement events occurred in the low undercooling range and high undercooling range, respectively. The EBSD test of grain refined microstructures showed that the microstructure in the low undercooling range has a high proportion of low-angle grain boundaries and high strength textures. However, there were a large proportion of high-angle grain boundaries and a high proportion of twin grain boundaries and more randomly oriented grains in the microstructure in the high undercooling range. The TEM test of the Cu55Ni45 alloy with the maximum undercooling of 284 K showed that there were high-density dislocation networks and stacking faults in the grains. Finally, the evolution relationship between microstructure hardness and undercooling was systematically studied. It was found that the microhardness of the three alloys decreased sharply near the critical undercooling. Combined with EBSD, TEM and microhardness analysis, it was confirmed that the grain refinement under low undercooling was caused by dendrite remelting, while the grain refinement under high undercooling was caused by stress-induced recrystallization.

Accurate interatomic potential for the nucleation in liquid Ti-Al binary alloy developed by deep neural network learning method

For the traditional empirical potentials, the accuracy of chemical bonding in a wide temperature range is regrettable, and thereby they are not an appropriate choice to explore the nucleation process of crystals. As a developed superalloy in aerospace, the liquid structure and nucleation mechanism of Ti-Al alloy desire to be investigated to comprehend the solidification process deeply. This work develops an accurate deep neural network (DNN) potential for Ti-Al binary system by employing a machine-learning method. Then, the local structure and the nucleation process of liquid Ti-48 %Al and Ti-52 %Al (atomic percentage) alloys from the normal to the undercooled state are performed by molecular dynamics simulation with the DNN potential. It is found that the formation of the locally ordered structures depends on the Ti-Al atom pair during the solidification. The Honeycutt-Andersen indices of 1431,1541 and 1551 are dominant, and the liquid structure is mainly characterized by perfect and distorted icosahedral clusters. By Voronoi Polyhedron (VP) analysis, Ti-centered VPs favor the formation of body-centered-cubic (BCC) structure, while Al-centered VPs have the main contribution to the icosahedral structure. Besides, liquid Ti-52 %Al alloy prefers to form face-centered-cube (FCC) nuclei than BCC nuclei in a high undercooling, which is closely related to the ability to capture Ti-Al bonds. The favor of the nucleation of the γ phase causes the nuclei with non-equilibrium composition, comprised of Ti-rich BCC and Al-rich FCC coordinated atoms, respectively. Novelty, the FCC nuclei in liquid Ti-Al alloys are independently generated without the precursor of BCC coordinated atoms. High undercooling can transform the nucleation mechanism from two steps to one step.

A comparative study on the microstructure development in Fe50Cu50 alloy prepared using aerodynamic levitation process and W-wire held process

The present lays emphasis on the effect of varying degrees of undercooling on the microstructure development of the Fe50Cu50 alloy system. A sufficiently high undercooling (>100 °C) was achieved using the aerodynamic levitation (ADL) technique whereas a W-wire holding method in the same ADL setups was employed to attain relatively low undercooling (87 °C) during solidification. In addition, solidification experiments were also performed without any undercooling in this alloy system. Thus, the effect of a wide range of undercooling on the phase separation and microstructural development of the Fe-Cu system was systematically investigated in this work. In a substantially undercooled ADL sample, a complete phase-separated micrograph was obtained, but in other situations, a discontinuous network of Cu in interconnected Fe dendritic areas was detected as determined by the scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) of the sample cross-section confirms that the phases present at the end of the solidification process are α-Fe and Cu, regardless of the degree of undercooling. In the instance of the ADL sample, the entrapment of Cu droplets in the inter-connected Fe dendritic area due to a combination of high undercooling and centrifugal force was detected using X-ray tomography, and the orientation distribution of the samples was determined using Electron Backscatter Diffraction (EBSD).

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.

Development, characterization and modification of mannitol-water based nanocomposite phase change materials for cold storage

Under the background of low carbon, phase change energy storage technology has been developed rapidly, which is widely used in solar energy utilization, industrial heat recovery, building temperature regulation, and cold chain logistics. In the fields of cold chain logistics, low-temperature phase change materials(PCMs) below 0 °C have great application space. However, the application of PCM in practice is still limited by high undercooling, low thermal conductivity, low cold storage efficiency and poor stability. In this paper, mannitol aqueous solution, which mannitol can reduce the undercooling degree, an organic-inorganic composite PCM, was used as the main energy storage. After adding MgCl2 as cooling agent, a base solution of salt with phase change temperature of −6 °C and latent heat of 240.1 J/g was prepared. In order to improve the thermal conductivity and cold storage efficiency, nano-copper oxide (Nano CuO) and covalently modified hydroxylated multi-walled carbon nanotubes (MWCNT-OH) were added as thermal conductivity enhancers to prepare nano-composite phase change materials (NCPCMs). In order to obtain good dispersion effect in base solution, sodium dodecyl benzene sulfonate (SDBS), polyacrylamide (PAM) and Guar gum (GG) were used as dispersants. The effects of different dispersants on the dispersion of nanoparticles under phase transition behavior were investigated. The results showed that the anionic dispersant SDBS had poor dispersion effect in salt solution, while the polymer dispersant was better than PAM and GG. The thermal properties and cycle stability of NPCMs were also explored and the result show that the phase change system of mannitol/MgCl2@MWCNT-OH/PAM had the best thermal performance, which has the thermal conductivity of 0.685 W/(m K), increased by 18.16 %, and the cold storage time was reduced by 57.3 %. Without supercooling, the NCPCM has high melting enthalpy, high thermal conductivity and good thermal stability, which has a broad development prospect in the field of cold chain.