Influence of nano-diamond content on the microstructure,mechanical and thermal properties of the ZK60 composites

Hongin M ,Jinhui Wng,? ,Hongyu Wng ,Ningning Dong ,Junwei Zhng ,Peipeng Jin ,Yong Peng

aQinghai Provincial Key Laboratory of New Light Alloys,Qinghai Provincial Engineering Research Center of High Performance Light Metal Alloys and Forming,Qinghai University,Xining 810016,PR China

b Key Laboratory of Magnetism and Magnetic Materials of Ministry of Education,Lanzhou University,Lanzhou Gansu 730000,China

Abstract The effect of nano diamond (ND) content on the microstructure,mechanical properties,and thermal conductivity of ZK60+x (x=0,0.05,0.1,0.15,0.2 wt.%) ND composites were investigated.The microstructures of ND/ZK60 composites were observed,which indicated that the nanoscale MgZn2 and ND particles distributed evenly in the α-Mg matrix.The tensile yield strength (TYS) and compressive yield strength(CYS) of the composites firs increased remarkably and then decreased with further increasing the ND content.Due to the surface area of the matrix-diamond interface increased and the grains sizes of composites decreased with the amount of ND increase,which cause the coefficient of thermal expansion(CTE)of the composites reduced significantly .Meanwhile,the thermal conductivity of the composite material decreases from 129 W·m-1·K-1 to 116 W·m-1·K-1 with the content of ND increasing from 0.05% to 2.0%.The thermal conductivity of the composites increases to the maximum and then decrease with the increase of temperature (in temperature range of 273 -573K).The ZK60+0.05 ND showed superior mechanical and thermal conductivity property,TYS of 343.97MPa,CYS of 341.74MPa,elongation of 15.71%,CTE of 7.3×10-6 K-1,and thermal conductivity of 129 W·m-1·K-1 at room temperature.It is demonstrated that the ND content has an obvious influence on the microstructure,mechanical properties,and thermal conductivity of ND/ZK60 composites.

Keywords: Nano diamond;Microstructure evolution;Cte;Thermal conductivity.

1.Introduction

Magnesium as heat dissipation materials used in industries,such as automation,electronics and aeronautics are attractive candidates because of its excellent properties,including lightweight,good thermal and electrical conductivity.The poor formability and mechanical properties of hexagonal close-packed (HCP) limited its application,and alloying is a common method to enhance the mechanical properties of magnesium,such as Mg-Zn-rare earth element,AM50,and AZ91,however,their thermal conductivities are seriously reduced,which restricts their applications in the engineering fields where dual mechanical and thermal properties are required [1-5].The development of nanostructure-reinforced metal matrix composites has opened new pathways for the design of new materials that exhibit a unique set of properties,such as high strength,low coefficient of thermal expansion(CTE),and high thermal conductivity[6-9].High thermal conductivity ensures uniform temperature distribution which reduces thermally induced stresses and therefore prolongs the service life and working stability of the materials.Therefore,the development of metal composites with both excellent thermal conductivity and mechanical properties is expected.

The ND particles are a promising reinforcement for metal matrix composites due to their high hardness,high thermal conductivity,and low CTE [10-14].Some researchers have studied the thermal conductivity and mechanical property of ND-reinforced metal matrix composites [15-19].For example,Xin et al.[16] found that the Diamond/Al composites with 45nmW coating showed the best thermal and mechanical stability.Xie et al.[17]reported that the maximum thermal conductivity value of Diamond/Cu composites is 853W/mK.Zhu et al.[18] study on surface modification of diamond particles and thermal conductivity properties of their reinforced metal-based (Cu or Mg) composites.Li et al.[19] prepared the ND reinforced ZK60 matrix composites by the powder metallurgy method,the results indicated that ND could efficiently improve the mechanical properties of ZK60 matrix.There are very sparse reports on the thermal properties of nanodiamond-based Mg matrix composites.Theoretically,the ND is expected to have a considerable effect on the microstructure,mechanical and thermal properties of the Mg composites,involving crystal boundaries,defects,voids,dislocation,twin,and atomic diffusion.These factors cause the mechanical and thermal properties response of the matrixdiamond interfaces to be extremely different from both Mg matrix and ND.Therefore,understanding how the ND affects the intrinsic microstructure,mechanical and thermal properties of ND/ZK60 is important in the development of more efficient ND/ZK60 composites.

In this paper,the ZK60+x(x=0,0.05,0.1,0.15,0.2 wt.%)ND composites were prepared by powder metallurgy.The mechanical and thermal conductivity properties of the ND/ZK60 are systematically investigated;the results are correlated with their microstructures.Such an investigation aims to provide an important basis for manufacturing and using of these Mg composites with high thermal conductivity and moderate mechanical as heat dissipation materials.

2.Experimental

The ZK60 powders with average size of 50μm were purchased from Sheng Tong Metal Powder Co.,Ltd.,the ND powders were purchased from XFNANO with the diameter of 5 -10nm.The ZK60+x (x=0,0.05,0.1,0.15,0.2 wt.%)ND composites were prepared by powder metallurgy.The ND particles were dispersed in alcohol solution by ultrasonic for 30min.The ZK60 alloy powders were slowly added into the alcohol solution with mechanical stirring for 20min.Then the mixed solution was dried at room temperature for 24h.In order to finel disperse,the mixed powders were milled by a DYXQM-12L planetary ball mill with the ball-to-powder weight ratio of 20:1,milling speed of 400 r/min,and milling time of 2h,respectively.Next,the mixed powders were compacted into Ф 45×30 mm in a vacuum sintering furnace(473K;150MPa;10-1Pa) for 1h.The compacts were placed on a four-column hydraulic press and heated to 573K for 1.5h.Finally,the samples with diameter of 10mm and lengths of 400mm were prepared by hot extrusion with an extrusion ratio of 13:1 at a speed of 0.1mm/s.

Fig.2.Structural analysis of the ND.(a) Low magnification TEM image.Inset,the SAED pattern of the ND in (a);(b) the HRTEM image of ND from (a).

The phase was investigated by X-ray diffraction (XRD,Bruker XD 8 ADVANCE-A25X) with Cu Kαradiation.The microstructures of the ND and ND/ZK60 composites were observed using electron back scattered diffraction (EBSD)and transmission electron microscopy (TEM,JEM-2010 HR)equipped with Energy Dispersive X-ray (EDX,Oxford Instruments).The tensile samples and compressive samples were tested by a Instron-5982 testing system,the dimensions of samples are show in Fig.1.The samples were processed into a cylindrical rod ofΦ6×25mm,and the CTE was measured by a thermal dilatometer (Netsch,DIL402 PC).The thermal conductivity of the samples with 10mm diameter and 2mm thickness were performed within the temperature range from 298K to 573K by the laser-flas method (Netsch,LFA 457 laser thermal conductivity meter).

3.Results and discussion

3.1.Microstructure and phase

Fig.2a shows a typical TEM image of ND.As can be seen,the ND particles with an average diameter of 8nm,and the corresponding SAED pattern is illustrated in the top right of Fig.2a.These diffraction rings can be well indexed into the(111),(220),and (311) planes (red semicircles) of ND.The lattice constant of ND is 0.206nm by using HRTEM (shown in Fig.2b),which is well consistent with the result of SAED pattern from Fig.2a.

Fig.3.XRD patterns from of ZK60+x (x=0,0.05,0.1,0.15,0.2) ND composites.

The X-ray patterns of the ZK60+x(x=0,0.05,0.1,0.15,0.2 wt.%) ND composites are depicted in Fig.3.The results showed that onlyα-Mg and second phase MgZn2can be detected in those samples,and theα-Mg phase is the main phase.However,no peaks of ND particles were found in ND/ZK60 composites.This phenomenon is ascribed to the low content of ND,and similar result has been reported in the literatures [19-21].

The microstructures of ND/ZK60 composites with different content of ND were obtained by EBSD,as shown in Fig.4.It can be found that the grain sizes of ZK60 alloy and ZK60+0.05ND are basically same with the size of 4.90μm.However,further increasing the ND content,the grain size of the composite decreases.The grain size of ZK60+0.02ND decreases to 1.92μm.The distribution of precipitates MgZn2were observed by TEM (shown in Fig.5).The particles pointed by the arrows are the second phase MgZn2.It was found that the volume fraction of MgZn2increased,which was attributed to the addition of ND can significant effect of refin the grains,the phenomenon was helpful to enhance the mechanical properties of the ND/ZK60 composites.The precipitates of MgZn2have a large distribution in theα-Mg matrix with an average diameter of approximately 150nm.It is consistent with the results of EBSD.

Fig.6 show the elemental mappings of the ZK60+0.05 ND from gray particle marked red dash line in Fig.5b.The maximum intensity of Zn occurs in the gray particle,which confir the distribution of precipitates observed in the XRD pattern and TEM images.Fig.6d shows a representative EDX line scans taken across the single precipitates MgZn2in Fig.6a (marked by a white line),acquired using a 365nm scan line along the MgZn2particle and a 1nm beam spotsize.These results indicate that the gray particles in Fig.5 are the precipitates of MgZn2in theα-Mg matrix,which is consistent with the EDX elemental mappings.It is implied that the nanoscale MgZn2particles distributed homogeneously in theα-Mg matrix,which leads to the relatively serious lattice distortion of theα-Mg matrix around the MgZn2particles.

A typical SAED pattern acquired from the ZK60+0.05ND was illustrated in Fig.7a.These diffraction rings can be well indexed into two sets of lattice planes,including a diamond cubic lattice structure with(111),(220)(red semicircle),and a Mg HCP structure with (0002),(0-111),and (0-110) planes.The MgZn2at the boundary is shown in Fig.7b (white rectangle).The crystal structure of the interfacial transition layers was studied using HRTEM as shown in Fig.7c.The thickness of the interlayer is measured to be approximately 8nm.More HRTEM images reveal that the interlayer indicates an epitaxial relationship.The ND embraced intoα-Mg matrix is shown in Fig.7d.It is implied that the nanoscale MgZn2and ND particles distributed homogeneously in theα-Mg matrix,which have an obvious influence on the mechanical properties and thermal conductivity of ND/ZK60 composites.However,it was difficult for ND particles to disperse evenly intoα-Mg with the increases of ND.The TYS and CYS of composites subsequently decreased.Meanwhile,the grains of composite materials refine which makes the opportunity of electrons being scattered become larger,which strengthens the distortion of theα-Mg matrix around the ND particles.Therefore,the thermal conductivity of ND/ZK60 composites decreases obviously.

Fig.4.Microstructures of ZK60+x ND composites with different ND addition:(a) x=0;(b) x=0.05;(c) x=0.1;(d) x=0.15;(e) x=0.2;(f) grain size statistics.

Fig.5.TEM images of ZK60+xND composites with different ND addition:(a) x=0;(b) x=0.05;(c) x=0.1;(d) x=0.15;(e) x=0.2.

Fig.6.Elemental distribution analysis of a MgZn2:(a) a HAADF-STEM image of MgZn2,where the white line marks the scanning track of the EDX line scan;(b)-(c) EDX elemental mappings of Mg-Kα and Zn-Kα,respectively;(d) EDX line scan spectra of Zn-Kα at 8.63keV,Mg-Kα at 1.25keV.

3.2.Mechanical property of composites

The tensile and compressive stress-strain curves of the asextruded ND/ZK60 composites are shown in Fig.8.The TYS and CYS of ZK60 is 320.43MPa and 314.22MPa,respectively.For the as-extruded composites,TYS and CYS firs increased remarkably and then decreased with increasing the ND content.When the content of ND is 0.1 wt.%,the mechanical properties of ND/ ZK60 composite are the best,the TYS of 348.42MPa,CYS of 353.46MPa,and EL of 15.98%.Meanwhile,the composites show a better tension-compression yield symmetry.

Sanaty et al.[22] proposed a modified model to predict the strength of metal matrix nanocomposites,which shows that Hall-Petch strengthening,Orowan strengthening and CTE mismatch are the three dominant strengthening mechanisms for the nanocomposites.According to the Hall-Petch rule,the composite strength is related to the grain size,and the smaller the grain size,the higher the mechanical properties.Based on the results of Fig.4,the grain size of the composites decreases with increasing the ND content.Meanwhile,theσOrowanis changed by the added ND,which shows the effect of second phases on the strength of composite [19].The difference in CTEs between ND and ZK60 alloy corresponds to the prismatic punching of dislocations at the interface,which leads to the increase in yield strength of the composite.Thus,the enhanced of strength of the as-extruded composites is mainly attributed to Hall-Petch strengthening,Orowan strengthening and CTE mismatch [19,23-26].As aforementioned,the grain size of the as-extruded composites was significantly reduced and the volume fraction of second phase increased and led to an improvement in TYS and CYS.While the ND addition is over 0.1wt.%,the TYS and CYS of composites was decreased.

Fig.7.Structural analysis of ZK60+0.05 ND composite.(a) Low magnification bright-field TEM image.Inset,the SAED pattern of the area in (a);(b)Enlarged bright field TEM image of MgZn2;(c) HRTEM image of the area marked by a white square in (b);(d) HRTEM image of ND.

Fig.8.Tensile and compressive stress-strain curves of the ZK60+x (x=0,0.05,0.1,0.15,0.2) ND composites along the as-extruded direction (a) Tension;(b) Compression.

3.3.CTE and thermal conductivity of composites

It can be seen from Fig 9a and 9b that the strains of ZK60 and composites show a cyclical change with a periodic change temperature of 300 -573K.The maximum strain of ZK60 is 8.4×10-3,while the maximum strains of composites are less than 2.6×10-3.The average CTE of ZK60 and composites can be calculated during each heating process (as shown in Fig 9c-d).The average CTE of ZK60 is 3.08×10-5K-1,the average CTE of ZK60+0.05ND composites is 8.62×10-6K-1,which reduced by 72%.As the number of thermal cycles increases,the CTE of ZK60 decreases by 6.5%,and the CTE of ZK60+0.2ND composite decreases by 18.1%.The CTE of the magnesium alloy is basically stable after the third thermal cycling,and that of the composite material tends to be stable after the second thermal cycling.The addition of ND can significantly reduce the CTE of the ZK60 matrix.That is because the ND has a low coefficient of thermal expansion.The addition of the ND results in the formation of a matrix-nano-diamond interface in the composite.The CTE of the composite depends on the degree of interface constraints on the matrix alloy [27-29].With the amount of ND increased,the matrix-nano-diamond interfaces of composites are increased,resulting in the CTE of the composites reduced.

Fig.10 shows the thermal cycle curves of ND/ZK60 composites during the temperature range of 300K-573K,the thermal strain hysteresis of materials is presented.Residual strain occurs after the firs cycle,the residual strain tends to be stable with the increase of the number of thermal cycle,and the shape of the curve remains basically unchanged.The thermal cyclic residual strain of the composites was obviously lower than that of the ZK60 alloy,which indicates that the composites have a higher dimensional stability than the matrix alloy.The dimensional stability of the material is enhanced with the increase of ND content.

Fig.9.5 thermal cycling process of ZK60 composites with temperature range between 300K-575K.(a) The relationship between the time and the thermal strain of ZK60 in thermal cycles;(b) the thermal cycles of ZK60+x (x=0.05,0.1,0.15,0.2) ND composites;(c) the CTE of ZK60;(d) the CTE of ZK60+x(x=0.05,0.1,0.15,0.2) ND composites.

Table 1The densities,specific heat capacity and thermal diffusivity of ND/ZK60 composites.

The thermal conductivity measurements were performed at room temperature as well as temperature dependent up to 573K.Through-plane measurements were performed directly[30,31].Thermal diffusivityαcan be calculated by the following equation [32]:

wherelis the thickness of the sample,and t1/2is the halfrise time,which is define as the interval required for the rear surface temperature to reach one-half of the maximum temperature increase.The density at room temperature was determined by the Archimedes method and the density at elevated temperature is calculated using the Eq.(2) [33].

whereρ0is the density at room temperature,Tis the absolute temperature.The specific heat capacities Cpof the composites in this work can be measured by the LFA 457 laser thermal conductivity meter.

The thermal conductivityκis calculated from the specific heat capacity,the thermal diffusivity (α) and the density (ρ)via the following equation [31]:

Fig.10.Relationship between strain and temperature in thermal cycling:(a).ZK60;(b) ZK60+0.05ND;(c) ZK60+0.1ND;(d) ZK60+0.15 ND;(e)ZK60+0.2ND.

The densities,specific heat capacity and thermal diffusivity of ZK60+x(x=0,0.05,0.1,0.15,0.2 wt.%) ND composites are shown in table 1.Phonons and electrons are the dominant heat transporter in alloys.Impurities and lattice phonons are obstacles to the flow of electron.When the impurity scattering is strong enough to take the dominating place,the thermal conductivity increases with the increase of temperature.It can be found that the ZK60+0.05ND has the highest Cpat room temperature.Fig.11 presents the thermal conductivity of the ZK60+x(x=0,0.05,0.1,0.15,0.2) ND composites.The thermal conductivity of ZK60 increases with increasing temperature.However,the thermal conductivity of the composites increases to the maximum and then decreases with the increase of temperature (temperature range from 273K to 573K).The thermal conductivity is proportional to the heat capacity and the average free path of the phonons,since the phonon mean free path is basically unchanged at low temperatures,the variation of thermal conductivity and heat capacity with temperature is similar;the heat capacity is basically constant at high temperature,and the umklapp scattering process is aggravated with temperature increased,which makes the phonons mean free path decrease,the thermal conductivity will decrease as the temperature increases [33-36].On the other hand,the highest thermal conductivity of carbon materials appears at about 450K [37],which maybe lead to the maximum thermal conductivities of the composites obtained at 473K.The thermal capacity of ND and ZK60 magnesium alloy increases with increasing temperature,and the average free path of phonons decreases with increasing temperature.They compete with each other and affect the change of thermal conductivity and the position of the highest point.It is believed that the ND and precipitates in the ZK60 alloys are both scattering the electrons and phonons.Comparing the thermal conductivity of composites,it can be found that the thermal conductivity of ZK60+0.05 ND is the largest,the higher thermal conductivity of ZK60+0.05ND mainly ascribe to the extraordinary highest specific heat capacity.Maybe due to the grains of composites refine and the proportion of small crystal grains increased with the increase of ND content,the Cpof composites decreased.The addition of the ND impeded the movement of electrons in the crystal lattice to reduce the mean free path of electrons,thereby reducing the thermal conductivity of the alloys.Therefore,the thermal conductivity of ND/ZK60 composites decreased with the increase of the ND content.Meanwhile,phonons vibration enhances and impedes the movement of both electrons and phonons with the increasing temperature,which can lower the thermal conductivity at elevated temperature.Comprehensive consideration of mechanical properties and thermal conductivity of composite materials,the ZK60+0.05ND showed superior mechanical and thermal conductivity property,TYS of 343.97MPa,CYS of 341.74MPa,and EL of 15.71%,CTE of 7.3×10-6K-1,thermal conductivity of 129 W·m-1·K-1at room temperature.

Fig.11.Comparison of thermal conductivity of ZK60+x (x=0,0.05,0.1,0.15,0.2) ND composites.

4.Conclusion

In this work,the effect of the ND addition on microstructural,mechanical properties,and thermal conductivities of ND/ZK60 composites were investigated.Following conclusions can be drawn:

1.The microstructure evolution indicated that the addition of ND has a significant effect of refining the grains.TEM images reveal that the ND and MgZn2particles distributed homogeneously in theα-Mg matrix,which has an important effect on the mechanical properties and thermal conductivity of ND/ZK60 composites.

2.The TYS and CYS of the as-extruded composites firs increased remarkably and then decreased with further ND addition.The strength enhancement mechanisms of the composites are mainly attributed to Hall-Petch strengthening,Orowan strengthening and CTE mismatch.

3.The addition of ND can significantly reduce the CTE of the ZK60 matrix.The CTE of ZK60+0.05ND is the largest due to its extraordinary Cp.The grains of composite materials refine and matrix-diamond interfaces increased with the increase content of ND,which makes the opportunity of electrons and phonons being scattered become larger.Thus,the thermal conductivity of ND/ZK60 composites decreased with the increase wt.% of the ND.

Conflict of Interest

The authors declared that they have no conflict of interest to this work.

Acknowledgement

Authors acknowledge the financial support of Qing Hai Provincial Natural Science Foundation (Grant No.2018-ZJ-949Q) to carry out this research work.