1.Introduction

Martensitic transformation and the microstructure of martensite in steels have been persistent issues owing to their technical importance in tailoring the microstructure and mechanical properties of steels.As is well known,carbon in steels is a crucially important alloying element which not only influences the morphology[1–6]of martensite but also exerts drastic impact on the hardness and strength of steels[7,8].It has been well recognized that with increasing carbon content the martensite morphologies evolve from low carbon lath martensite to high carbon lenticular martensite.Additionally,numerous experimental investigations have indicated that the carbon content plays a key role in the formation of the substructures of carbon martensites.For example,low carbon martensites tend to have dislocations[9,10]as their substructure and the martensites with medium and high carbon content always possess twined substructures[4,11,12].

It has been conventionally believed that the outstanding harness of carbon martensite arises from the supersaturation of carbon atoms in the interstitial sites of α-Fe lattice[13–15].However,very recent experimental studies by transmitting electron microscopy suggested that the quenched martensites in mild(0.2 mass%C)and medium(0.6 mass%C)are composed of twins and nanoscale ω particles in twin boundaries,instead of uniform solid solutions[16–19].In addition,similar results were achieved in high carbon(0.98 mass%C and 1.4 mass%C)steels[20,21],although detailed characterization of twins and ω particles was not presented.Referring to these experimental studies and the early results on the morphologies of martensite in carbon steels,the microstructures such as twins and the volume fraction of ω particles are closely associated with the carbon content of the steels.Particularly for the steels with low carbon content,the dislocations,instead of twins,are the substructure of the lath martensites[3].In another word,carbon content in steels play key role in the formation of substructures during the martensitic transformations.

In spite of abundant experimental studies on the microstructures morphologies of the martensite in high carbon steels(>0.6 mass%C),the reports on the microstructures,especially the twins and ω phase of the martensite in high carbon steels have been lacking.Investigating the substructural details of high carbon martensite,such as twins and ω phase,is helpful to fully understand the martensitic transformations and mechanical properties of carbon steels.In the present study,a novel strategy was employed to obtain the martensite containing maximum carbon content(approximate 2.1 mass%C)by quenching a ductile cast iron and characterize the substructure of the ultrahigh carbon martensite.In fact,the microstructure of the martensite in ductile cast iron has received few attention,because the ductile irons developed in industry are often subjected to normalizing treatment and their regularstructures are characterized by pearlite,ferrite and spheroidal(or granular)graphite.Clarifying the microstructure of the martensite in ductile cast iron not only benefit understanding the substructures of ultrahigh carbon martensite,but also provide new insight into the properties of high carbon steels and ductile cast irons.

Table 1 The chemical compositions of the ductile cast iron(mass%).

C Si Mn Mo Cr Fe 3.44 2.1 0.66 0.41 1.52 Balance

2.Materials and methods

In the present study,a ductile cast iron was employed to investigate the microstructures of the martensite with ultrahigh carbon content.Table 1 lists the detailed chemical compositions of the ductile cast iron.

按如图2(a)所示的从中心到边缘1-9的位置，采用棱镜耦合仪Metricon Prism Coupler 2010进行逐点测试，可直接测量薄膜在1 539 nm波长下的折射率与厚度；采用应力测试仪Dektak 150测定晶圆在镀膜前后的形变量，图2(b)和(c)为晶圆弯曲半径测量和晶圆整体应力分布图.

The plate specimens of 0.5 mm and 2 mm in thickness were sealed into quartz tube under Ar atmosphere and austenized at 1130°C for 3 h,followed by quenching into liquid nitrogen(-196°C).Different scanning calorimetry(DSC)was applied to measure the phase transformation temperatures of ductile iron specimen by using synchronized thermal analyzer(449F3)with heating/cooling rate of 10°C/min.The phase constitution of the specimens was determined by a Regaku D/MAX 2500×-Ray diffractometer(XRD)at voltage of 40 kV and current of 200 mA.The morphologies of the quenched ductile iron were observed on a LEICA DM4000 optical microscope and a JSM 6010 scanning electron microscope(SEM).The fine microstructure of the quenched specimen was observed on a JEM-2011F transmission electron microscope(TEM).These specimens for TEM characterization were ground to approximately 100µm and subsequently thinned by ion milling technique at-30°C.The simulation of the electron diffraction patterns was carried out with the software of CaRIne Crystallography 3.1.The tensile of dog-bone specimens of 2 mm in thickness was performed at a strain rate of 1×10-3 s-1 on a Sans 5504 mechanical tester.

3.Results and discussion

In order to produce the martensite with maximum carbon content by quenching the ductile cast iron,DSC measurement was first carried out to determine the temperature at which the austenite possess maximum solubility of carbon,i.e.,approximate 2.1 mass%C.Fig.1(a)shows the DSC curves of ductile cast iron specimen and binary Fe-C phase diagram[22].The endothermic and exothermic peaks of the specimen indicate that the temperature at which the austenite possesses maximum solubility of carbon is located between 1120°C and 1137°C.As such,an austenization at 1130°C was employed to achieve the austenite with the maximum carbon content,approximate 2.1 mass%C.According to the Fe-C phase diagram and the DSC results,ultrahigh carbon martensite could be produced by quenching the specimen after full austenization at 1130°C.

通过深化设计+施工方案的策划应用，最大程度的降低了现场施工的返工率以及重复率，保障了施工的工期和规范性。

One can see from Fig.4(a)that the width of twin boundaries where ω particles exist ranges from 2 nm to 4 nm.This means that the width of the twin boundaries in the ultrahigh carbon martensite is compatible with the width of the nanotwins.As is well known,the martensites in high carbon steels perform outstanding hardness and poor ductility,although the martensites have twin structures which are generally regarded as an important configuration for plastic deformation in metals and alloys.In spite of persistent attention on the microstructure and mechanical behavior of the martensites in steels,the mechanism for the brittleness of twinned martensite is not clear up to date.According to the TEM results of the ultrahigh carbon martensite in ductile iron,it seems that the twins in the martensite of carbon steels are different from the twins in other metals or alloys in which the twins have sharp boundaries. Considering the specific configuration of high carbon martensite, i.e. large amount ω nanoparticles embedded in twin boundaries and cohere with the twins,it is reasonable to conclude that the deformation of the ultra-fine twins could be hindered remarkably by the dense ω particles.This might be the origin responsible for the immobility of ultra-fine twins and the poor ductility of martensites in high carbon steels and in ductile iron. In this context, we could understand the reason why a normalizing or annealing treatment must be applied in the production of ductile cast iron.During the normalizing or annealing treatment,a tempering occurs in the ultrahigh carbon martensites,leading to good ductility and toughness of the ductile cast iron.

To probe the microstructure of the ultrahigh carbon martensite in the quenched ductile iron,intensive TEM observations were carried out and the results is shown in Fig.3.In the bright-field micrograph shown in Fig.3(a),the martensite contains numerous ultrafine twins.The corresponding selected area electron diffraction pattern by[1¯13]α zone axis demonstrates that these transformation twins in ultrahigh carbon martensite are of{112}<111>type,as shown in Fig.3(b).The darkfield micrograph imaged using(110)α,t twin diffraction spot outlined by white doted circle in Fig.3(b)reveals clearly the substructure of the martensite,the ultrafine{112}<111>type twins with twin width of 3–9 nm,as shown in Fig.3(c).Note that the width of the twins in the ultrahigh carbon martensite is much thinner than that in the martensite of low or medium carbon steels(tens nanometers).

In the selected area electron diffraction pattern shown in Fig.3(b),in addition to the main diffraction spots ascribed to body-centered cubic(bcc)matrix and the nanotwins,extra diffraction spots at 1/3(12¯1)α and 2/3(12¯1)α are observable.Fig.3(d)presents a dark-field micrograph imaged using the extra spot at 1/3(12¯1)α outlined by yellow dashed circle in Fig.3(b),showing a large number of nanometer scale particles which are parallel to nanotwins.By comparing the configurations of the nanotwins in Fig. 3(c)and the nanoparticle distributions in Fig.3(d),it is easily to find that these nanoparticles imbed themselves coincidently in the boundaries of the nanotwins.Referring to recent TEM investigations on the martensite in carbon steels[16,17,19–21],these extra diffraction spots and the nanometersized particles can be indexed as ω phase with primitive hexagonal structure,a metastable phase frequently appeared in bcc structured metals and alloys.Compared with the martensites in mild and medium carbon steels[16,19],the martensite in the ductile cast iron of the present study contains much more ω nanoparticles in twin boundaries.Ping et al.proposed that carbon atoms can facilitate the transformation from bcc matrix to ω phase[16].This measure that higher carbon content in martensite could be beneficial to forming large amount of ω phase during martensitic transformation.Obviously,ultrahigh carbon martensite in ductile iron of the present study contains large amount of ω nanoparticles embedded in twin boundaries,lending support to Ping's proposition.

Fig.2(a,b)and Fig.2(c)show,respectively,the optical and SEM micrographs of the quenched ductile iron specimen,in which graphite nodules,lenticular martensites and retained austenite are clearly observable.Referring to the micrographs with higher magnification,as shown in Fig.2(b)and(c),one can see a large amount of retained austenite around zigzag arrays of martensite plates in which midribs are visible.These features are consistent with the typical morphology of high carbon martensite in carbon steels[24].Fig.2(d)shows the strainstress curve of quenched ductile cast iron.By contrast with the industrial ductile cast iron with good strength and considerable ductility,the ductile iron undergoing austenization and quenching treatments performs very poor ductility and strength.

Fig.4 shows the high-resolution TEM results of the ultrahigh carbon martensite in quenched ductile cast iron.The TEM micrograph in Fig.4(a)and corresponding inverse fast Fourier transformed image in Fig.4(b)clearly demonstrate that the nanoscale ω particles in twin boundaries are coherent with ferrite matrix and twins.The fast Fourier transformed diffraction pattern from ω phase zone(the square outlined by yellow dashed line)in Fig.4(b)is in good agreement with the simulated electron diffraction pattern of primitive hexagonal lattice,as shown in Fig.4(c)and(d).In addition,the interplanar spacings of and planes derived by simulation is consistent with the values by TEM results of Fig.4(b).For the ultrahigh carbon martensite in ductile cast iron of the present study,the lattice parameters of ω phase are in accordance with the relationship of°and

Fig.1.DSC curves(a)and the XRD patterns(b)of ductile cast iron undergoing austenization at 1130°C and quenching into liquid nitrogen.

Fig.2.Optical(a,b)and SEM(c)micrographs and stress-strain curves(d)of quenched ductile cast iron.

Fig.3.TEM results of the ultrahigh carbon martensite in quenched ductile cast iron.(a)Bright-field micrograph of martensite plate.(b)The selected area electron diffraction pattern of the martensite in(a)with[1¯13]α zone axis.(c)The dark field micrograph imaged using(110)α,t twin diffraction spot outlined by white dotted circle in(b).(d)The dark field micrograph by the extra diffraction spot at 1/3(12¯1)α outlined by yellow dashed circle in(b).

Fig.4.High resolution TEM results of the ultrahigh carbon martensite in quenched ductile cast iron.(a)High resolution TEM micrograph of[1¯13]α zone axis.(b)The inverse Fourier transformed image from the yellow square zone in(a),showing twins and ω phase in twin boundary.(c)The fast Fourier transform(FFT)image of yellow square zone in(b).(d)The simulated electron diffraction pattern of[21¯1¯3¯]ω zone axis from primitive hexagonal lattice with parameters of aω=4.046 Å and cω=2.49 Å,where aω is the parameter for x-axis,and cω the parameter for z-axis.

丁达仍旧趴在地上一动不动。在他体内，壶天晓和镜心羽衣瘫软在向导室的座椅上，一边设法恢复精力，一边等待同伴创造奇迹。他们搭建的感应网络依然十分稳定，因此，同伴的行踪他俩都一清二楚，这无疑是个好势头。壶天晓已把自己最新的经验库通过感应网络分享给了蓝蓝，他相信这个已有他大部分经验库的机器人有能力在地面上保护幽之谷的居民，并狙击飞鼠小分队。

独活种子在采收时，应把植株分成上部、中部、底部3个部位分别采收，优先使用上部和中部的种子，弃用底部种子。在独活留种田管理过程中，应在独活末花期摘除植株底部分枝上的花序，使植株全部营养集中供给上部和中部种子，有助于提高独活种子质量。

Fig.1(b)shows the XRD pattern of the specimen undergoing austenization at 1130°C for 3 h and then quenching into liquid nitrogen(-196°C),suggesting that the quenched ductile iron specimen consists of martensite,austenite and graphite.Evidently,the split diffraction peaks of(002)α/(200)α doublet and(112)α/(211)α doublet ascribed to martensite phase exhibit typical characteristics of body centered tetragonal (bct) structure. The interplanar spacing parameters of the martensite can be determined by the XRD pattern with reference to Bragg equation[23],λ=2d·sinθ,where λ is wavelength of diffracted X rays,θ is the incident angle of the X rays and d is the spacing of lattice planes.As such,the lattice parameters of the tetragonal martensite a and c were evaluated to be 2.844 Å and 3.031 Å,respectively,corresponding to a significant tetragonality(ratio of c to a)of 1.066.

3、T3时刻下，最大沉降量增大至1.90m，可见进行堆载后，土体含水进一步排出，排水板下方的淤泥层也在堆载作用下得到加固。

4.Conclusions

Ultrahigh carbon martensite of approximate 2.1 mass%C was obtained by quenching ductile cast iron,and the microstructure of the martensite was intensively characterized by high resolution transmitting electron microscopy.The microstructure of the ultrahigh carbon martensites is composed of nanoscale{112}<111>-type twins and large number of ω nanoparticles embedded in twin boundaries and coherent with the twins.The dense ω nanoparticles in twin boundaries could remarkably hinder the deformation of the nanotwins,leading to poor ductility and strength of quenched high carbon steels and ductile cast iron.The results achieved in the study not only elucidate the substructures of ultrahigh carbon martensite,but also provide new insight into the mechanical behavior of high carbon steels and ductile cast irons.

Acknowledgments

This work is supported by the National Natural Science Foundation of China(Grant nos.51771012,51431007 and 51671043).

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