Structure and Magnetic Properties of Fe_(76.5)Si_(13.5)B_9Cu_1 Alloy with Nanoscale Grain Size

The structure and magnetic properties of Fe76.5Si13.5B9Cu1 alloys with a nanocrystalline (NC) bcc Fe(Si) phase trom about 23 to 46 nm in diameter, which were first formed into amorphous ribbons and then annealed at various temperatures between 703 and 773 K, have been investigated. At annealing temperatures from 703 to 748 K, the single NC bcc(Si) phase is obtained in the crystallized alloys. The grain size and the Si-content in the NC bcc Fe(Si) phase for the alloys annealed at different temperatures are presented. The soft magnetic properties and the saturation magnetostriction for the alloys with the NC bcc Fe(Si) phase are also measured. The results show that, the saturation magnetizotion and the permeability are improved for the alloys with only the NC bcc Fe(Si) phase and become better with decreasing of the NC bcc phase size, and the saturation magnetostriction declines for the alloys with increasing Si-content in the NC bcc Fe(Si) phase.

Grain size and structure distortion characterization of α-MgAgSb thermoelectric material by powder diffraction

Nanostructuring, structure distortion, and/or disorder are the main manipulation techniques to reduce the lattice thermal conductivity and improve the figure of merit of thermoelectric materials. A single-phase α-MgAgSb sample, MgAg0.97Sb0.99, with high thermoelectric performance in near room temperature region was synthesized through a high-energy ball milling with a hot-pressing method. Here, we report the average grain size of 24–28 nm and the accurate structure distortion, which are characterized by high-resolution neutron diffraction and synchrotron x-ray diffraction with Rietveld refinement data analysis. Both the small grain size and the structure distortion have a contribution to the low lattice thermal conductivity in MgAg0.97Sb0.99.

Microstructure evolution and mechanical properties of brazing joint for ultra-thin-walled Inconel 718 considering grain size effect and brazing temperature

The systematic investigation of the mechanical properties and microstructure evolution process of ultra-thin-walled Inconel 718 capillary brazing joints is of great significance because of the exceptionally high demands on its application.To achieve this objective,this study investigates the impact of three distinct brazing temperatures and five typical grain sizes on the brazed joints’mechanical properties and microstructure evolution process.Microstructural evolution analysis was conducted based on Electron Back Scatter Diffraction(EBSD),Scanning Electron Microscopy(SEM),X-Ray Diffraction(XRD),High-Resolution Transmission Electron Microscopy(HRTEM),and Focused Ion Beam(FIB).Besides,the mechanical properties and fracture behavior were studied based on the uniaxial tension tests and in-situ tension tests.The findings reveal that the brazing joint’s strength is higher for the fine-grain capillary than the coarse-grain one,primarily due to the formation of a dense branch structure composed of G-phase in the brazing seam.The effects of grain size,such as pinning and splitting,are amplified at higher brazing temperatures.Additionally,micro-cracks initiate around brittle intermetallic compounds and propagate through the eutectic zone,leading to a cleavage fracture mode.The fracture stress of fine-grain specimens is higher than that of coarse-grain due to the complex micro-crack path.Therefore,this study contributes significantly to the literature by highlighting the crucial impact of grain size on the brazing properties of ultra-thin-walled Inconel 718 structures.

Influence of initial microstructure and grain size on transformation of bainite to austenite in large size forgings

The kinetics of austenite formation in the surface and center regions of a 40 t forged ingot of a high-strength medium- carbon low-alloy steel was studied using high-resolution dilatometry. The starting microstructures from the surface or center regions had different proportions of bainite and residual austenite as well as different prior austenite grain sizes. Two heating rates representing the actual heating rates in the surface （5℃ s -1） and center regions （0.5℃ s -1） of large size forged blocks were utilized. Dilatometric curves revealed only one transformation step of austenite formation at both heating rates independent of grain size or proportion of phases. Optical microscopy, field emission gun scanning electron microscopy and X-ray diffraction were used to study microstructure evolution and confirm the results obtained by dilatometry. The kinetic parameters for austenite formation were determined from the dilatometry data by Johnson-Mehl- Avrami-Kolmogorov （JMAK） equation. The JMAK coefficients were determined for each condition of the investigated steels. The calculations indicated that the nucleation and growth of austenite in the surface region were accelerated more than 10,000 times due to a significantly smaller average prior austenite grain size, stability of initial retained austenite, and accumulation of coarse carbides at the surface. The results were discussed in the framework of classical nucleation and growth theories using the kinetic parameters for austenite formation.

Designing hetero-structured ultra-strong and ductile Zr-2.5Nb alloys:Utilizing the grain size-dependent martensite transformation during quenching

To further improve the service performance of Zr-2.5Nb alloy worked as pressure tubes in pressurized heavy water reactors,more investigation about the microstructure and thermomechanical processing route of Zr-2.5Nb alloy need to be conducted.In this work,a hetero-structured Zr-2.5Nb alloy was prepared by applying a novel technique.Microstructure analysis reveals that the alloy exhibits a grain sizedependent martensite substructure transition during post-rolling quenching.The hetero-structure consists of equiaxed primaryαgrains and the lamellae groups containing both parallelα’dislocation martensite andα’twin martensite.Compared with the previously reported Zr-Nb alloys,the present Zr-2.5Nb alloys manifest the highest yield strength(∼710 MPa),together with a high ultimate tensile strength(∼844 MPa)and good ductility(∼17.1%).The enhanced mechanical properties are found to arise from the properly controlled fraction/size of the two types of martensite,which not only significantly strengthens the alloy but also contributes to a stronger strain hardening.A model based on the grain-size-dependent critical resolved shear stress for dislocation slip and twinning has been proposed to explain theα’martensite substructures transition at a critical grain size dc=3.3μm.Below this size,the critical resolved shear stress(CRSS)for twinning is higher than that for the<c+a>slip.Thus,theα’dislocation martensite is more favorable to form.Otherwise,theα’twin martensite would exhibit a high activity.The present work indicates that making use of the grain size-dependent martensite transformation to tailor the heterostructure in Zr alloys is an effective strategy to overcome the strength–ductility trade-off in the material.

Microscopic origin and relevant grain size effect of discontinuous grain growth in BaTiO_(3)-based ferroelectric ceramics

Barium titanate[BaTiO_(3)(BT)]-based ceramics are typical ferroelectric materials.Here,the discontinuous grain growth(DGG)and relevant grain size effect are deeply studied.An obvious DGG phenomenon is observed in a paradigmatic Zr^(4+)-doped BT-based ceramic,with grains growing from∼2.2–6.6 to∼121.8–198.4μm discontinuously near 1320℃.It is found that fine grains can get together and grow into large ones with liquid phase surrounding them above eutectic temperature.Then the grain boundary density(D g)is quantitatively studied and shows a first-order reciprocal relationship with grain size,and the grain size effect is dependent on D g.Fine grains lead to high D g,and then cause fine domains and pseudocubic-like phase structure because of the interrupted long-range ferroelectric orders by grain boundary.High D g also causes the diffusion phase transition and low Curie dielectric peak due to the distribution of phase transition temperature induced by internal stress.Local domain switching experiments reveal that the polarization orientation is more difficult near the grain boundary,implying that the grain boundary inhibition dominates the process of polarization orientation in fine-grain ceramics,which leads to low polarization but a high coercive field.However,large-grain ceramics exhibit easy domain switching and high&similar ferroelectricity.This work reveals that the grain boundary effect dominates the grain size effect in fine-grain ceramics,and expands current knowledge on DGG and grain size effect in polycrystalline materials.

Grain size effect on piezoelectric properties of Sr_(2)Nb_(2)O_(7)ceramics

Grain size effect on piezoelectric properties and thermal stability of perovskite layer structured(PLS)Sr_(2)Nb_(2)O_(7) ceramics are investigated.The Sr2Nb2O7 ceramics with different average grain sizes from 1.2μm to 3.6μm were prepared in different sintering temperatures by solid state reaction method.The average grain size increases,accompanied by a higher relative density of up to 96%.Pure Sr_(2)Nb_(2)O_(7) ceramics with larger grain size show a remarkable d33 of(1:7
0:1)pC/N while still with a very high T_(c) of(1340±4)℃.The thermal depolarization temperature of samples with large grain sizes reach over 1200℃ and the thermal stability increased with increasing of grain size.The ferroelectric domains structure was observed by PFM and larger grain is easy to form ferroelectric domain then enhance piezoelectric properties.This study demonstrates enhanced piezoelectric properties can be achieved in pure Sr_(2)Nb_(2)O_(7) by solid state reaction method and bring great revitalization to the Sr_(2)Nb_(2)O_(7)-based ceramics as a promising high-temperature piezoelectric material.

Solidified structure of thin-walled titanium parts by vertical centrifugal casting

The solidified structure of the thin-walled and complicated Ti-6AI-4V castings produced by the vertical centrifugal casting process was studied in the present work. The results show that the wall thickness of the section is featured with homogeneously distributed fine equiaxial grains, compared with the microstructure of the thick-walled section. The grain size of the castings has a tendency to decrease gradually with the increasing of the centrifugal radius. The inter-lamellar space in thick-walled casting parts is bigger than that of the thin-walled parts, and the profile of inter-lamellar space is not susceptible to the centrifugal radius.

Surface grain refinement mechanism of SMA490BW steel cross joints by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a postweld technique for improving the fatigue strength of welded joints. This technique makes use of ultrasonic vibration to impact and plastically deform a weld toe and can achieve surface grain refinement of the weld toe, which is considered as the main reason for the improvement of fatigue strength. In this paper, the microstructure of the surface of a treated weld toe was observed by metallographic microscopy and transmission electron microscopy (TEM). The results show that UIT could produce severe plastic deformation on the surface layer of the weld toe and the maximum depth of plastic deformation extended to approximately 260 μm beneath the treated surface. Repeated processing could exacerbate the plastic deformation on the surface layer, resulting in finer grains. We can conclude that the surface grain refinement mechanism of SMA490BW welded joints is related to the high density of dislocation tangles and dislocation walls. © 2017, University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg.

Dependence of Structure and Magnetic Properties on Annealing Temperature in Fe(72.5)Cu1Nb2V2Si(13.5)B9 Alloy

The magnetic properties of Fe_(72.5)Cu_1Nb_2V_2Si_(13.5)B_9 alloy are investigated from an amorphous to a nanocrystalline and complete crystalline state. The sample annealed at 550℃ for 0.5 h shows a homogeneous nanocrystalline structure and presents excellent soft magnetic properties. When the specimens were annealed at a temperature above 600℃, the magnetic properties are obviously deteriorated because the grain size grows up, exceeding the exchange length.

Numerical simulation and experimental validation on fabrication of nickel-based superalloy Kagome lattice sandwich structures

Nickel-based superalloy lattice sandwich structures present higher stiffness,higher strength and higher temperature resistance in comparison with other metals.In this study,the Kagome unit was adopted to design the lattice sandwich structure and ProCAST software was used to simulate the filling and solidification processes of the nickel-based superalloy.Grain morphology and sizes of the nickel-based superalloy lattice sandwich structures were simulated by using of cellular automaton coupled with finite element model(CAFE),and indirect additive manufacture combining with investment casting were carried out to fabricate the nickel-based superalloy lattice sandwich structures.The calculated grain morphology and sizes are in good agreement with the experimental results.The grains are mainly equiaxed with an average size of about 500µm.The simulated results also show that the superheat of melting and the mold preheated temperature have significant influence on the grain size of the Kagome lattice sandwich structures,lower superheat of melting and mold preheated temperatures are encouraged to obtain the fine grains while assuring the integrity of the Kagome lattice sandwich structures for industrial application.

Ni_(38)Co_(25)Cr_(15)Fe_(10)Al_(10)Ti_(1)Nb_(1)非等原子比高熵合金的拉伸性能

为了进一步提高FCC高熵合金的强度,利用真空电弧熔炼法制备了Ni_(38)Co_(25)Cr_(15)Fe_(10)Al_(10)Ti_(1)Nb_(1)非等原子比高熵合金。结果表明:高熵合金的凝固组织由单相FCC结构组成,铸带具有典型枝晶结构且存在元素偏析。高熵合金具有较高的抗再结晶性能,退火温度为1000℃时再结晶分数仅约为45.1%,退火温度为1050℃时发生完全再结晶,此时晶粒尺寸细小均匀,平均晶粒尺寸约为4.25μm。退火Ni_(38)Co_(25)Cr_(15)Fe_(10)Al_(10)Ti_(1)Nb_(1)高熵合金具有优异的拉伸性能。经过1000℃退火后合金抗拉强度可达1640 MPa,伸长率约为20%,经过1050℃退火后合金抗拉强度和伸长率分别约为1580 MPa和28%。

基于仿生蛾眼结构制备减反射微晶玻璃

目的采用湿法刻蚀制备减反射微晶玻璃,分析晶化时间对透过率、反射率和形貌的影响。方法通过熔融法制备得到基础玻璃,调整晶化时间制备得到一系列不同晶粒尺寸(12.2~27.4nm)的微晶玻璃,将制备得到的基础玻璃及不同晶粒尺寸的微晶玻璃放置在配制好的5%(质量分数)HF、2%(质量分数)SiO_(2)、1.5%(质量分数)BaSO_(4)、2%(质量分数)羧甲基纤维素钠、89.5%(质量分数)H_(2)O的刻蚀液中,在25℃条件下,以40 kHz的超声频率双面蚀刻60 min,通过分光光度计测试刻蚀前后样品的透过率和反射率,利用扫描电子显微镜对刻蚀前后样品的表面和断面形貌进行表征。结果基础玻璃在500℃核化230 min,665℃晶化135 min后,制备得到微晶玻璃的晶粒尺寸约为25.1 nm,进一步经过湿法刻蚀后,刻蚀深度约为150 nm,蛾眼玻璃透过率最高为98.02%。结论微晶玻璃经过湿法刻蚀后增透效果明显,对X射线衍射(XRD)、掠入射X射线衍射(GIXRD)、扫描电子显微镜(SEM)的测试结果进行分析可知,微晶玻璃中存在晶体相和残余玻璃相,经过酸蚀处理后,表面能形成以透锂长石晶体和二硅酸锂晶体均匀分布的蛾眼结构,晶体尺寸为12.2~27.4 nm的微晶玻璃进行湿法刻蚀后透过率分布在95.99%~98.02%。

微合金元素对大规格热轧棒材强化作用的研究

混晶缺陷一般由再结晶、加工比(变形量)、二次加热和异常长大引起,会大幅度降低结构钢的机械性能。为研究微合金成分设计导致实际晶粒混晶的问题,按照标准GB/T 6394—2017对加工比为6.2的同规格钢材进行边缘、半径1/2处、心部三个位置晶粒的研究。通过对照分析25炉相同工艺生产的35CrMoA、39NiCrMo3、45^(#)、S355J2这4种钢中Al、N、Ti、V的含量与实际晶粒度和混晶的关系,结果表明:Al、N高的钢,晶粒得到细化;Ti、V含量低的钢(45#钢和S355J2钢),出现了混晶的现象。

考虑第二相粒子的晶粒尺寸梯度镍基合金晶体塑性有限元分析

晶粒尺寸梯度结构镍基合金具有优异的强度和延展性,但存在于合金内第二相粒子对这些性能的影响尚不明确。为了阐明第二相粒子对梯度结构镍基合金力学性能的影响,将晶体塑性有限元方法结合Hall–Petch法则通过用户子程序方式嵌入至Abaqus软件中,并对晶粒尺寸梯度结构镍基合金的强度和延展性进行了研究,得到了晶粒尺寸梯度结构镍基合金有限元模型的应力和应变云图。进一步探究了位于表层或芯部的不同颗粒尺寸第二相粒子对该合金宏观力学性能的影响,得到了含第二相粒子的晶粒尺寸梯度结构镍基合金的应力-应变曲线图和应力和应变分布图。结果表明:位于表层1号尺寸第二相粒子的镍基合金表现出较高的强度和较好的延展性,强度为1804.96 MPa,应变为20.49%;位于芯部1号尺寸第二相粒子的镍基合金抗拉强度最大,为1902.34 MPa,但延展性最差,为9.68%。随着第二相粒子尺寸的增大,镍基合金的强度逐渐降低。研究结果有助于理解镍基合金的强塑性,并为合金的设计和应用提供参考。

High-throughput screening of critical size of grain growth in gradient structured nickel

Nanocrystalline metals with high Gibbs free energy have a strong tendency towards thermally driven grain growth,thus understanding the critical size or temperature of grain growth is vital for their applications.The investigations of thermal stability were usually conducted on the materials with a homogeneous structure;however,these methods are time-consuming and expensive.In the present work,we reveal a high-throughput experimental strategy to characterize the size-dependent thermal stability via annealing the gradient structured Ni.Employing this method,the critical size of grain growth(d_(c))at a given annealing temperature was rapidly determined.The critical size of grain growth was~95 nm when annealed at 503 K for 3 h,which is consistent with the value reported in the homogeneous structured Ni.Furthermore,this critical size was found to be identical in three types of gradient structured Ni,i.e.,independent on the gradient structure.Our present work demonstrates a high-throughput strategy for exploring the critical size of grain growth and size-dependent thermal stability of metals.

550 MPa级微合金化高强结构钢设计与生产

介绍了屈服强度550 MPa级微合金化高强钢在中板坯连铸-连轧生产线上的设计开发与生产实践情况。3种实验方案对比结果显示:高锰低铌钛体系采用低温卷取,晶粒度11级,屈服强度668 MPa,碳化物弥散析出,冲击断口呈现明显的混合断裂形貌,聚集韧窝和准解理断裂交叉分布,冲击吸收功较低。中锰、中铌、高钛成分体系采用高温轧制和卷取,晶粒度10级,屈服强度649 MPa,碳化物在晶界析出充分,提升了韧性,但是强度降低明显;降低卷取温度后,组织细化显著,晶粒度增加至12级,屈服强度721 MPa,但纵向组织出现粗化的条状铁素体,材料组织均匀性变差;高温卷取冲击试样断口基本为韧窝形貌,而低温卷取的试样断口韧窝小且浅,且有少量解理断口。低锰、高铌、中钛成分体系下,晶粒度11级,屈服强度620 MPa,碳化物弥散析出效果最好,珠光体含量少且尺寸均匀、细小;冲击试样断口也呈现显著韧性断裂形貌,部分位置韧窝大且深,呈现微孔聚集断裂的典型形貌,但仍可观察到少量解理撕裂的形貌。试制结果表明:通过添加Nb、Ti等强化元素,可优化材料组织组成、尺寸和均匀性,实现550 MPa级产品材料强度和塑韧性最佳匹配。

戈壁风沙流结构特性及其意义

通过砾质戈壁风沙流野外实测数据的分析以及风洞模拟实验研究发现:戈壁风沙流结构具有与沙漠风沙流完全不同的风沙流特征,戈壁风沙地表的粗糙度随风速的增大而增加,其表面风沙流输沙量高度分布表现出独特的"象鼻"效应,在一定高度处呈现最大值,并随风速的增加而增高。该"象鼻"效应导致戈壁风沙流结构特征值λ远大于1,不论风速多大,风沙流都处于未饱和状态的非堆积搬运状态。这种特殊性质比较清楚地解释了敦煌莫高窟千年来不被沙山埋没的谜底。并且在风沙防治工程实践中,采用砾石压沙措施,构造类似与砾质戈壁的下垫面,人工促使风沙流结构呈现"象鼻"形状,可使防沙工程达到理想的输导作用。

粗细晶WC基体对WC-Co非均匀结构显微组织形成的影响

本文研究了粗晶WC基体与细晶WC基体对WC-8Co合金的非均匀结构组织形成的影响。结果表明,粗晶WC为基体,细晶WC添加量35%以上时,形成粗、细WC晶粒尺寸分别为7.0～8.0μm、2.0～2.5μm的非均匀结构显微组织。细晶WC为基体,粗晶WC添加量为45%时,形成粗、细WC晶粒尺寸分别为6.0～7.0μm、0.4～0.5μm的非均匀结构显微组织。在合适的工艺参数条件下,可获得粗、细晶分别为8.0～9.0μm、2.0～3.0μm的非均匀结构的合金组织。

35CrMo钢动态再结晶过程数值模拟与试验研究

以热物理模拟试验研究为基础 ,得出 35CrMo钢发生动态再结晶时的数学模型。采用热-力耦合的弹塑性有限元法对 35CrMo结构钢在热变形过程进行了数值模拟。变形的不均匀性导致动态再结晶进行的不等时性 ,动态再结晶的发生初始于大变形区 ,随着应变的增加 ,逐渐向粘着区和自由变形区延伸。同时预测热变形过程的形变量、形变速率和形变温度对再结晶微观组织演变的影响。在一定温度下 ,再结晶晶粒尺寸的大小与应变速率呈反方向变化 ,随着变形的进行 ,试样内的晶粒尺寸趋于细化和均匀化。在一定应变速率下 ,随着形变温度的降低 ,再结晶晶粒尺寸趋于细化 ,导致了锻件的综合性能提高。为了观察显微组织演化过程 ,对模拟结果进行了金相法验证 ,模拟结果与实验结果比较吻合 。