同素异构转变对退火态Fe-15Mn-10Al-0.3C双相钢塑性的影响

将经过不同冷轧处理的Fe-15Mn-10Al-0.3C钢在900℃退火,使用SEM、XRD以及EBSD等手段研究了退火过程中钢的组织和性能的演变。结果表明：在退火过程中冷轧钢的奥氏体带发生了同素异构转变,γ相转变为α相,且转变量随着退火时间的延长而增加;同素异构转变影响退火试样的拉伸变形行为。随着退火时间延长α相和γ相的取向从近邻关系到满足K-S,有利于位错穿过相界滑移,使塑性提高;对于退火时间足够长的冷轧钢,两相之间的K-S关系失去相关性,塑性下降。通过该转变能调控α-铁素体与γ-奥氏体之间滑移系的平行程度,改善Fe-Mn-Al-C钢的塑性。     

微裂纹对相变陶瓷的相变条件及强度的影响

相变临界条件的研究是相变增韧的关键，材料内大量存在的微裂纹是影响相变陶瓷性能的重要因素。本文用作者改进的等效夹杂理论，研究相变陶瓷中微裂纹对材料强度的影响，建立在外载作用下的相变临界条件。通过Ａｌ_２Ｏ_３／ＺｒＯ_２陶瓷的三点弯曲实验，验证了本文理论模型的合理性。     

Thermodynamic Functions and Phase Transformation of Metal Nanocrystals

1.IntroductionNanocrystalline materials have attracted increasingscientific interests since they emerged in the late80s',be-cause of their unusual physical and chemical propertiesthat are normally attributed to the ultrafine grains.Thespecial properties of nanocrystalline materials are essen-tially connected with both thermodynamics and dynam-ics of the formation of nano materials.However,it hasshown that traditional thermodynamic and phase trans-formation models cannot satisfactorily describe…     

Solution of an Inverse Problem of Heat Conduction of 45 Steel with Martensite Phase Transformation in High Pressure during Gas Quenching

In order to simulate thermal strains,thermal stresses,residual stresses and microstructure of the steel during gas quenching by means of the numerical method,it is necessary to obtain an accurate boundary condition of temperature field.The surface heat transfer coefficient is a key parameter.The explicit finite difference method,nonlinear estimation method and the experimental relation between temperature and time during gas quenching have been used to solve the inverse problem of heat conduction.The relationship between surface temperature and surface heat transfer coefficient of a cylinder has been given.The nonlinear surface heat transfer coefficients include the coupled effects between martensitic phase transformation and temperature.     

Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above 4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.     

固溶态Cu-Ni-Si合金的相变动力学

通过测量固溶态Cu Ni Si合金的导电率变化研究了该合金的相变动力学,并由合金导电率与新相体积分数之间的线性关系,确定了该合金固溶及其60%变形态于550℃时效时的相变动力学方程。     

Modelling of Phase Transformation of Plain Carbon Steels during Continuous Cooling

A model based on Avrami equation and Scheil‘s additivity rule was proposed to simulate the phase transformation in plain carbon steels during continuous cooling in hot strip mill. In this model, a wide range of composition, cooling rate, primary austenite grain size and retained strain has been taken into account. It can be used to calculate the phase fraction transformed at different temperatures during continuous cooling. The phase equilibrium and transformation starting temperature can be determined by using Thermo-Calc and DICTRA. The simulated results containing the transformation at starting and finishing temperatures, Ae1, Ae3 and the maximum volume fraction for Q235B, were obtained. The calculated phase volume fractions are in good agreement with the experimental results.     

Phase Transformation under Continuous Cooling Conditions in Medium Carbon Microalloyed Steels

Several 35CrMo4 and 38MnV7 steels with different additions of Ti and V were manufactured by electroslag remelting. The influence of the alloying and microalloying elements on phase transformation at different cooling rates was studied and the continuous cooling transformation diagrams were plotted. In order to optimize the heat treatment and improve the mechanical properties, the range of cooling rates leading to a fully bainitic microstructure （without ferrite, pearlite and especially without martensite） was determined. Bainite and martensite transformation start temperatures （Bs, Ms） were also established and compared with the values predicted by empirical equations. The important role of precipitates （especially V carbonitride particles） on final microstructure and mechanical properties was assessed.     

Numerical Simulation of Radiation-Induced Chemical Segregation and Phase Transformation in a Binary System

We present the development of a hybrid Monte Carlo-phase field model that is able to simulate radiation induced chemical segregation and the corresponding phase transformation and nano-structure evolution. Under irradiation by a lowenergy ion beam, defects (vacancies) are created and accumulate. In a binary crystalline material, AB, studied in this work, these defects are of the two types A and B and diffuse at different rates. These differential diffusivities are sufficient driving mechanisms for the formation of chemically distinct regions with accompany changes in phases and nano-structure. In this work, we present a model that can simulate these changes by treating the differential diffusion of the vacancies of the two components.     

Applied Strain Field on Microstructure Optimization of Ti-Al-Nb Alloy Computer Simulated by Phase Field Approach

The effects of applied tensile strain on the coherent α2→O-phase transformation in Ti-Al-Nb alloys are explored by computer simulation using a phase-field method. The focus is on the influence of the applied strain direction on the microstructure and volume fraction of the O-phase precipitates. It is found that altering applied strain direction can modify microstructure of Ti-25Al-10～12Nb (at. Pct) alloy during α2→O-phase transformation effectively and full laminate microstructure in the Ti-25Al-10Nb (at. Pct) alloy can be realized by an applied strain only along the direction 30° away from the α2 phase <1010> in magnitude equivalent to the stress-free transformation strain. The simulation also shows that not only the magnitude of applied strain but also the applied strain direction influences the O-phase volume fraction and the effect of strain direction on the volume fraction is up to 25%.     

Alumina Effect on the Phase Transformation of 3Y-TZP Ceramics

The isothermal tetragonal-to-monoclinic phase transformation of 3 mol fraction Y2O3-ZrO2 ceramics contain- ing different amounts of Al2O3 during ageing in water at 130℃ for periods of time up to 40 h was investigated to explore the effect of Al2O3 addition on this transformation. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al2O3. The transformation kinetics showed a nucleation and growth mechanism on the specimen surface to be dominant in the low temperature ageing in water environment.     

Numerical Simulations of Equiaxed Dendrite Growth Using Phase Field Method

Phase field method offers the prospect of being able to perform realistic numerical experiments on dendrite growth in a metallic system.In this paper, the equiaxed dendrite evolution during the solidification of a pure material was numerically simulated using the phase field model.The equiaxed dendrite growth in a two-dimensional square domain of undercooled melt(nickel) with four-fold anisotropy was simulated.The phase field model equations was solved using the explicit finite difference method on a uniform mesh.The formation of various equiaxed dendrite patterns was shown by a series of simulations,and the effect of anisotropy on equiaxed dendrite morphology was investigated.     

Electrical Resistivity and Phase Transformation in Steels

Changes in electrical resistance accompanying transformations in steels with magnetic change (e.g. γ → martensite/bainite) and without magnetic change (e.g. γ → α, above Curie temperature) have been examined; the former class affects the resistivity the latter does not. Next, while the efficacy of electrical resistivity measurement in capturing the well‐known features of austenite stabilization (e.g. over – ageing, reversibility, and influence of prior martensite amount and so on) in high carbon steels has been reported in an earlier publication, new features (e.g. increase in resistance ‐increase at very low temperatures, change in temperature co‐efficient of resistivity in the stabilized material etc.) are highlighted here. Finally, the work shows that a quantitative estimate of precipitation in the copper bearing, age‐hardenable HSLA‐100 steel during tempering can be done by continuous electrical resistivity measurement. These data also allow an in‐depth kinetic analysis using the Johnson‐Mehl‐Avrami equation.     

掺杂镧和铈的TiO2纳米粒子的结构相变

以钛酸四丁酯,La2O3和Ce(NO3)3@6H2O为原料,采用溶胶-凝胶法制备了纯的和分别掺杂5mol%镧和铈的TiO2纳米粒子,并利用XRD,TG-DTA,TEM和XPS等测试技术对样品的晶型、尺寸和形貌等进行了表征,主要考察了镧和铈的掺杂对TiO2的晶粒尺寸和锐钛矿→金红石结构相变的影响,并初步的探讨了镧和铈的作用机理.结果表明,600℃焙烧而获得的TiO2纳米粒子具有与P-25型TiO2粒子相类似的组成、结构、形貌和粒子尺寸;掺杂剂镧和铈分别以La2O3和CeO2小团簇形式存在,并比较均匀的弥散在TiO2纳米粒子中,二者的掺杂抑制了锐钛矿晶粒的增长和锐钛矿→金红石的结构相变,使相变温度显著提高,晶格畸变增大,而镧比铈的作用更加显著.镧和铈的作用机理可能主要与掺杂离子的半径和价态以及稀土氧化物的存在和熔点等有关.     

Microstructure and Phase Transformation of a Niobium-rich TiAl-based Alloy Containing Boron and Carbon

Microstructure and phase transformation of Ti46Al8Nb0.5B0.2C alloy have been investigated.X-ray diffraction (XRD),optical microscopy (OM),scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that as-cast and hot isostatic pressing (HIP) alloy mainly composed of γ and α 2 phase have fully lamellar microstructure with point-like or ribbon-like TiB 2 distributing in lamellar colony or at grain boundary.The mean size of lamellar colony is about 150 and 450 μm for as-cast and HIP alloy,respectively.The lamellar spacing is about 550 and 600 nm for as-cast and HIP alloy,respectively.It has been found that cooling rates and quenching temperatures have significant effect on phase transformation of Ti46Al8Nb0.5B0.2C alloy.When the alloy is treated at 1380℃ for 1 h and cooled from α domain,water cooling leads to complete α→α 2 transformation,oil cooling leads to predominant α→α 2 and part α→γ m transformation,air cooling leads to α→α + γ p2 →L(α + γ) →L(α 2 + γ) transformation,and furnace cooling leads to α→α + γ p3 →L(α + γ) →L(α 2 + γ) transformation.However,when the alloy is treated at 1400℃ for 1 h and cooled from α domain,water cooling leads to predominant α→α 2 and part α→α + γ p4 →γ m transformation,oil cooling leads to α→α + γ p5 →γ m transformation,air cooling leads to α→α + γ p6 →L(α + γ) →L(α 2 + γ) transformation,and furnace cooling leads to α→α + γ p7 →L(α + γ) →L(α 2 + γ) transformation.Microstructural evolution of the alloy during various heat treatments has been examined and the phase transformation mechanisms have been elucidated.Based on the experimental observation,schematic CCT diagrams for the alloy have been given.     

各向异性条件下晶粒长大过程的相场法模拟

引入与时间有关的取向错配场变量,模拟了晶界能各向异性条件下晶粒长大的演化行为.模拟结果表明,与晶界能各向同性系统相比,随演化时间的延长,晶界能各向异性延迟晶粒的生长,使得晶粒的平均面积呈非线性变化;在相同的演化时间下,各向异性系统的晶粒尺寸分布比各向同性系统宽;晶界边数少的晶粒所占的比例明显增加;进入准稳态后,各向异性和各向同性系统中的晶粒相对尺寸分布随时间皆无明显变化.     

Phase Transformation and Magnetic Properties of Mechanically Alloyed PrCo5-based Magnets

PrCo5-based nanograin Pr=CO100-x （x=14-22） alloys with high coercivity were synthesized by mechanical alloying and subsequent annealing. The crystallization, phase components and magnetic properties of the alloys were investigated systematically. The main phase of the alloy for x=14 is Pr2CO17 with rhombohedral Th2Zn17-type. The amount of the Pr2CO17 phase decreases with increasing Pr content, and a nearly single phase PrCo5 with hexagonal CaCu5-type is formed in Pr18Co81 alloy. Further increase in the Pr content leads to the formation of another magnetically hard Pr2Co7 phase with its Curie temperature about 350℃. Remanences decrease monotonously with increasing Pr content, whereas the coercivities increase, reaching a maximum of 2040 kA/m （25.6 kOe） in Pr19CO81 powders milled for 5 h and annealed at 973 K for 2 min, and then decrease for higher Pr content. The high coercivity is attributed to the high anisotropy field of the PrCo~ phase and its nanoscale grain size.     

Phase transformation and microstructure evolution of pearlite heat-resistant steel during heating

ABSTRACT

The phase transformation and microstructure evolution of pearlite heat-resistant steel during heating were observed with an ultra-high temperature confocal scanning laser microscope. The α-ferrite completely disappeared earlier than Fe₃C during the formation of γ-austenite, which is inconsistent with the fact that the Fe₃C should disappear completely earlier under equilibrium conditions. After the Fe₃C?+?α→γ transformation, static recrystallisation of γ-austenite occurred, accompanied by the dissolution of cementite. During the γ→δ transformation, the δ-cell first precipitated at the triple point of the γ-austenite grain boundaries, and then the δ-cell platelet with one tip appeared in the γ-austenite grain. The law of δ/γ inter-phase boundaries was analysed based on inter-phase boundary types and element diffusion.     

ECAP形变对高纯铝微结构及冲击层裂损伤的影响

在高应变率冲击载荷下,金属材料的主要失效方式之一是层裂损伤。为探讨微结构对层裂损伤的影响,本文利用等径角挤压(Equal Channel Angular Pressing,ECAP)技术制备了不同微结构的高纯铝,并对改性后的高纯铝进行平板撞击实验,通过电子背散射衍射表征加载后样品的孔洞分布和晶界取向差。研究发现:高纯铝在ECAP挤压变形过程中发生动态再结晶和动态回复,导致采用ECAP技术无法获得超细晶高纯铝;降低挤压温度可抑制动态再结晶,并得到尺寸相对较小的晶粒(约50 μm)。平板撞击实验结果表明,晶粒尺寸对弹性极限、层裂强度等宏观力学性能影响较小。回收样品分析表明,大晶粒样品中孔洞尺寸大而数量少,小晶粒样品中孔洞尺寸小,但数量多。孔洞在晶界(沿晶孔洞)及晶界附近(晶内孔洞)均可成核。晶内孔洞比沿晶孔洞数量多但尺寸小。大角度晶界比小角度晶界更容易导致沿晶孔洞成核,而晶内孔洞倾向于在45°～55°晶界附近成核。     

纳米氧化铝的相变研究

以硫酸铝为原料,用沉淀法制取纳米氧化铝,研究了其相变过程。用XRD、SEM、AFM及IR等手段对不同温度下煅烧所得的产品进行了表征。结果表明,该方法制备的氧化铝粉体呈球形、团聚程度轻、粒度分布较均匀、γ相和δ相平均粒径20-30nm、α相平均粒径53nm,其物相变化次序为：非晶态Al2O3→γ-Al2O3→δ-A12O3→α-Al2O3。