Ni75Al7.5V17.5合金时效沉淀D022相的孪生相场模拟

为揭示合金固态相变初期新相形成机制,尤其是D022相变孪晶形成机制,利用三元微观相场模型对Ni75Al7.5V17.5合金于1 273 K下等温时效沉淀过程的前3个阶段进行了模拟.计算结果表明,L12结构的Ni3Al有序相首先沿惯析面(100)从母相中共格形核,而后D022结构的Ni3V有序相在先析出的L12相和母相的相界形核.由于{100}的反相畴界上形成较大的共格失配,导致能量较高且不稳定,反相畴界在两相生长中遭到破坏,而晶面{110}的界面能比{100}的反相畴界能低,生长过程中D022的3种变体互相正交排列形成阶梯状结构的{110}孪晶面.     

时效成形对2324铝合金组织及性能的影响

研究了时效成形对2324铝合金力学性能的影响,利用透射电镜观察了时效成形过程中的微观组织.研究了时效成形产生的"应力位向效应"及其对力学性能的影响,对"应力位向效应"的特征和产生原因进行了分析.结果表明:由于外加应力造成了平行应力方向和垂直应力方向扩散系数的差异,导致析出相择优生长.无应力时效时析出相沿{201}晶面族均匀析出,时效成形过程中析出相沿平行(111)方向择优生长,择优生长方向主要取决于外加应力的方向.     

合金沉淀过程的微观相场法计算机模拟

讨论了连续体相场动力学与微观相场动力学理论在描述合金相变方面各自的特点.基于微观相场动力学理论建立的微扩散方程,以原子占位几率和序参数描述合金沉淀过程的原子簇聚和有序化.该模型将三维问题进行二维投影,在倒易空间求解,直观给出原子演化图像,并可描述形核、长大、和粗化在内的全过程.微扩散模型已成功用于二元及三元合金沉淀机制及微观组织的模拟.     

可控电磁能(CEME)时效处理下Al-Zn-Mg-Cu合金的析出及强化机理研究

文中提出了可控电磁能(CEME)铝合金辅助时效处理技术,研究了脉冲电磁场产生的可控电磁能对Al-Zn-Mg-Cu合金析出相密度、尺寸分布及力学性能的影响并分析了组织强化机制.引入磁-热扩散模型和量子力学探讨外加脉冲磁能作用下Al-Zn-Mg-Cu合金的扩散析出机理.结果表明,施加脉冲电磁场有利于增加析出质点数量,而延长时效可促进析出相长大,经脉冲电磁场时效1 h后的析出相分布密度为1.124μm-2,约为传统时效12 h的3倍,其抗拉强度分别为506 MPa和542 MPa;随着析出相尺寸的增大,强化机理由共格应变场强化机制(OS)过渡至Orowan绕过强化模型(CS),理论临界析出颗粒尺寸为20 nm.动力学分析认为,外加磁能对扩散激活能的Gtot作用较小,提高原子振动频率v0是增加析出质点数量的主要原因.     

高能超声对Mg2 Si / AZ91D 复合材料的影响

为改善原位颗粒增强镁基复合材料的性能,采用原位合成技术制备了Mg2Si/AZ91D复合材料,通过在熔体中施加高能超声,研究了超声时间和超声功率对复合材料组织性能的影响.结果表明:随着超声时间的延长或超声功率的增大,复合材料中粗大的汉字状Mg2Si相变得细小、分布均匀,同时细小分布均匀的球化状β-Mg17Al12相增多;超声时间为6 min、超声功率为1.2 kW时,组织中呈短棒状的Mg2Si颗粒和球化状β-Mg17Al12相分布均匀,且复合材料的抗拉强度和伸长率达到最大,分别为220.5 MPa和2.6%,较未施加超声的复合材料试样提高了22.3%和38.9%;再延长处理时间或增大输出功率,组织有粗化的趋势,复合材料的抗拉性能及伸长率也呈现先升后降趋势.     

B_4C增强铝基复合材料力学性能有限元模拟

建立了颗粒增强铝基复合材料的轴对称单胞模型,并通过有限元方法模拟了B_4C颗粒增强5083铝基复合材料的力学性能和微观应力分布。结果表明,模拟结果与实验结果吻合较好,模拟椭球体颗粒增强复合材料的抗拉强度为485 MPa,而实验值为477 MPa,相对误差仅为1.7%。颗粒形状对复合材料微观应力场有很大影响:圆柱体颗粒的尖角处容易造成应力集中,而球体颗粒界面处应力分布较为均匀。在一定范围内,复合材料的弹性模量和抗拉强度随着B_4C颗粒体积分数的增加而增加。在颗粒体积分数不变的情况下,不同长径比的颗粒沿复合材料受力方向定向排列时,颗粒的长径比越大,复合材料的弹性模量、强度等力学性能也越高。     

相场法模拟共格两相系统中有序粒子的形貌演化和反向粗化

在共格错配系统中,基体和析出相之间的弹性应力会对系统的微观组织结构和粗化过程产生显著影响。采用相场法模拟了三粒子系统的粗化过程,着重探讨了析出相粒子的相对位置和有序结构对其形貌和粗化过程的影响。模拟结果发现,改变析出相粒子的相对位置,粒子的粗化行为会发生变化。如果粒子沿着弹性软方向排列,其中的小粒子就可能发生反向粗化,反向粗化的驱动力是系统弹性能的降低。处于反相态的有序粒子相互接触时不会发生合并,粒子会沿着弹性软方向排列成链条状组织。此外,在两粒子系统中,即使粒子的面积相差较大,小粒子也可能发生反向粗化。     

TiB2/Al-Cu-Li复合材料时效析出及组织演变对力学性能的影响

研究了TiB₂/Al-Cu-Li复合材料T6工艺的微观组织演变和时效析出对力学性能的影响。通过气氛保护熔炼法制备了TiB₂/Al-Cu-Li复合材料。结果表明：在铸态合金的微观组织中,TiB₂颗粒和共晶相主要分布在晶界周围。均匀化处理后,大部分共晶相回溶。轧制变形后,TiB₂颗粒沿着轧制方向被拉长,产生了大量位错。固溶处理削弱了轧制产生的Brass织构和S织构,回溶了轧制产生的析出相。在175℃温度下进行时效,欠时效过程中,δ’(Al₃Li)/β’(Al₃Zr)为主要析出相。随着时效时间的增加,到22 h峰时效时,T1相为主要析出强化相。通过位错强化和析出强化的共同作用,随时效时间增加,屈服强度和抗拉强度先上升后下降,延伸率持续下降。复合材料峰时效的极限抗拉强度为562.7 MPa,屈服强度为475.9 MPa,延伸率为4.5%。     

时效组织演化的计算机模拟理论与模型

总结了时效组织的计算机模拟研究,基于Cahm-Hillard的非经典转变理论,提出一种新的动力学模型,不同于以往的连续介质模型,为离散格点形式,可租用于时效过程的所有阶段,包括形核、长大、粗化等,模型还可描述原子有序化和相分离,并考虑到弹性能对形貌演化和粗化过程的影响,且体积分数的影响、沉淀颗粒间的相互作用也在方程中自动体现。     

Inconel 740H蠕变过程中晶界M23C6的粗化行为

为了解700℃超临界电站锅炉过/再热器用主要候选材料Inconel 740H晶界M_(23)C_6相在蠕变过程中的粗化行为,对通过真空感应熔炼和真空电弧重熔双联工艺制备的合金,在750℃/200 MPa条件下分别开展蠕变中断与蠕变断裂试验,并利用扫描电镜、透射电镜、能谱分析仪等测试方法对晶界M_(23)C_6相进行表征.采用Image-Pro Plus软件对相参量进行定量统计,进而分析其变化趋势.研究表明:在所研究温度、应力及时间范围内,随着蠕变时间的延长,晶界富Cr的M_(23)C_6相逐渐由粒状、短棒状发展成为长条、半连续的链状分布;粒子沿平行于晶界方向的长大速率明显高于沿垂直于晶界方向的长大速率,但二者的粗化均表现出尺寸三次方与时间之间呈线性关系的规律;相数量的增加与蠕变时间亦呈线性关系;与尺寸的粗化相比,相数量的增长较为缓慢,外加应力在一定程度上促进了M_(23)C_6的析出.     

Computer Simulation of the Interphase Boundary Evolution in Ni75AlxV25-x Alloy

The interphase boundary evolution of ordered phase in Ni75AlxV25-x alloy during precipitation was simulated on atomic-scale based on the microscopic phase-field dynamic model. The results show that the second phase precipitated from the interphase boundary formed by the first phase and the disordered matrix at high temperature, and from the interphase boundaries of the first phase at low temperature. L12 phase had obvious selective orientation when precipitated from the interphase boundaries of D022- L12 phase nucleated easily at the interphase boundaries formed by [10] and [01] directions of D022 projection along [001] direction, and grew easily at [10] direction. There was no the selective orientation when L12 phase precipitated from the interphase boundary formed by D022 and the disordered matrix. D022 phase had the selective orientation when precipitated from the interphase boundaries of L12, and grew along the [10] direction.     

Modeling the microstructural evolution of Ni-base superalloys by phase field method combined with CALPHAD and CVM

A quantitative simulation with phase field method (PFM) in a real alloy system was performed with a new strategy of modeling the microstructure evolution of Ni-base superalloys, in which, the calculation of phase diagrams (CALPHAD) method and cluster variation method (CVM) are combined with the PFM. In this strategy, the four sub-lattice model is used to evaluate the chemical free energy density while CVM to calculate some parameters, such as the lattice misfit and gradient coefficients in phase field equations. With this strategy, the microstructure evolution of a Ni–Al binary alloy at the temperature of 1000 K has been simulated; the elastic energy due to the lattice misfit and elastic inhomogeneity between γ and γ′ phases has also been taken into account. Moreover, the directional coarsening phenomenon of Ni–Al alloy has been reoccurred when the various external stress conditions are applied, which shows good agreement with Pineau’s directional coarsening map.     

Microscopic Phase-field Simulation of Competition Mechanism Between L12 and D022 Structure in Ni-Cr-Al Alloy

Simulations are performed on temporal evolution of atom morphology and ordering parameters of Ni-14.5 Cr-16.5 Al alloy during early precipitation process at different temperatures based on microscopic phase-field theory; the relationship between precipitation sequence and mechanism of L12 and D022 structure and precipitation temperature are illuminated. The nonstoichiometric ordered L12 phases appear first with congruent ordering＋spinodal decomposition mechanism which is then followed by precipitation of D022 phases at ordering domain boundaries of L12 phases by spinodal decomposition mechanism at 1073 K and 1223 K. The nonstoichiometric L12 phases transform to stoichiometric ordering phases gradually. The incubation period of L12 and D022 phases is shorter at 1073 K than that 1223 K, and growth speed is higher at 1073 K. At 1373 K, L12 and D022 phases appear simultaneously by non-classical nucleation and growth mechanism. After that the particles of D022 phases diminish and disappear gradually; L12 phases grow and single L12 phases are remained at last.     

Coarsening kinetics of γ' precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r³ vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r² vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.     

Microscopic Phase-field Simulation of Competition Mechanism Between Ll_2 and D0_(22) Structure in Ni-Cr-Al Alloy

Simulations are performed on temporal evolution of atom morphology and ordering parameters of Ni-14.5 Cr-16.5 Al alloy during early precipitation process at different temperatures based on microscopic phase-field theory; the relationship between precipitation sequence and mechanism of L12 and D022 structure and precipitation temperature are illuminated. The nonstoichiometric ordered L12 phases appear first with congruent ordering+spinodal decomposition mechanism which is then followed by precipitation of D022 phases at ordering domain boundaries of L12 phases by spinodal decomposition mechanism at 1073 K and 1223 K. The nonstoichiometric Ll2 phases transform to stoichiometric ordering phases gradually. The incubation period of L12 and D022 phases is shorter at 1073 K than that 1223 K, and growth speed is higher at 1073 K. At 1373 K, L12 and D022 phases appear simultaneously by non-classical nucleation and growth mechanism. After that the particles of D022 phases diminish and disappear gradually; L12 phases grow and single L12 phases are remained at last.     

Process model for strength of age hardenable aluminium alloy welds

Abstract

An existing process model for hardness prediction in age hardenable aluminium alloy welds is presented and analysed. One of the key criticisms of this model is that its derivation assumes softening is due to precipitate dissolution alone. The influence of precipitate coarsening has been determined by developing an equivalent model for softening owing to coarsening. It is shown that the experimentally derived master curves that form the basis of the model are capable of representing softening by a mixture of precipitate coarsening and dissolution. Methods to predict post-weld natural aging are discussed, and a new method is presented based on direct prediction of the Guinier–Preston zone fraction. The model has been applied to friction stir welding. Model predictions agree well with measured hardness profiles, and the sensitivity of the predictions to temperature is discussed.     

Microstructure evolution and analysis of a single crystal nickel-based superalloy during compressive creep

During compressive creep, the cubical γ′ phase in [0 0 1] orientation single crystal nickel-based superalloy is transformed into the rafted structure along the direction parallel to the applied stress axis. By means of the elastic stress-strain finite element method (FEM), the von Mises stress distributions of the cubical γ′/γ phases are calculated for investigating the influence of the applied stress on the stress distribution and the directional coarsening regularity of γ′ phase. Results show that the stress distribution of the cubical γ/γ′ phases may be changed by the applied compressive stress, and the coarsening orientation of γ′ phase is related to the von Mises stress distribution of the γ matrix channel. Thereinto, under the action of applied compressive stress, the bigger von Mises stress produced on (0 0 1) plane of the cubical γ′ phase is thought to be a main reason of the microstructure evolution. The expression of the driving force for the elements diffusion and the directional growing of γ′ phase during compressive creep are also proposed.     

γ″Precipitate in Inconel 718

The γ"precipitate in Inconel 718 Ni-base superalloy has been investigated using TEM. Based on a calculation of diffraction pattern. the orientation relations between precipitates and matrix are given in detail. The influence of solution temperature on growth of γ" precipitates was investigated. and a γ" precipitate free area near δ phase was found. The coarsening behaviour of γ" precipitates during short time aging is discussed     

Effects of heat treatment on microstructure and mechanical properties of extruded AZ80 magnesium alloy

Microstructure evolution of the extruded AZ80 magnesium alloy and different precipitation behavior of β-Mg₁₇Al₁₂ phase in different heat treatment conditions were investigated. Solution treatment caused the dissolution of β-Mg₁₇Al₁₂ phase and the grain coarsening. During aging, discontinuous precipitates were preferentially initiated at some of the grain boundaries and then continuous precipitates appeared in the grain interiors. In the period of direct-aging, discontinuous precipitates formed between the banded structure and no continuous precipitate appeared. After solution and aging treatment, an improved combination of strength and elongation was obtained. The highest strength was achieved for the extruded AZ80 sample after directing-aging treatment.     

Microstructural control of maraging steel C300

Abstract

The microstructure evolution and precipitation kinetics of maraging steel C300 have been studied in the aging temperature range from 400 to 600°C. The relation between mechanical properties and precipitation hardening response is explained, and modelling is used to optimise the properties. Ultrafine needle shaped Ni₃Ti phase is the main strengthening precipitate in maraging C300, and it shows very high resistance to coarsening. A spherically shaped Fe₂Mo phase is formed at higher temperatures and in the overaged condition. Inter- and intralath reverted austenite nucleates at higher temperature (～600°C). Rolling and aging treatment can produce the highest hardness by a combination of work hardening and precipitation strengthening. Microstructural evolution simulation using Monte Carlo modelling has been applied to this alloy, and the modelling has been validated by the experimental results.