Microstructure And Its Scales Of Cu--70%Sn Peritectic Alloy Under High—Temperature Gradient Directional Solidification

在凝固速率1-500 μm／s的范围内,Cu-70％Sn包晶合金定向凝固组织由ε相、包晶η相和共晶体组成,ε相为 领先相与最高界面生长温度假设的分析一致.理论计算结果显示,当凝固速率大于22.35 mm／s时,η相可直接从液相中析出, 无需通过包晶反应进行.凝固速率越低,ε向η相固相转变系数越大,造成ε相尺寸在1-5μm／s范围内变化很小,而包晶η 相的体积分数随凝固速率的增加呈现先减后增的变化趋势.在凝固速率小于50#m／s时, Cu-70％Sn包晶合金一次枝晶间距 满足λV0.325=199.5μm1.325·s-0.325;在凝固速率高于50μm／s时,一次枝晶间距满足λV0.528=676 μm1.528·s-0.528.     

不同速率定向凝固条件Mg-1.5Gd镁合金的微观结构及微观偏析

本文研究了在定向凝固条件下凝固速率对Mg-1.5Gd镁合金微观结构的影响。试样通过Bridgman定向凝固炉来制备,温度梯度恒定为40K/mm,凝固速率为10-200μm/s。研究发现,Mg-1.5Gd镁合金凝固组织为典型胞晶结构,通过线性拟合得到胞晶间距与凝固速率关系为：λ= 130.2827V-0.222,此结论与Trivedi模型拟合较好。通过Scheil模型进行热力学凝固路径计算,结合试验观察可以确定凝固组织为α(Mg)相和α(Mg) Mg5Gd二元共晶相。同时,通过Scheil模型计算所得的Gd元素的微观偏析与EPMA测量结果基本一致。     

定向凝固下Mg-1.5Gd合金的微观结构与微观偏析

研究了不同速率下定向凝固Mg-1.5Gd合金的微观结构和微观偏析。研究发现,Mg-1.5Gd合金的凝固组织为典型胞晶结构,通过线性拟合得到胞晶间距与凝固速率的关系为：λ= 130.2827V^-0.2228,此结论与Trivedi模型拟合较好。通过Scheil模型计算热力学凝固路径,结合试验观察可确定凝固组织为α(Mg)相和α(Mg)+Mg₅Gd二元共晶相。同时,通过Scheil模型计算所得Gd元素的微观偏析与EPMA测量结果基本一致。     

冷却速率对Ni-Nb合金非平衡凝固组织的影响

采用阶梯铜模喷铸方式开展了Ni-10%Nb(质量分数)合金非平衡凝固实验,并利用Procast软件对不同铜模内径所对应的冷却速率进行了计算。综合采用光学显微镜、扫描电镜及能谱分析仪对不同冷速条件下合金二次枝晶间距、溶质含量等组织参量进行表征。实验结果表明,随铜模内径减小,合金冷却速率增加,二次枝晶间距减小,枝晶主干中溶质含量得到明显提高。在此基础上,利用枝晶生长模型对不同过冷度条件下的枝晶生长速率进行了计算。分析结果表明,随冷却速率增加,凝固发生时过冷度增大,枝晶生长速度提高,溶质截留现象趋于严重,导致溶质过饱和现象发生。此外,冷却速率的提高还有利于减小液/固界面前沿溶质扩散区长度,导致二次枝晶间距减小。     

Cu含量对Al-Cu合金定向凝固组织及一次枝晶间距影响

选用Al-Cu合金进行定向凝固试验,对不同铜含量的Al-Cu合金一次枝晶微观组织形貌进行分析,并测定了一次枝晶间距.实验结果表明:在铜含量较低时,适当增加铜含量能促进枝晶生长的规整性;当含铜量为1.77％时,定向凝固组织并不明显.随着含铜量的增加,可观察到较为细小的一次枝晶组织,一次枝干呈平行生长趋势,且枝干间距较均匀,二次枝晶呈点状分布在一次枝干上.当试样铜含量从1.77％增加到3.00％时,一次枝晶间距逐步增大,但枝晶间距增加幅度较小;当铜含量较小时,试样铜含量对枝晶间距的影响不明显.     

Effect of Heat Treatment on Microstructure and Mechanical Properties of Ultra-fine Grained Ti-55511 Near β Titanium Alloy

采用热轧制备晶粒尺寸为0.1~0.5μm超细晶Ti-55511近β钛合金,利用SEM、TEM等手段研究热处理工艺对超细晶合金显微组织和力学性能的影响。结果表明,在350~650℃的温度条件下,合金强度和硬度随温度的增加呈现先增高后降低的趋势,在450℃达到峰值1486 MPa;在450℃温度退火时,随着退火时间的延长,合金强度首先急剧上升至1536 MPa后趋于稳定,延伸率呈现先增加后下降的趋势。显微组织分析表明,合金在退火过程中主要发生回复现象,未发生明显的粗化长大现象,晶粒尺寸均<1μm。回复过程在消除加工硬化的同时促进了晶界/相界的稳定化,提高细晶强化作用;退火过程中发生的α→α2和β→ω→α相变过程,第二相粒子弥散强化效应增强。但是当第二相粒子尺寸增加至一定尺寸时,会显著降低合金的塑性。退火过程中合金力学性能的变化与强化机制的作用有关。     

冷却速率对Al-4.6Cu-Mn合金凝固组织的影响

以添加不同Mn含量的Al-4.6Cu-Mn合金为对象,利用FE-SEM、VNT Quant Lab-MG专业定量金相分析系统软件研究了不同冷却速率下Al-4.6Cu-Mn合金的二次枝晶间距、非平衡共晶相体积分数以及微观凝固组织形貌特征。结果表明:冷却速率在540～106 K.s-1范围时,随着冷却速率的提高,Al-4.6Cu-Mn合金非平衡共晶相体积分数随之降低,二次枝晶间距减小,非平衡共晶相形态细化;当Mn含量增加时,Al-4.6Cu-Mn合金中非平衡共晶相体积分数随之增大。     

Pb-30%Bi包晶合金定向凝固的两相微观组织研究

对Pb-30%Bi包晶合金进行定向凝固试验,研究了在不同生长速率、不同试样直径条件下获得的两相微观组织,并对其组织的形成及微观组织尺度进行分析。试验结果表明,当生长速率V由1μm/s逐渐增大到100μm/s时,微观组织经历胞状初生α相+胞间包晶β相到枝晶状α相+枝晶间β相微观组织的转变。当生长速率V≤0.5μm/s时,在小直径(1.8 mm)试样中发现不完全带状组织,而在大直径(7 mm)试样中却得到了振荡组织。同时发现,初生α相的一次枝晶间距λ与凝固速率V之间存在λV0.16=241.0μm1.16s-0.16的关系,这与Hunt模型和Trivedi模型较好地吻合。     

Microstructural evolution of Al-20Si-5Fe alloy during rapid solidification and hot consolidation

Al-20Si-5Fe melt was rapidly solidified into particles and ribbons and then consolidated to near full density by hot pressing at 400°C/250 MPa/1 h. According to the eutectic-growth and dendritic-growth velocity models, the solidification front velocity and the amount of undercooling were estimated for the particles with different sizes. Values of 0.43−1.2 cm/s and 15–28 K were obtained. The secondary dendrite arm spacing revealed a cooling rate of 6 × 10⁵ K/s for the particles with an average size of 20 μm. Solidification models for the ribbons yielded a cooling rate of 5 × 10⁷ K/s. As a result of the higher cooling rate, the melt-spun ribbons exhibited considerable microstructural refinement and modification. The size of the primary silicon decreased from approximately 1 μm to 30 nm while the formation of iron-containing intermetallic compounds was suppressed. Supersaturation of the aluminum matrix in an amount of ∼7 at.% Si was noticed from the XRD patterns. During the hot consolidation process, coarsening of the primary silicon particles and precipitation of β-Al₅FeSi phase were observed. Evaluation of the compressive strength and hardness of the alloy indicated an improvement in mechanical properties due to the microstructural modification.     

Microstructure and Mechanical Behavior of a New α+β- Type Mg-9Li-3Al-2.5Sr Alloy

采用传统铸造方法分别制备了Φ10 mm和Φ90 mm Mg-9Li-3Al-2.5Sr(LAJ932)合金锭。在挤压温度260℃,挤压比28条件下对Φ90 mm合金锭进行挤压。分别分析和报道了铸态和挤压态LAJ932镁合金的微观组织和力学性能。探讨了该合金在挤压过程中的组织演变规律。研究结果表明:铸态和挤压态LAJ932镁合金均包括α-Mg(hcp)相,β-Li(bcc)相和Al4Sr相。Φ10 mm铸锭的组织比Φ90 mm铸锭组织细小得多。挤压过程中α-Mg相发生连续动态再结晶,而β-Li相发生非连续动态再结晶。挤压过程中,在hcpα-Mg相中形成{10 1 0}<10 1 0>织构,而bccβ-Li相中则形成{110}<101>织构。挤压过程中,LAJ932镁合金的强度和塑性均得到改善。挤压态Mg-9Li-3Al-2.5Sr(LAJ932)合金的抗拉强度达到235 MPa,屈服强度为221 MPa,延伸率为19.4%,合金展现出良好的力学性能。     

Effect of cooling rates on dendrite spacings of directionally solidified DZ125 alloy under high thermal gradient

The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfine dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings λ₁ and the cooling rate V _c is λ₁ = 0.013V _c^−0.32. The regression equation of the secondary dendrite arm spacing λ₂ and the cooling rate V _c is λ₂ = 0.00258V _c^−0.31, which gives good agreement with the Feurer/Wunderlin model.     

Microstructure and Strain Hardening of a Friction Stir Welded High-Strength Al–Zn–Mg Alloy

Microstructural evolution and strain hardening behavior of a friction stir welded（FSWed） high-strength7075Al-T651 alloy were evaluated.The nugget zone was observed to consist of fine and equiaxed recrystallized grains with a low dislocation density and free of original precipitates,but containing uniformly distributed dispersoids.The strength,joint efficiency,and ductility of the FSWed joints increased with increasing welding speed.A joint efficiency of ＊91% was achieved at a welding speed of 400 mm/min and rotational rate of 800 r/min,while the ductility remained basically the same as that of the base metal.There was no obvious strain rate sensitivity observed in both base metal and welded joints.While both the base metal and FSWed joints exhibited stage III and IV hardening characteristics,the hardening capacity,strain hardening exponent,and strain hardening rate all increased after friction stir welding.     

钒微合金化对718合金中铌元素偏析行为的影响

利用金相显微镜、电子探针、能谱仪等分析了含钒元素（V）0.066%的718合金铸锭凝固组织,对其微观组织和析出相特征进行分析,研究其中易偏析铌元素（Nb）的分布状态及偏析规律。结果表明,V元素的微合金化使718合金铸锭枝晶组织得到细化,二次枝晶间距及Nb元素的偏析系数均有所降低;由于Nb元素主要分布在Laves相及碳化物中,通过对二者体积分数的定量表征,V元素的加入大大降低Laves相及碳化物的析出,体积分数降低;线扫描分析表明,距Laves相越远,Nb元素含量越低,而含V样品中,这种含量降低的趋势有所减缓。所有分析结果均表明,V元素的合金化改善了718合金中Nb元素的偏析状态,细化了枝晶组织,对降低718合金均匀化热处理难度、提高合金成分和组织均匀性有利。含V碳化物在凝固过程中成为异质形核点,减小了凝固形核的驱动力,使得二次枝晶间距变小,表现出更小的Laves相体积分数和较细的枝晶组织。     

Effect of Phase Transformation During Long-Term Solution Treatment on Microstructure,Mechanical Properties,and Bio-corrosion Behavior of Mg–5Zn–1.5Y Cast Alloy

The influence of long-term solution treatment for various intervals on the microstructure,mechanical properties,and corrosion resistance of the as-cast Mg–5Zn–1.5Y alloy was investigated.Variation of secondary phases was studied during solution treatment through thermal analysis test and thermodynamic calculations.Tensile and hardness tests,as well as polarization and immersion tests,were performed to evaluate the mechanical properties and corrosion behavior of the ascast and heat-treated alloy,respectively.Results show that solution treatment transforms I-phaseinto W-phaseas well as dissolves it into the a-Mg matrix to some extent;therefore,the amount of W-phase increases.Moreover,prolonged solution treatment decreases the volume fraction of the phases.In the first stage of solution treatment for 14 h,the tensile properties significantly increase due to the incomplete phase transformation.Although long-term solution treatment sharply decreases the tensile and hardness properties of the alloy,it improves the corrosion resistance due to the transformation of I-phase into W-phase.In fact,it decreases corrosion potential and simultaneously dissolves intermetallic compounds into the a-Mg matrix,resulting in the reduction in galvanic microcells between the matrix and compounds.It is found that the optimum time for long-term solution treatment is 14 h,which improves both corrosion behavior and mechanical properties.     

Numerical simulation of recalescence of 3-dimensional isothermal solidification for binary alloy using phase-field approach

A accelerated arithmetic algorithm of the dynamic computing regions was designed, and 3-dimensional numerical simulation of isothermal solidification for a binary alloy was implemented. The dendritic growth and the recalescence of Ni-Cu binary alloy during the solidification at different cooling rates were investigated. The effects of cooling rate on dendritic patterns and microsegregation patterns were studied. The computed results indicate that, with the increment of the cooling rate, the dendritic growth velocity increases, both the main branch and side-branches become slender, the secondary dendrite arm spacing becomes smaller, the inadequate solute diffusion in solid aggravates, and the severity of microsegregation ahead of interface aggravates. At a higher cooling rate, the binary alloy presents recalescence; while the cooling rate is small, no recalescence occurs.     

激光熔化沉积定向快速凝固高温合金组织及性能

采用激光熔化沉积定向快速凝固工艺,制备出了具有快速定向生长微细柱晶组织的Rene95高温合金板状试样,其一次枝晶间距约为7 μm、枝晶间完全无γ/γ'共晶组织析出.结果表明,激光熔化沉积定向快速凝固微细柱晶Rene95高温合金具有优异的力学性能.