Cu元素对Al-Zn-Mg-Er-Sc-Zr合金组织演变和腐蚀性能的影响

利用Cu元素的含量变化研究了Al₈Cu₄Er相的形成与演变规律及其对Al-Zn-Mg-Er-Sc-Zr合金腐蚀性能的影响。结果表明：随着Cu含量的增加,合金晶粒得到显著细化,但同时固溶态合金不同类型的残余相增多;Al₈Cu₄Er相与Al-Fe相存在伴生关系,二者通过Cu与Fe交互作用实现相的转化,且可表述为如下关系式：;不同成分合金的晶间腐蚀均表现出与残余相密切相关的点蚀特征,含Cu、Er的Al-Fe相虽然具有更小的腐蚀坑尺寸,但网状分布特征使腐蚀深度有所增加;而具有更好耐蚀性能的Al₈Cu₄Er则因相的粗化和它与Al-Fe相的伴生关系导致合金耐蚀性能严重下降。     

温度梯度对定向凝固Al-Zn-Mg-Cu合金微观组织和硬度的影响

采用定向凝固方法制备不同温度梯度下的高锌Al-Zn-Mg-Cu合金,表征了该合金的一次枝晶臂间距λ₁、二次枝晶臂间距λ₂以及其维氏硬度。在此基础上,采用线性回归和曲线拟合分析方法建立了温度梯度、枝晶间距和显微硬度之间的关系,结果与枝晶生长理论模型吻合,并获得了高锌Al-Zn-Mg-Cu合金的凝固特征参数,同时分析了温度梯度对显微硬度的影响机制。研究结果对高锌Al-Zn-Mg-Cu合金制备工艺优化有指导作用。     

Ho添加对Al-Zn-Mg-Cu合金均匀化热处理过程中微观组织和性能演变的影响

通过SEM、OM和DSC,研究添加Ho的Al-Zn-Mg-Cu合金均匀化热处理制度,测试不同均匀化热处理过程中合金的电导率和硬度变化。结果表明,铸态合金中存在4种第二相：T(AlZnMgCu),Al₇Cu₂Fe,Al₈Cu₄Ho 及S (Al₂CuMg),第二相导致合金元素分布存在严重微观偏析。合金在475 ℃均匀化热处理20 h后,T相完全回溶基体且未观察到S相,仅剩余Al₇Cu₂Fe和Al₈Cu₄Ho。硬度和电导率随T相的回溶而变化,T相的回溶使得合金硬度升高,电导率降低。同时,在475 ℃均匀化热处理5~20 h过程中,Al₃Ho相析出,这一现象引起硬度和电导率的升高。结合均匀化动力学分析,确定合金适宜的均匀化热处理制度为470~475 ℃/20~25 h。     

多组元替换Zr-Al-Ni-Cu-Ag金属玻璃的热稳定性和晶化行为

为提高金属玻璃的热稳定性并获得大过冷液相区,研究了成分为Zr_65-x(Al_0.21Ni_0.29Cu_0.04Ag_0.46)_35+x（x=0,7.5,15.0,22.5）的金属玻璃,重点分析了组分浓度对合金热稳定性、热诱导沉淀相以及力学性能的影响。结果表明,随着合金组分浓度的增加,非晶漫散射峰的峰位向更高角度偏移,出现了玻璃转变现象。随着玻璃转变温度（T_g）和晶化温度（T_x）增加,液相线温度（T_l）降低,导致T_x和T_g之间的温度差（ΔT_x）减小,约化玻璃转变温度（T_rg）增大。此外,形核激活能（E_x）和长大激活能（E_p1）随着溶质浓度的增加而增加。初晶从四方Zr₂Ni、Zr₂(Cu, Ag)、ZrAg和六方Zr₅Al₃相的组合转变为单一四方ZrAg相,维氏硬度呈现出增加的趋势。通过研究,发现了具有141 K过冷液相区（ΔT_x）和高热稳定性的新型金属玻璃Zr_65-x(Al_0.21Ni_0.29Cu_0.04Ag_0.46)_35+x（x=7.5）,且具有较强的抵抗晶化的特性。本研究采用的多组元替换策略对提高金属玻璃的热稳定性具有重要意义。     

微量Gd添加对Mg-8Zn-1Mn-3Sn合金组织转变及性能的影响

研究了微量Gd的添加对Mg-8Zn-1Mn-3Sn合金显微组织及性能的影响。结果表明,Mg-8Zn-1Mn-3Sn-xGd主要由α-Mg基体、MgZn₂、Mg₇Zn₃、Mg₂Sn相、MgSnGd相组成。MgSnGd相为高温相,在合金凝固过程中最先形成,改变了凝固过程,使晶界处半连续第二相转变为断网状。MgSnGd相与α-Mg基体存在共格位向关系,能作为异质形核核心细化合金晶粒。Mg-8Zn-1Mn-3Sn-0.5Gd合金的综合力学性能最佳,合金力学性能得到显著提高的机制为通过添加Gd元素细化晶粒组织、MgSnGd相钉扎晶界阻碍位错运动以及晶界第二相形貌转变。     

Tm对锆基块体非晶合金力学性能和腐蚀性能的影响

采用铜模负压吸铸工艺制备了(Zr_0.6336Cu_0.1452Ni_0.1012Al_0.12)_100-xTm_x（x=0~5,原子分数）块体金属玻璃（BMG）合金,研究了Tm对合金力学性能和抗腐蚀性能的影响。结果表明,当Tm含量增加到3%时,其玻璃形成能力（GFA）和压缩塑性显著提高,但过量Tm会降低GFA。x=3时合金的最大过冷液相区宽度为100 K,抗压强度为1669 MPa,塑性应变为21.01%,远高于Zr_0.6336Cu_0.1452Ni_0.1012Al_0.12 BMG的各项性能（67 K、1439 MPa和5.90%）。然而,电化学测试结果表明,x=3时的合金在3.5%（质量分数）NaCl溶液中的耐腐蚀性不佳,且其耐腐蚀性和力学性能随Tm含量的变化趋势与预期不同。可能是由于过量添加稀土元素Tm,容易形成更多的氧化物,导致点蚀加剧。进一步添加Tm可以提高Zr基BMG钝化膜的完整性和耐点蚀性能,但力学性能不理想。     

Nb含量对Fe0.5MnNi1.5CrNb x 高熵合金微观结构和力学性能的影响

采用真空感应熔炼法制备了Fe_0.5MnNi_1.5CrNb_x（x=0,0.05,0.1,摩尔比）高熵合金,并分析了不同Nb含量对其组织和力学性能的影响。结果表明,不含Nb元素的合金具有单相fcc结构,其抗拉强度和断裂延伸率（即延展性）分别为519 MPa和47%。添加少量的Nb(x=0.05)后出现(200)织构和少量Fe₂Nb Laves相,合金的延展性增加到55%,并且抗拉强度增加到570 MPa。当Nb含量增加到x=0.1时,织构减少,而Fe₂Nb Laves相增多,抗拉强度和延展性分别为650 MPa和45%。     

冷却速率对Al-Cu二元合金凝固组织和性能的影响

为了研究冷却速率对Al-Cu二元合金凝固组织和性能的影响,通过楔形铜模铸造制备了Al-6%Cu合金铸锭。结果表明,当冷却速率从100 K/s降低到2 K/s时,铸锭晶粒形态的转变过程为：全部柱状晶→柱状晶与等轴晶混合→全部等轴晶。同时,靠近模壁处的柱状晶宽度从244.7 μm增加到408.2 μm,铸锭心部等轴晶的平均晶粒尺寸从629.8 μm减小到152.8 μm,并且平均枝晶臂间距从10.1 μm增加到52.8 μm。计算得出Al-6%Cu合金平均枝晶臂间距和冷却速率经验公式中的参数,其中A和n的值分别为78.75和0.41。当冷却速率从100 K/s降低到25 K/s时,共晶Al₂Cu的形态从骨骼状变为片层状,在共晶Al₂Cu附近的α-Al的形态呈蜂窝状。当冷却速率由2 K/s增加到100 K/s时,Al-6%Cu合金的硬度由618 MPa增加到726 MPa。     

Mo添加对Al19Fe20- x Co20- x Ni41Mo2 x 共晶高熵合金摩擦学性能的影响

研究了Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x（x=0,1,2,3,4,5）共晶高熵合金（EHEAs）的摩擦学性能。结果表明,添加微量Mo的EHEA可形成面心立方（fcc）+B2共晶组织,而添加相对较高含量Mo的EHEAs可形成fcc+B2+μ树枝状组织。Mo元素有利于提高L1₂相的强度和B2相的延性。然而,随着Mo含量的增加,生成的富Mo μ相降低了EHEAs的强度和塑性。Al₁₉Fe₁₈Co₁₈Ni₄₁Mo₄ EHEA具有高强度和高延展性的最佳组合。增加Mo含量可以提高EHEAs的抗氧化性。随着Mo含量的增加,EHEA在滑动过程中形成了抗氧化性增强的摩擦氧化物层,摩擦系数单调下降。本研究为Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs的摩擦学性能研究提供了指导。     

Co-Sn单相合金深过冷凝固时Co3Sn2 相生长形貌的演变机制

开展了(Co₆₀Sn₄₀)_100-xNb_x (x=0,0.4,0.6,0.8,at%)单相合金的深过冷凝固实验,研究了Co₃Sn₂相生长形貌的演变机制。结果表明,在小过冷度下,Co₃Sn₂相在x=0,0.4以海藻状的模式进行生长,随着添加的Nb含量增加至0.6at%,其生长形貌转变为树枝晶,并在x=0.8进一步转变为分形海藻晶,这主要是由于界面能各向异性和动力学各向异性的变化。随着过冷度的增加,(Co₆₀Sn₄₀)_99.4Nb_0.6合金中Co₃Sn₂相生长形貌在过冷度大于28 K时从树枝晶转变为分形海藻,当过冷度高于143 K时转变为密集海藻。少量的Nb添加在小过冷度和中间过冷度时能提高Co₃Sn₂相的生长速度,但是在大过冷度下会显著降低生长速度。Co₃Sn₂相生长速度随过冷度变化规律的转变对应其生长形貌从分形海藻向密集海藻的转变。     

Tensile behavior of directionally solidified Ni3Al intermetallics with different Al contents and solidification rates

Despite the excellent high temperature mechanical properties of the Ni3Al intermetallic compound, its application is still limited due to its inherently weak grain boundary. Recent research advances have demonstrated that the tensile ductility can be enhanced by controlling the grain morphology using a directional solidification. In this study, a series of directional solidification experiments were carried out to increase both the tensile ductility and the strength of Ni₃Al alloys by arraying either the ductile phase of γ-Ni-rich dendrite fibers or the hard phase of β-NiAl dendrite fibers in the γ′-Ni₃Al matrix. The dendrite arm spacing could be controlled by the solidification rate, and the volume fraction of the γ or β phase could be altered by the Al content, ranging from 23 at.% to 27 at.%. With an increasing Al content, the γ dendritic microstructure was transformed into the β dendrite in the γ′ matrix, thereby reducing the tensile ductility by increasing the volume fraction of brittle β dendrites in the γ′ matrix. With an increasing solidification rate, the dendrite arm spacing decreased and the tensile properties of Ni₃Al varied in a complex manner. The microstructural evolution affecting the tensile behavior of directionally solidified Ni₃Al alloy specimens with different solidification rates and Al contents is discussed.     

Solidification structures and phase selection of iron-bearing eutectic particles in a DC-cast AA5182 alloy

The solidification structures of a commercial DC-cast AA5182 alloy have been studied through the thickness of a rolling ingot. The dendrite arm spacing, grain size, size of primary eutectic particles and macro segregation in the ingot have been measured. The types of the iron-bearing primary particles in the as-cast alloy have been identified by using electron microprobe and selected area electron diffraction pattern in TEM. It has been found, for the first time, that the dominant iron-bearing primary particles are Al_m(Fe,Mn) and Al₃(Fe,Mn) instead of Al₆(Fe,Mn) and Al₃(Fe,Mn). The evolution of the relative fraction of the Al_m(Fe,Mn) particles in the alloy through the thickness of the ingot has been measured. The phase selection principle for the iron-bearing primary particles in the alloy is discussed with reference to the phase diagram and growth kinetics of the different intermetallic phases.     

Thermodynamic calculation of high zinc-containing Al-Zn-Mg-Cu alloy

Phase fraction and solidification path of high Zn-containing Al-Zn-Mg-Cu series aluminum alloy were calculated by calculation of phase diagram (CALPHAD) method. Microstructure and phases of Al-9.2Zn-1.7Mg-2.3Cu alloy were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The calculation results show that η(MgZn₂) phase is influenced by Zn and Mg. Mass fractions of η(MgZn₂) in Al-xZn-1.7Mg-2.3Cu are 10.0%, 9.8% and 9.2% for x=9.6, 9.4, 8.8 (mass fraction, %), respectively. The intervals of Mg composition were achieved for θ(Al₂Cu)+η(MgZn₂), S(Al₂CuMg)+η(MgZn₂) and θ(Al₂Cu)+S(Al₂CuMg)+η(MgZn₂) phase regions. Al₃Zr, α(Al), Al₁₃Fe₄, η(MgZn₂), α-AlFeSi, Al₇Cu₂Fe, θ(Al₂Cu), Al₅Cu₂Mg₈Si₆ precipitate in sequence by no-equilibrium calculation. The SEM and XRD analyses reveal that α(Al), η(MgZn₂), Mg(Al,Cu,Zn)₂, θ(Al₂Cu) and Al₇Cu₂Fe phases are discovered in Al-9.2Zn-1.7Mg-2.3Cu alloy. The thermodynamic calculation can be used to predict the major phases present in experiment.     

Influence of a high magnetic field on columnar dendrite growth during directional solidification

The influence of a high magnetic field on the growth of MnBi, α-Al and Al₃Ni dendrites in directionally solidified Bi–Mn, Al–Cu and Al–Ni alloys have been investigated. Results indicate that the magnetic field changes the dendrite growth significantly. Indeed, the magnetic field aligns the primary dendrite arm and the effect is different for different dendrites. For the MnBi dendrite, an axial high magnetic field enhanced the growth of the primary dendrite arm along the solidification direction; however, for the α-Al and Al₃Ni dendrites, the magnetic field caused the primary dendrite arm to deviate from the solidification direction. At a lower growth speed, a high magnetic field is capable of causing the occurrence of the columnar-to-equiaxed transition (CET). Moreover, it has also been observed that a high magnetic field affects the growth of the high-order (i.e., secondary and tertiary) dendrite arms of the α-Al dendrite at a higher growth speed; as a consequence, the field enhances the branching of the dendrite and the formation of the (1 1 1)-twin planes. The above results may be attributed to the alignment of the primary dendrite arm under a high magnetic field and the effect of a high magnetic field on crystalline anisotropy during directional solidification.     

The effects of the distribution aspect of precipitate on the corrosion behavior of as-cast magnesium alloys

In the present study, the corrosion behavior of AZ91D as-cast alloy was investigated form the viewpoint of the distribution aspect of precipitate (Mg₁₇Al₁₂) and the variation of Al concentration in the Mg-rich matrix. The dendrite arm spacing (DAS) of an as-cast specimen was measured as a function of degree which describes the distribution aspect of the precipitate, and the salt spray test was conducted for various grain-sized specimens for 20 days. The dendrite arm spacing increased as the grain size increased to about 150 μm, but a constant value is indicated when the grain size exceeds that range. Although the relationship between the corrosion rate and grain size is of a nonlinear type, the linear trend between the corrosion rate and the dendrite arm spacing is maintained for the overall range of dendrite arm spacing. Since the precipitate in the as-cast alloy is discontinuously distributed, this linear relationship means that the variation of Al-solute concentration in the Mg-rich matrix has a more potent effect than the protective action of the precipitate on the corrosion behavior of an as-cast alloy.     

Dendritic Growth in an Aluminum-Silicon Alloy

Unidirectional solidification experiments have been carried out on an Al-3 wt.% Si alloy as a function of temperature gradient, G and growth rate, V. The samples were solidified under steady-state conditions with a constant growth rate of 8.20 μm/s at different temperature gradients (1.97-6.84 K/mm) and with a constant temperature gradient (6.84 K/mm) at different growth rates (8.20-492.76 μm/s). Microstructure parameters (primary dendrite arm spacing, λ₁, secondary dendrite arm spacing, λ₂, dendrite tip radius, R and mushy zone depth, d) were measured as a function of temperature gradient and growth rate. The experimental results have been compared with the current theoretical models and similar experimental works.     

Heat Treatment Development for a Rapidly Solidified Heat Resistant Cast Al-Si Alloy

Existing heat treatment standards do not properly define tempers for thin-walled castings that solidified with high solidification rates. Recently emerged casting processes such as vacuum high pressure die casting should not require long solution treatment times due to the fine microstructures arising from rapid solidification rates. The heat treatment studies involving rapidly solidified samples with secondary dendrite arm spacing between 10 and 35 μm were conducted for solution times between 30 min and 9 h and temperatures of 510 and 525 °C and for various aging parameters. The metallurgical analysis revealed that an increase in microstructure refinement could enable a reduction of solution time up to 88%. Solution treatment resulted in the dissolution of Al₂Cu and Al₅Mg₈Si₆Cu₂, while Fe- and TiZrV-based phases remained partially in the microstructure. The highest strength of approximately 351 ± 9.7 and 309 ± 3.4 MPa for the UTS and YS, respectively, was achieved for a 2-step solution treatment at 510 and 525 °C in the T6 peak aging conditions, i.e., 150 °C for 100 h. The T6 temper did not yield dimensionally stable microstructure since exceeding 250 °C during in-service operation could result in phase transformation corresponding to the over-aging reaction. The microstructure refinement had a statistically stronger effect on the alloy strength than the increase in solutionizing time. Additionally, thermal analysis and dilatometer results were presented to assess the dissolution of phases during solution treatment, aging kinetics as well as dimensional stability.     

Microstructures and mechanical properties of Er modified AA7075 alloy

The effect of erbium and homogenization on microstructures and mechanical properties of AA7075 alloy were investigated using optical microscope (LIMI), scanning electronic microscope (SEM) equipped with energy dispersive X-ray detection (EDS) and mechanical testing. It was found that during solidification most of the elements Er and Cu segregated at the grain boundaries in the form of Al₃ErCu or Al₈Cu₄Er and a very small part of Er dissolved in the FCC-Al matrix. The latter is not completely reclaimable for heterogeneous nucleation and refine grain FCC-Al matrix in its as-cast state. Er and Cu- containing phases fragmented into fine particles after extrusion and these led to a slight increase in grain boundary strengthening. The process of homogenizing, extruding and aging the Er containing alloys considerably improved the strength due to precipitation. The analysis of the extruded sample showed that the addition of Er can slightly retard the recrystallizing behavior of AA7075 alloy.     

Influence of directional solidification variables on primary dendrite arm spacing of Ni-based superalloy DZ125

The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm. The results reveal that the primary dendrite arm spacing decreases from 94 μm to 35.8 μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s. The regression equation of the primary dendrite arm spacings λ1 versus cooling rate is λ1=0.013(GV)-0.32. The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data. The influence of directional solidification rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.