W含量对一种高W镍基高温合金显微组织的影响

通过OM、SEM和XRD对高W镍基高温合金进行组织观察与分析,研究了W含量对镍基高温合金凝固组织的影响规律。结果表明,当W含量为14%(质量分数,下同)时,镍基合金中无α-W相析出。当W含量高于16%时,合金凝固期间可析出α-W,并且随W含量提高,合金的晶粒尺寸由1.04 mm减小至0.17 mm,共晶含量由6%增至10%;W含量对在枝晶间/枝晶干内的γ'相尺寸及形态无明显影响。由于α-W的析出温度较高,在凝固期间首先析出,并在残余液相收缩作用下,α-W向液相核心处发生转移并长大;同时α-W可作为异质形核的核心,降低枝晶形核的临界形核功,使18%W合金获得较小的晶粒尺寸。此外,在不同取向枝晶汇聚生长的作用下,残余液相中Al、Ti等元素形成较高的浓度梯度而发生共晶转变,这是18%W合金中共晶含量较高的主要原因。     

冷却速率对A357合金凝固组织的影响

以不同冷却速率铸造A357合金试样,用光学显微镜和扫描电镜(SEM)分析其凝固组织.通过不同冷却速率下的DTA试验研究了A357合金的凝固行为;采用DSC试验研究了不同冷却速率所得试样中相的变化;测定了这些试样的微观硬度.结果表明,随着冷却速率的提高,A357合金的宏观晶粒尺寸和二次枝晶臂间距减小,共晶Si相的形态更加细小,且分布愈加弥散;合金液相线温度和二元共晶的反应温度降低,三元或四元共晶反应被抑制,合金凝固组织中Mg2Si相的析出受到抑制,微观硬度明显提高.     

过共晶Cu-Cr合金定向凝固组织中α相的形成机制研究

考察了Cu-1.7%Cr过共晶合金的定向凝固组织,对过共晶组织中出现亚共晶合金的初生α相的生长进行了探讨.研究表明:Cu-1.7%Cr过共晶合金定向凝固组织由初生β相、α相和(α-β)共晶组成;α相和(α+β)共晶的成分较大偏离平衡相图中的各相成分;初生β相分布在α相基体上.Cu-1.7%Cr凝固组织中α相的形成是非平衡凝固过程中枝晶α相和共晶相之间竞争生长的结果.α相是一种非平衡凝固组织,(α+β)共晶是偏离了平衡共晶成分的伪共晶组织.随着冷却速率的增加,α相的形貌尺寸减小,而数量有所增加.     

AZ61镁合金的差热分析及凝固组织观察

通过差热分析并结合金相组织观察的方法研究了AZ61镁合金相变温度点和凝固组织的特征,并对凝固组织演变机理做了理论分析。结果表明:AZ61镁合金固液转变温度在587.10~608.43℃之间,共晶转变温度在413℃左右;AZ61镁合金在凝固过程中首先析出α-Mg树枝晶,α-Mg树枝晶间富Al的残余液体发生共晶转变,形成α-Mg+β-Mg17Al12共晶组织。共晶组织中的α-Mg相往往依附初晶α-Mg形核生长,并将β-Mg17Al12相推向α-Mg枝晶的晶界,形成离异共晶组织。     

冷却速度对Al-3Fe合金凝固特性和微观组织的影响

采用冷却曲线测定、光学显微镜(OM)和X射线衍射仪(XRD)研究了冷却速度对Al-3Fe合金凝固特性和凝固组织的影响。结果表明,在冷却速度为0.69~11.8℃/s的范围内,Al-3Fe合金的显微组织均由α-Al枝晶、初生富Fe相及枝晶间的共晶组织组成,其中富Fe相主要为Al₃Fe、Al₅Fe₂和Al₆Fe相。冷却速度对Al-3Fe合金的凝固特性和凝固组织有明显的影响。随着冷却速度的提高,合金初生相形核温度和共晶反应温度降低,合金的凝固组织明显细化。     

Mg-Gd-Y-Zr合金可控冷却速率实验方法及凝固组织的X射线断层扫描表征

建立了可控冷却速率的实验方法、凝固过程测温方案和合金内部温度推算方法,对Mg-6Gd-3Y-0.5Zr(质量分数,%)(GW63K)合金开展可控冷却速率实验,结合X射线断层扫描技术、扫描电子显微镜和X射线能谱分析等实验表征手段,表征可控冷速镁合金的凝固组织形貌并获取定量信息,研究了冷却速率对GW63K合金凝固组织微观形貌、定量信息的影响和变化规律。结果表明：在GW63K合金凝固组织中,共晶呈网络状分布在晶界处,形状不规则的第二相呈岛状分布在共晶组织中;平均冷却速率Rc在0.13～0.33℃/s范围内,随着冷却速率的增加,网状共晶组织更密集、均匀和连续,第二相分布更均匀、尺寸更小,微观溶质偏析减小,第二相和共晶体积分数均呈下降趋势。     

Al含量对一种高W铸造高温合金组织与持久性能的影响

以不同Al含量的高W铸造高温合金K416为研究对象,对不同Al含量合金在975℃/240 MPa条件下的持久性能和室温基体显微硬度进行了测试。结果表明:随着Al含量的提高,合金的持久寿命和显微硬度均呈下降趋势,Al含量中限和高限合金在975℃/240MPa条件下的持久寿命低于技术条件规定值。进一步的组织表征结果表明:当Al含量为5.60%(质量分数,下同)时,合金中仅有极少量α-W相和初生M_6C型碳化物析出。随着Al含量提高至6.20%,合金中α-W相和初生M_6C型碳化物的面积分数由0.08%提高至2.1%,同时γ/γ′共晶的面积分数由12.6%提高至21.8%。Al含量的提高对合金晶粒、枝晶、MC型碳化物以及γ′相的尺寸和形貌影响不大。硬脆的初生M_6C型碳化物的大量析出以及固溶强化元素W的消耗是其持久性能降低的主要原因。同时,对不同Al含量K416合金的持久断裂机制也进行了讨论。     

A356铝合金的凝固组织特征对固溶处理工艺的影响（英文）

为了提高性能且降低成本,设计末端激冷的砂型铸造实验,研究A356铝合金的凝固组织随冷却速率的演化特征以及不同凝固组织对后续固溶处理过程的影响。实验结果表明:初生相α(Al)的二次枝晶间距、共晶Si的尺寸以及Al-Si共晶体积分数随冷却速率的提高而减小。经过540℃固溶处理1 h后淬火,当冷却速率为2.6 K/s时,共晶Si完全球化;当冷却速率为0.22 K/s时,共晶Si发生部分球化;当冷却速率为0.22和0.12 K/s时,仅共晶Si的边缘发生钝化。同时,当冷却速率为2.6 K/s时,合金具有最大的显微硬度。由此可见,经过高冷却速率凝固后,细化的共晶Si更容易发生球化,从而在给定固溶温度条件下,降低了固溶处理所需要的时间,即降低了生产成本。     

Al-Si-Cu-Ni-Mg合金凝固组织调控及对高温拉伸性能的影响

应用JMat Pro软件、金相显微镜、高温拉伸试验机和扫描电镜研究了浇注温度和模具温度对Al-Si-Cu-Ni-Mg合金凝固组织及高温拉伸性能的影响。结果表明：Al-Si-Cu-Ni-Mg合金的凝固组织主要为α-Al和共晶硅，合金中的强化析出相主要为δ-Al₃Cu Ni、Q-Al₅Cu₂Mg₈Si₆、γ-Al₇Cu₄Ni、ε-Al₃Ni和Mg₂Si相；当控制浇注温度710℃，模具温度150℃时，Al-Si-Cu-NiMg合金中α-Al晶粒、共晶硅、富Ni相、Q相、Mg₂Si相尺寸最小，组织均匀程度和高温拉伸性能达到最佳。     

雾化Al-Si合金液滴冷却凝固过程模拟

在群体动力学模型基础上，提出了描述雾化共晶合金液滴组织演变的数学模型，并将其与液滴的运动与传热方程相耦合，对合金的冷却凝固过程进行研究，探讨液滴尺寸对合金冷却凝固过程的影响，结合A390合金进行了计算。结果表明：随着液滴尺寸的减小，所获得的冷却速度及过冷度增大，而析出初生Si相的尺寸及体积分数减小，当液滴尺寸足够小时，液滴温度的变化趋势及微观组织将发生突变。     

Effect of Zr on solidification and microstructure of a Ni-based superalloy with high Al and Ti contents

The total content of Al and Ti in advanced Ni-based wrought superalloys is up to 7.5wt.%,which makes it easier to form harmful nonequilibrium eutectic(γ+γ′)andηphase.It has been reported that the addition of certain amount of Zr can modify precipitation of the nonequilibrium phases obviously,but the mechanism is still controversial.The effect of Zr ranging from<0.0006wt.%to 0.150wt.%on solidification behavior,segregation and microstructure of a Ni-based superalloy with high Al and Ti contents was investigated,eliminating the interferences of C and B.Results show that increase in Zr content significantly promotes the formation of eutectic(γ+γ′),ηand Zr-rich phase in the interdendritic region.Besides the Zr-rich phase,Zr dissolves slightly in the eutecticγ′and obviously in theηphase.An interesting phenomenon is discovered that the Zr addition significantly increases the area fraction of liquid pools and enlarges the forming range ofγdendrites,which suggests that Zr markedly retards the solidification.Zr affects the eutectic(γ+γ′)andηformation mainly due to the retard of solidification and dissolution of Zr in them.The retard of solidification obviously increases the residual liquid fraction and undercooling.Zr can serve as a forming element for the eutectic(γ+γ′)andηphase,and the obvious dissolution of Zr inηphase significantly decreases the critical concentration of Ti for its precipitation.     

Solidification behavior and rheo-diecasting microstructure of A356 aluminum alloy prepared by self-inoculation method

Semisolid slurry of A356 aluminum alloy was prepared by self-inoculation method, and the microstructure and solidification behavior during rheo-diecasting process were investigated. The results indicate that the semisolid slurry of A356 aluminum alloy can be prepared by self-inoculation method at 600 °C. Primary α-Al particles with fine and spherical morphologies are uniformly distributed when the isothermal holding time of slurry is 3 min. Liquid phase segregation occurs during rheo-diecasting process of semisolid slurry and the primary particles(α1) show obvious plastic deformation in the area of high stress and low cooling rate. A small amount of dendrites resulting from the relatively low temperature of the shot chamber at the initial stage of secondary solidification are fragmented as they pass through the in-gate during the mould filling process. The amount of dendrite fragments decreases with the increase of filling distance. During the solidification process of the remaining liquid, the nucleation rate of secondary particles(α2) increases with the increase of cooling rate, and the content of Si in secondary particles(α2) are larger than primary particles(α1). With the increase of cooling rate, the content of Si in secondary particles(α2) gradually increases. The morphologies of eutectic Si in different parts of die casting are noticeably different. The low cooling rate in the first filling positions leads to coarse eutectic structures, while the high cooling rate in the post filling positions promotes small and compact eutectic structures.     

Effect of cooling rate on solidification parameters and microstructure of Al-7Si-0.3Mg-0.15Fe alloy

The effects of cooling rate on the solidification parameters and microstructure of Al-7Si-0.3Mg-0.15Fe alloy during solidification process were studied. To obtain different cooling rates, the step casting with five different thicknesses was used and the cooling rates and solidification parameters were determined by computer-aided thermal analysis method. The results show that at higher cooling rates, the primary α(Al) dendrite nucleation temperature, eutectic reaction temperature and solidus temperature shift to lower temperatures. Besides, with increasing cooling rate from 0.19 °C/s up to 6.25 °C/s, the secondary dendritic arm spacing decreases from 68 μm to 20 μm, and the primary dendritic volume fraction declines by approximately 5%. In addition, it reduces the length of Fe-bearing phase from 28 μm to 18 μm with a better uniform distribution. It is also found that high cooling rates make for modifying eutectic silicon into fibrous branched morphology, and decreasing block or lamella shape eutectic silicon.     

Modeling of eutectic macrosegregation in centrifugal casting of thin walled ZA8 zinc alloy

The maximum segregation zone and microstructure formation during the solidification of thin walled ZA8 zinc–aluminum alloy produced by centrifugal casting are investigated. From the results obtained, it is seen that the maximum segregation zone of the eutectic through the part section corresponds to the zone of final solidification point. The concentration of eutectic through the section changes depending on the initial mold temperature, pouring temperature, and cooling rate. A high cooling rate reduces the rate of change in eutectic concentration across the section. The distance separating the maximum segregation zone from the inner and outer faces of the casting can be controlled by controlling the ratio between the speeds of the solidification fronts advancing from opposite sides. The microstructure obtained becomes finer as the cooling rate increases. The structure of eutectic changes according to the cooling rate, and may be granular or lamellar.     

Effect of cooling rate and Fe/Mn weight ratio on volume fractions of α-AlFeSi and β-AlFeSi phases in Al−7.3Si−3.5Cu alloy

The crystallization behavior of iron intermetallic compounds in Al?7.3Si?3.5Cu alloy was investigated using thermal analysis, metallography, and a thermodynamic simulation performed with the Scheil module of ThermoCalc^®. We observed that β-AlFeSi disappeared when both high (>3.5°C/s) and low cooling rates (<0.1°C/s) were used. The elimination of β-AlFeSi at low cooling rates may be attributable to a decrease in the magnitude of microsegregation associated with low cooling rates. The disappearance of β-AlFeSi at high cooling rates may be related to growth difficulties when this phase precipitates during solidification of eutectic Al?Si: β-AlFeSi precipitation approaches the solidification onset of eutectic Al?Si when the cooling rate is increased. Moreover, the cooling rate at which the β-AlFeSi phase is suppressed should depend on the chemical composition of the alloy. According to thermodynamic simulations, the alloy composition determines the precipitation onset temperature of both β-AlFeSi (T _β) and eutectic Al?Si (T _eut). The cooling rate at which the β-AlFeSi phase disappears may be even higher as the difference betweenT _β andT _eut increases.     

铸态GH742合金的凝固特点及枝晶偏析

高合金化GH742合金存在严重的枝晶偏析,Nb、Ti大量偏聚于枝晶间,导致MC碳化物、(γ+γ′)共晶、Laves相、δ相等析出;高Mo含量及其枝晶间偏析是析出σ相的重要原因;La、Ce在枝晶间的富集促使含氧硫稀土相和Ni5Ce相的析出。与一般合金凝固特点不同,高含量Al、Ti、Nb导致GH742合金凝固过程中先后发生(γ+γ′)和(γ+Laves)两种类型的共晶反应。结合差热分析技术和凝固组织特点确定了合金的凝固温度区间为1346-1190℃,凝固顺序为γ、MC、(γ+γ′)、Laves、Ni5Ce。     

增压涡轮用镍基高温合金的凝固特性和热裂倾向性

研究了增压涡轮用镍基高温合金K418和K419的凝固特性和热裂倾向性。同时研究了合金元素的偏析行为和析出相。结果表明,凝固末期多种强枝晶间偏析元素在液相中的偏聚导致K419合金的凝固行为较K418复杂。多种元素在枝晶间剩余液相中的偏聚导致K419合金的液相线极低。K419合金凝固过程中漫长的剩余液相期的存在严重削弱了枝晶间结合力,增加了其热裂倾向性。基于一种热裂敏感区模型提出热裂倾向性系数判据,K419合金的热裂倾向性系数高于K418合金。     

Solidification behavior of Pt-containing 718Plus superalloy

Solidification behaviors of Pt-containing 718Plus superalloyswere studied by scanning electron microscopy (SEM), energy dispersive spectrum (EDS), differential scanning calorimetry (DSC) and simulation calculations. It is found that Pt increases solidification range and decreases solidus temperature of the alloy and precipitation temperature of Laves + γ eutectic phase since Pt enlarges the region of γ phase by increasing Nb solubility. In addition, Pt segregates to the interdendritic region and increases the segregation of Nb and Tiin the interdendriticregion due to the strong attractive interactions between Pt and Nb/Ti. As a result, Pt promotes the precipitation of the Laves + γ eutectic phase and η phase around eutectic phase. The increase of solidification range and segregation degrees of Nb and Al caused by Pt also promotes the precipitation and growth of γ’ + γ” phase around eutectic phase. These results provide experimental bases for understanding the mechanism of Pt in solidification behavior of superalloys.