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。     

冷却过程中共晶组织的形成和转变

采用液淬方法，探讨了共晶成分铝硅合金在冷却过程中共晶组织的形成和转变规律。研究表明：在共晶温度以上的激冷试件中，呈现亚共晶组织，存在着初生α和共晶硅的粗化过程；共晶α首先依附在初生α四周形成，然后两相共同伸向共晶液体；同时，在初生α内部也有硅相析出；从凝固结束到室温的缓慢冷却过程中，高于固溶度的硅继续析出，硅片继续粗化，使初生α与共晶α的差别消失，最后形成粗大硅片均匀分布在固熔体基体上的共晶组织。     

铸态Al-7Si-0．3Mg合金的连续冷却析出图

采用冷却速率为0．01～3K／s的差示扫描量热法（DSC）和冷却速率更高的淬火膨胀法研究铸态Al-7Si-0．3Mg合金淬火的析出行为。在合金冷却的过程中发生了两种析出反应,高温反应开始于淬火起始温度540℃,低温反应始于400℃左右。3K／s的淬火冷却速率已经显著抑制淬火过程中相的析出。合金T6态的硬度随着淬火速率的增快而增加,这是由合金过饱和固溶度增加而导致的。通过膨胀实验和硬度实验的结果可以估计临界冷却速率大约为60K／s。通过光学显微镜观察淬火态的铸态Al-7Si-0．3Mg合金的显微组织。结果表明：根据淬火冷却速率的不同合金的显微组织由铝一硅共晶组织、铝固溶体枝晶及枝晶间的析出相组成。     

半固态浆料的冷却速率对组织的影响

为了研究半固态浆料的冷却速率对微观组织的影响,采用水冷冷却斜槽来制备半固态A356铝合金浆料.结果表明:当冷却速率大于26.4℃/s时,半固态浆料中的初生α(Al)全部由细小的枝晶和蔷薇晶组成;当冷却速率为14.0～20.6℃/s时,初生α(Al)主要由球形、近球形晶粒和一些细小的蔷薇晶组成;当冷却速率为5.0～14.0℃/s时,初生α(Al)主要由球形和近球形晶粒组成.适宜的冷却速率为5.0～15.0℃/s.冷却速率越快,晶粒平均直径越细小,但形状因子越低.     

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。     

过共晶球铁凝固过程中奥氏体的生长方式与形貌特征

采用着色腐蚀技术显示出过共晶球铁中的高温凝固组织，观察分析过共晶球铁凝固过程中奥氏体的生长方式与形持征。结果表明：在非平衡凝固条件下，过共晶球铁凝固时通常会析出初生枝晶和晕圈枝晶，其形成条件主要与冷却速率和熔体的过冷有关。随铸件模数Mc增大，枝晶数量减少，二次臂间距显著增大，形态趋于不发达，由初生枝晶向晕圈枝晶过渡。当Mc≤0.3cm时，二次臂间距的实测值与理论预测值基本吻合。初生石墨球周围往往形成环状封闭奥氏体壳；共晶前期石墨球周围形成封闭或不封闭的框架奥氏体壳；共晶后期石墨球往往被周边生长着的共晶奥氏体所包覆，最终成为共晶奥氏体的一部分。     

一种带状细晶铁素体的成因及其影响因素

以温轧铁素体、珠光体为原始组织，研究了加热速度和冷却速度对带状细晶铁素体形成的影响；结果表明：带状细品铁素体是当加热速度≥50℃／s时（冷却速度10℃／s）所形成的一种特征组织。随着加热速度的增加，细晶铁素体量随之增加，且带状特征更加明显；当冷却速度≥10℃／s时，其对该特征组织的影响与加热速度的影响类似。形成带状细晶组织的原因：一是原始组织中经温轧后的形变铁素体及部分破碎球化的珠光体提供了带状细晶奥氏体的形核环境；二是快速加热及冷却过程中奥氏体品粒的不均匀性。     

冷却速度对高铬铸铁热处理过程中组织转变的影响

研究了冷却速度对高铬铸铁热处理冷却过程中其微观组织转交的影响,并观察其微观组织.结果表明,在较低冷速下微观组织为典型的亚共晶白口铸铁组织形态,由初生奥氏体的低温转变组织和共晶体组成;在冷却速度为3℃/s时开始出现马氏体组织,并优先在共晶奥氏体区域大量形成;随着冷却速度的增加,马氏体量逐步增多,在10℃/s冷速下为连续的马氏体基体组织和共晶碳化物.二次碳化物在初生奥氏体区大量弥散析出,而在共晶奥氏体区没有二次碳化物生成.随着冷速的增加,初生奥氏体和共晶奥氏体区的珠光体片层间距均逐渐减小.     

定向凝固恒速和跃迁加速下Cu-12.58%Mg过共晶合金的凝固组织演变

在定向凝固恒速和跃迁加速下,研究了Cu-Cu2Mg过共晶合金中Cu2Mg初生相和共晶组织的变化。结果表明,在定向凝固速率5～100μm／s下,Cu2Mgg初生相领先共晶组织生长,生长方向与定向凝固方向一致,界面形态从侧枝较少的树枝晶变为间距较小的细枝晶形态。在定向凝固速率从2μm／s跃迁加速到20μm／s下,生长Cu2Mg初生相枝晶有的被抑制停止生长,有的出现细化,面共晶组织细化是通过层片中Cu相分枝进行的。研究表明,通过定向凝固跃迁加速工艺方法,可使Cu—Cu2Mg共晶组织层片间距更加细化。     

过冷Fe_(82)B_(17)Si_1合金的再辉效应模拟及组织演化

通过熔融玻璃净化与循环过热相结合的方法获得过冷度ΔT=6~280 K范围内的Fe82B17Si1共晶合金的凝固组织演变;结合突变方程和JMAK模型拟合凝固过程的冷却曲线,拟合结果符合Fe82B17Si1共晶合金的组织类型及形态随过冷度的变化规律.结果表明,当6 K≤ΔT75 K时,Fe82B17Si1合金中形成了复杂规则共晶及准规则共晶组成的混合共晶组织;当75 K≤ΔT180 K时,凝固组织由混合共晶组织和深过冷非规则共晶组织组成;当180 K≤ΔT250 K时,凝固组织由不同含量的初生a-Fe相和枝晶间深过冷非规则共晶组织组成;当ΔT250K时,凝固组织为完全非规则共晶组织.     

MICROSTRUCTURE AND EXTENDED SOLID SOLUBILITY OF Tb IN RAPIDLY SOLIDIFIED Ag-Tb ALLOY

The 0,05—0.08 mm thick splat foils of Ag-Tb alloy,containing Tb up to 10 at.-%,were made by the hammer-anvil technique.The cooling rate was in order of 10~6 K/s.The metastable extension of solid solubility of Tb in the alloy was measured to be up to 5 at.-% by lattice parameter method.The secondary phase in metastable eutectic structure was identi- fied as Ag_3Tb from diffraction pattern.The microstructure and micro-defect in as-quenched Ag-Tb splat foil were also observed.     

深过冷Ni-21.4%Si共晶合金的凝固特性研究

对熔融玻璃净化后深过冷Ni-21.4%Si（原子分数,下同）共晶合金的凝固特性进行了实验研究,并对其均质形核过冷度进行了理论预测.结果发现,采用熔融玻璃净化可使Ni-21.4%Si共晶合金获得318 K的过冷度.理论计算表明,此过冷度达到了Ni-21.4%Si共晶合金的均质形核过冷度.Ni-21.4%Si共晶合金凝固特性与过冷度△T有关:当过冷度小于250 K时,冷却曲线有2个再辉峰,其中当过冷度小于206 K时,凝固组织由Ni3Si相和规则共晶组成,当过冷度在206 K到250 K之间时,凝固组织由α-Ni相和规则共晶组成;过冷度大于250 K后,冷却曲线只有1个再辉峰,凝固组织为反常共晶.过冷度会影响初生相Ni3Si的生长方式.随着过冷度的增大,初生相Ni₃Si的生长会由小平面生长方式转为非小平面生长方式.     

Effects of cooling rate and B content on microstructure of near-eutectic Al-13wt%Si Alloy

Under cooling rates of 2 ℃/s and 10℃/s, the influences of B content on the microstructure of near eutectic Al-13.0wt%Si alloy have been investigated. Results showed that the addition of boron resulted in refinement of eutectic grains, and to some extent, had an inhibiting effect on precipitation of the primary phases, and the refining and inhibiting effects are much more obvious at higher cooling rate. When B was not added, higher cooling rate promoted the α-Al dendrites formation. At lower cooling rate, the addition of B did not cause the so called ＂columnar to equiaxed transition （CET）＂, however, at higher cooling rate, this transition was obvious. After the addition of B, the nucleation temperature TN ascended and nucleation mode changed from nucleation mode of from wall towards centre （without B addition） to a nucleation mode that the eutectic nucleated evenly throughout whole sample （with B added）. It can be concluded that the addition of B offers a large amount of nuclei for eutectic solidification, as a result, the eutectic grains was refined. Higher cooling rate will lead to more nuclei, so the effects on the refinement of eutectic grains and on suppression of primary phases are increased.     

Microstructure transition from stable to metastable eutectic growth in Ni–25%Al alloy

The microstructural evolution during directional solidification of the Ni–25%Al (mole fraction) alloy was investigated in the range of growth velocity from 10 to 100 μm/s under a given thermal gradient of 10 K/mm. The solidification microstructures reveal a transition from γ‘–β equilibrium eutectic to γ–β metastable eutectic plus β dendrites. A mixed microstructure of γ‘–β and γ–β eutectics produced at a growth velocity of 25 μm/s illustrates that the transition occurs during the competitive growth between γ and γ' phases. The growth temperature for each phase was considered to understand the microstructure selection during solidification. The experimental results show that a phase or a microstructure solidifying with the highest temperature under a given growth condition is preferentially selected upon solidification. In addition, both stable eutectic and metastable eutectic are shown to coexist and simultaneously grow in the velocity range between 25 and 60 μm/s due to their similar growth temperatures.     

Microstructure and Mechanical Properties Determined in Compressive Tests of Quasi-Rapidly Solidified NiAl-Cr(Mo)-Hf Eutectic Alloy After Hot Isostatic Pressure and High Temperature Treatments

The effect of high cooling rate of approximately 10² K/s and subsequent hot isostatic pressure (HIP) and high temperature (HT) treatment on the microstructure and mechanical properties of NiAl-based intermetallic alloys was investigated. The results reveal that rapid solidification refines the microstructure of the NiAl-Cr(Mo)-0.5Hf eutectic alloy and transforms the Ni₂AlHf Heusler phase, which is present in the equilibrium state to a metastable Hf(Ni, Al, Cr) solid solution phase. Simultaneously, the shape and distribution of the Hf(Ni, Al, Cr) solid solution phase were considerably improved. After the HIP treatment, the Hf(Ni, Al, Cr) solid solution phase has changed from continuous distribution along eutectic cell boundaries into semicontinuous distribution, and the primary NiAl(Cr, Mo) phase has coarsened. The HT treatment reduces the volume fraction of the primary NiAl(Cr, Mo) phase and optimizes the distribution of the Hf(Ni, Al, Cr) solid solution phase. Rapid solidification and the resulting fine-grained microstructure will significantly improve the mechanical properties of the alloy in compression tests. However, additional HIP and HT treatments enhance the high-temperature strength properties obviously.     

Tb在快速凝固Ag合金中固溶度扩展

采用电弧熔化快速锤砧急冷技术制备含Tb直到10at.-%的Ag-Tb合金箔,厚度0.05—0.08mm,其冷却速度为10~6K/s数量级,采用晶格参数法确定Tb在快速凝固Ag-Tb合金中亚稳扩展固溶度可达到5at-%,确定亚稳共晶中的第二相为Ag_3Tb,观察了快凝Ag-Tb合金的显微结构与缺陷。     

内生晶相对Cu46Zr46Al8非晶复合材料力学性能的影响

通过不同退火工艺分别制备出了含内生晶相的Cu46Zr46Al8非晶复合材料,并研究了其显微组织及室温压缩性能.结果表明,在淬火态非晶复合材料的晶化组织中出现了亚稳初生相与共晶共存的形态,这种组织形态是导致其力学能大幅度降低的主要原因.通过控制退火工艺在Cu46Zr46Al8非晶中内生出不同形态和尺寸的晶体相.枝晶臂细且长的树枝晶对材料的强度有较大的危害作用,而细小的胞状初生相晶粒则对材料的强度提高有一定的作用.     

定向凝固Cu-1.0%Cr合金中带状组织及其形成机制

在温度梯度为200 K/cm和凝固速率为2-10μm/s下, Cu-1.0%Cr亚共晶合金的凝固组织中分别存在单相平界面和共晶、单相平界面和胞晶及胞间共晶、单相胞晶和共晶3种带状组织.从实验和理论两方面阐明了上述带状组织的产生与合金定向凝固界面前沿的溶质分布有关;形成的原因是合金未达到稳态凝固,凝固速率和合金成分一直处于变化之中.另外,单相平界面和共晶的带状组织中,合金凝固速率在单相平界面凝固中随着界面液相成分的增加而逐渐增大,而在共晶组织中随着界面前沿液相成分的减少而逐渐减小.     

Computation of metastable phases in tungsten-carbon system

Metastable phase equilibria in the W-C system are presented in the vicinity of the metastable reactions involving W₂C, WC_1−x , and WC. Metastable phase boundaries were obtained by reproducing the stable boundaries using optimized Gibbs energy formulations and extrapolating them into regions of metastability. Four metastable reactions were obtained: a metastable congruent melting reaction of WC at 3106 K, a metastable eutectic reaction between WC_1−x and graphite at 2995 K, a metastable eutectic reaction between W₂C and WC at 2976 K, and a metastable eutectic reaction between W₂C and graphite at 2925 K. The reaction enthalpies and entropies associated with these transitions are also computed using the available Gibbs energy data. Furthermore, possible kinetic paths that could lead to metastability are discussed.