EFFECT OF Zr ON DUCTILITY OF CAST Ni_3 Al-BASE ALLOYS

Many compressive and tensile tests showed that the room temperature ductility of boron-freeNi_3Al alloy may be markedly improved by a moderate addition of Zr.The ductility of thestoichiometric Ni_3Al and even slightly enriched with Al,may be improved fairly between 400—600℃,which behaves much better if Zr is added together with B.However,the high tem-perature ductility of Ni_3Al alloys containing Cr may be significantly increased by Zr whichsegregates slightly along grain boundaries.Thus,the toughening mechanism of Zr alloyingseems to be different from that of B addition.     

STRENGTH-DUCTILITY AND γ'PRECIPITATES IN Ni-BASE ALLOYS

ＳＴＲＥＮＧＴＨ－ＤＵＣＴＩＬＩＴＹＡＮＤγ＇ＰＲＥＣＩＰＩＴＡＴＥＳＩＮＮｉ－ＢＡＳＥＡＬＬＯＹＳ￥ＷＡＮＧＺｈｉｘｉｎｇ;ＸＩＥＳｈｉｓｈｕ;ＬＵＪｕｎｙｉｎｇ;ＷＡＮＧＬｅｉ;ＹＡＮＧＨｏｎｇｃａｉ（ＮｏｒｔｈｅａｓｔｅｒｎＵｎｉｖｅｒｓｉｔｙ...     

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。     

NiAl力学性质合金化效应的第一原理计算

采用第一原理赝势平面波方法,计算几种合金化元素X(X=Cr、Mn、Fe、Co和Cu)与不同Fe含量(0,3.125%,4.167%,6.25%,摩尔分数)(NiFe)Al超胞的几何与电子结构,并采用如下几个力学参数:弹性常数C44、Cauchy压力参数(C12-C44)、弹性模量E和剪切模量G及比值G/B0等,表征和评判了合金化元素X与不同Fe含量对NiAl金属间化合物延性与硬度的影响。结果表明:高浓度(6.25%)合金化虽可提高NiAl晶体的硬度,但却导致NiAl延展性降低,合金化后NiAl硬度增加的次序为:(Ni7Mn)Al8>(Ni7Co)Al8>(Ni7Fe)Al8>(Ni7Cr)Al8>(Ni7Cu)Al8>NiAl,其延性降低次序则与硬度增加次序相反;随着Fe含量的升高,NiAl晶体的硬度增加,但其使硬度增加的上限约为4%,而随着Fe含量的降低,NiAl晶体的延性逐步增大,当Fe含量低到一定程度时,可改善NiAl晶体的本征脆性。     

热等静压工艺对K4125镍基高温合金显微组织的影响

采用3组不同参数的热等静压(HIP)工艺对K4125镍基高温合金进行显微组织演变研究。结果表明,3种热等静压工艺制备的合金中Hf、Mo等在MC碳化物中的分布区域略有不同,Ta、Mo、Co、Cr、Ti及Al等为正偏析元素,W、Ni为负偏析元素,与其他元素相比较,Ni、Co、Cr偏析程度较小,提高热等静压温度及压力,各元素的偏析程度均有所降低,γ/γ′共晶组织含量逐渐减少,同时枝晶干γ′相的尺寸显著降低,但面积分数无明显变化。此外,3种热等静压合金均出现大块状MC碳化物碎化、二次MC碳化物析出及晶界细小碳化物形成等现象,提高热等静压温度及压力后,这种现象加剧,且碳化物中Ta、Ti含量降低,TaC、TiC的分解倾向增加。     

Fe和Mo元素对TiAl金属间化合物显微组织和性能的影响(英文)

β相可以提高TiAl金属间化合物的塑性。通过显微组织分析和变形行为的评估研究β稳定性元素Fe和Mo对Ti-45Al-xFe-yMo(x,y=1,2,3,4)合金的影响。结果表明:合金中的B2(β)相随着Fe和Mo元素含量的增加而增多,Mo表现出强的β稳定性。加入3%Fe和2%Mo后,合金的晶粒得到细化,其尺寸达到12-m。由于具有一定量的β相,细化后的Ti-45Al-3Fe-2Mo合金在790℃具有良好的塑性。     

粉末冶金TiAl基合金显微组织及力学性能的研究

采用粉末冶金方法制备多种成分的ＴｉＡｌ基合金,并研究其显微组织及室温、高温力学性能,结果表明,采用粉末冶金方法能制备成分均匀、显微组织细小的Ｔｉ－Ａｌ－Ｃｒ－Ｎｂ系列合金。添加合金元素对粉末冶金ＴｉＡｌ基合金的显微组织具有显著影响。粉末冶金ＴｉＡｌ基合金的力学性能与其显微组织有密切的关系,显微组织越细小,其室温强度及延性越高,但在高温下,其屈服强度随晶粒尺寸增加而增加。所制备出的Ｔｉ－４７Ａｌ－３     

Atomic site location by channelling enhanced microanalysis (ALCHEMI) in γ′-strengthened Ni- and Pt-base alloys

The additions of alloying elements to Ni- and Pt-base alloys influence the microstructure and thereby the creep properties, whereas the mechanism is uncertain. Therefore atomic site location by channelling enhanced microanalysis (ALCHEMI) was used to determine the site partitioning of ternary and quaternary alloying elements in the L1₂-ordered γ′-phase. Two ternary Ni–Al alloys with Cr and Ti additions were investigated. The measured site partitioning showed that Cr and Ti atoms prefer the Al-sublattice sites. For a ternary Pt–Al–Cr alloy, it was found that Cr atoms occupy Al sites. The influence of Ni as a fourth alloying element in a Pt–Al–Cr–Ni alloy on the site partitioning was also investigated. The detected results give evidence that in the quaternary alloy Cr and Ni atoms prefer the Pt sublattice. First principles calculations were used to support the experimental data.     

Microstructure and mechanical properties of hypereutectic Al-Fe alloys prepared by semi-solid formation

The effects of alloying elements, electromagnetic stirring, reheating and semi-solid formation on the microstructure and mechanical properties of Al-Fe alloys prepared by semi-solid formation were studied. It was found that alloying elements and electromagnetic stirring can alter the morphology and growth mode of the iron-rich phase in Al-Fe alloys; and effectively refine the primary Al3Fe phase. In contrast to the microstructure obtained in conventional casting, the Al3Fe phase becomes thin short rod-like instead of thick needle-like; and the dendritic grain structure almost disappears in the semi-solid formation. The Al3Fe phase can be further refined through being dissolved or fused during subsequent reheating. It was also found that the larger extrusion ratio of semi-solid formation causes a greater crushing effect and therefore the Al3Fe phase is more refined and has more uniform distribution. Moreover, Al-Fe alloys prepared by semi-solid formation exhibit excellent mechanical properties at both room and high temperatures.     

Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl

The microstructure and mechanical properties of eight γ-TiAl based alloys with compositions in the range Ti–44Al–8(Nb,Ta,Zr,Hf)–(0–0.2)Si–(0–1)B have been investigated to assess the possibility of improving the properties of γ-TiAl through heavy alloying. It has been shown that the microstructures of these alloys can be significantly different from those of the binary or 48–2–2 type alloys as a result of differences in the phase equilibria. As expected with large additions of beta stabilisers such as Nb, Zr and Ta, the beta phase was stabilised to much lower temperatures than that in the Ti–44Al binary alloy. In some of the alloys the ω phase, which is a transformed product of the beta phase, is stable at room temperature and up to >900°C. In alloys which contain both beta- and gamma- stabilisers, there is no single α phase field in the transformation sequence and instead there is a (α+β+γ) three phase regime. The mechanical data obtained from these alloys indicate that heavy alloying can be used to increase the strength and creep resistance of γ-TiAl significantly although ductility generally remains poor. The addition of boron appears to be beneficial in that both strength and ductility are improved, particularly for materials with the duplex microstructure.     

CONCENTRATION DISTRIBUTION OF ALLOYING ELEMENTS IN SURFACE LAYER OF Ti ALLOY MELTS

The concentration distribution of alloying elements such as Al,Sn,V,Si and Mo in surfacelayer of quenched Ti alloy melts(TC4,TA 7 and TC9)has been determined by EPMA.Ti al-loy samples were melted and evaporized by electron beam in water cooled copper curcible.Theactivity coefficient of alloy elements in Ti alloy melts are:γ_(Al)=0.009—0.018 and γ_(Sn)=0.066—0.123 at 1921—2106℃;γ_V=0.713 at 2021℃;γ_(Si)=0.020 and γ_(Mo)=0.913 at 1921℃.Therate controlling steps of the evaporation of alloying elements Al,Sn,V,Si and Mo from Ti al-loy melts have been discussed with the data of evaporation activation energies of such alloy el-ements.     

Effects of alloying elements on thermal expansions of γ-Ni and γ′-Ni3Al by first-principles calculations

The effects of alloying elements (Al, Cr, Hf, Pt, Y, and Zr) on the coefficients of thermal expansion (CTE) of face-centered-cubic Ni (γ) and L1₂-Ni₃Al (γ′) phases have been investigated in terms of the first-principles quasi-harmonic approach by considering both the vibrational and thermal electronic contributions. By combining the present CTE data and the compositions and phase fractions of γ and γ′, the CTE for some Ni-Al-base alloys are predicted and compared with available experimental measurements in the literature, showing good agreement. It is observed that the addition of Pt is effective in reducing the CTE of both γ and γ′ phases, resulting in lower expansion mismatch between the bond coat and the thermally grown oxide alumina layer, as shown by the predicted CTE of Ni-22 Al-x Pt (at.%) alloys (x = 5, 10, 15, 20, 25, 30).     

Fe_3Al系合金环境氢脆研究SCIEI

研究了晶粒尺寸、形状、应变速率及预氧化对Ｆｅ_３Ａｌ系合金环境氢脆的影响，结果表明，细化晶粒能显著提高Ｆｅ_３Ａｌ合金在纯氧中的塑性及强度，但不能根本改善其环境氢脆敏感性；随着应变速率的提高，Ｆｅ_３Ａｌ合金在空气中的延伸率增加；Ｆｅ_３Ａｌ在空气及氢气中的脆化程度相似；预氧化能部分消除Ｆｅ_３（Ａｌ，Ｃｒ，Ｂ）合金的环境氢脆。实验结果可用表面化学反应是Ｆｅ_３Ａｌ材料环境致脆的控制步骤这一假设予以解释．     

Partition behavior of alloying elements and phase transformation temperatures in Co–Al–W-base quaternary systems

The phase equilibria among γ (A1), γ′ (L1₂), χ (D0₁₉), β (B2) and μ (D8₅) phases and the γ′ solvus and γ solidus temperatures were investigated in the Co–Al–W-based quaternary systems with alloying elements of Ti, V, Nb, Ta, Cr, Mo, Mn, Fe, Ni, Si, Zr, Hf, Ru and Ir by electron probe microanalysis (EPMA) using multiphase alloys and by differential scanning calorimetry (DSC). It was found that Ta, Nb, Ti, V, Mo and W are partitioned to the γ′ or χ phase rather than to the γ phase, while Cr, Mn and Fe tend to be distributed to the γ phase. The correlation between the partition coefficient of alloying elements between γ/γ′, γ/χ and γ/β phases and ab initio formation energy of Co₃X (L1₂), Co₃X (D0₁₉) and CoX (B2) was respectively obtained. It was also found that the γ′ solvus temperature increases by the addition of the γ′ former elements such as Ta, Nb and Ti, which decreases the γ solidus temperature.     

冶炼工艺对0Cr15Ni25WMoTi2NbAl合金力学性能的影响

研究了真空感应 电渣和非真空感应 电渣两种工艺冶炼的0Cr1 5Ni2 5WMoTi2NbAl合金的杂质元素含量、组织与性能。因真空感应冶炼过程中晶界偏聚元素Pb得到了有效挥发 ,减轻了晶界的弱化效应 ,因而该合金中高温塑性、持久强度均不同程度提高。随着中温形变速率的降低 (拉伸、大应力持久、小应力持久 ) ,两工艺合金的晶界弱化效应加剧。未发现两者的基本组织以及晶界沉淀相的种类、数量、形貌和分布有明显差异。     

Ni3Al中Al含量对硼韧化作用的影响

本文利用“有序—无序”模型对Ni_3Al中的点阵空位与成分的关系进行了计算,结果证明,当Al含量超过化学计量值时,Ni_2Al中有大量的点阵空位存在,称之非化学计量空位。正电子湮没技术证实了这种空位的存在。由此解释了Al含量对硼韧化作用的影响。     

SOLIDIFICATION OF NICKEL-BASED SINGLE CRYSTAL SUPERALLOY BY ELECTRIC FIELD

The crystal growth of a nickel-based single crystal superalloy DD3 was researched via controlled directional solidification under the action of a DC electric field. The cellular or dendrite spacing of the single crystal superalloy is refined and microsegregation of alloying elements Al, Ti, Mo and W, is reduced by the electric field. The electric field decreases the interface stability and reduces the critical growth rate of the ceUular-dendritic translation because of Thomson effect and Joule heating. The precipitation of the γ＇ phase is more uniform and the size of the γ＇ phase is smaller with the electric field than that without the electric field.     

SLIP FEATURE AND GRAIN BOUNDARY BEHAVIOURS OF Ni_3Al ALLOYS DURING DEFORMATION

The deformation behaviours of Ni_3Al alloys with various chemical compositions and subjectedto different heat treatments were in situ observed under SEM. Moreover,in situ observationsof slip trace were supplemented by the direct observation of dislocation arrangements underTEM.In B-doped Ni_3Al alloys it is shown that close to the grain boundary there exists a thinslip transition region,within which slip lines are reoriented or other slip systems are operatedto produce a local strain accommodation and to relax the stress concentration at grain bound-aries.Boron seems to lower the stress for dislocation generation from the grain boundaries orto ease the cross-slip close to the boundaries,and also to increase the number of dislocationsources.However,B-enhenced ductility is seriously affected by stoichiometry,the addition ofa tertiary alloy element and heat treatment,etc..     

Mn,B复合合金化对Ni3Al室温塑性的影响

本文通过宏合金化和微合金化结合的方法,进一步改善了Ni_3Al的室温塑性,在本实验条件下,按最佳硼合金化配制的Ni_3Al合金的室温拉伸延伸率为14%,而复合合金化得到的Ni_3Al则为35%。同时,实验结果还表明,只有Al当量含量(Mn%+Al%)小于化学计量值时,才能发挥复合合金化的优势。     

Grain-boundary segregation of impurities in iridium and effects on mechanical properties

Impurity effects were investigated in an Ir alloy of nominal composition Ir–0.3 W–0.006 Th–0.005 Al¹. The impurities that were added included Fe, Ni, Cr, Al, and Si, at levels ranging from 50 to 5000 ppm. Of the elements investigated (impurities as well as alloying additions), only Si and Th were found to segregate to the grain boundaries. In alloys doped with low levels of Si (<200 ppm), both Si and Th segregated to the grant boundaries. In addition, Th-containing precipitates were identified in these alloys, but not Si-containing precipitates. When added at higher bulk levels, Si displaced (beneficial) Th from the boundaries. This caused the segregated Th at the boundaries to decrease (to practically zero) when the amount of grain-boundary Si increased (to its saturation level). Silicon also reacted with Th to form Ir–Th–Si precipitates and promoted the formation of a low-melting eutectic on the grain boundaries of the highest Si alloys. As for the other elements, not only did they not segregate to the grain boundaries, but they also did not affect Th segregation. Together, these results explain why Fe, Ni, Cr, Al (and Si at low levels) do not embrittle iridium, whereas Si at high levels causes severe embrittlement.