Structure and strength of Al,Cu-Al3Ni directionally solidified composites

A series of Al₃Ni fiber reinforced composites with a matrix composition varying from pure aluminum to Al-3.3 wt pct Cu were prepared by directional solidification of Al-Ni-Cu alloys. The solidification conditions were kept constant in all cases atG/R ≃ 10⁴ °C · s/mm² (G is the temperature gradient andR is the growth rate). The mechanical properties of the composites were studied in the as grown and in the heat treated conditions and the results were discussed in terms of the structure and composition. With the techniques used, it was possible to preserve the Al-Al₃Ni eutectic composite structure while strengthening the matrix by copper addition. The addition of 1 wt copper to the matrix caused a considerable increase in the mechanical strength, especially after heat treatment, without affecting the ductility. Strength values of the order of 530 MN/m² were reached in the heat treated composites which is higher than predicted by the rule of mixtures. This is attributed to the high work hardening capacity of the matrix especially in the presence of θ’ phase. Massive Al₃Ni rods and dendrites caused premature fracture and reduction in the strength of the composites containing 2 and 3 wt pct copper. Eliminating these defects by using higherG/R values can produce composites with exceptionally high strength.     

Interfacial shear strength of cast and directionally solidified NiAl-sapphire fiber composites

The feasibility of fabricating intermetallic NiAl-sapphire fiber composites by casting and zone directional solidification has been examined. The fiber-matrix interfacial shear strengths measured using a fiber push-out technique in both cast and directionally solidified composites are greater than the strengths reported for composites fabricated by powder cloth process using organic binders. Microscopic examination of fibers extracted from cast, directionally solidified (DS), and thermally cycled composites, and the high values of interfacial shear strengths suggest that the fiber-matrix interface does not degrade due to casting and directional solidification. Sapphire fibers do not pin grain boundaries during directional solidification, suggesting that this technique can be used to fabricate sapphire fiber reinforced NiAl composites with single crystal matrices.     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C.     

An analysis of the mechanical behavior of Al-Al3Ni composites

Commercial purity Al-Al₃Ni eutectic composites have been prepared by directional solidification at growth rates ranging from 9.63 x 10^-3 to 1.0 mm/s. The composites were tested in tension and in compression and the results were analyzed using a simple model taking into consideration the difference in Poisson’s ratio of the phases, interfiber spacing, and discontinuity and premature fracture of fibers. The theoretically predicted values of the tangent modulus and strength in tension and compression were shown to closely fit the experimental results up to a growth rate of about 0.3 mm/s. Beyond this value, the excessive misalignment of the fibers caused some deterioration in the mechanical properties and a change in the mode of fracture. It has been concluded that the elastic constrained matrix exerts considerable effect on the mechanical properties thus providing an effective means of improving them by increasing the surface area of the fiber-matrix interface.     

An analysis of the fatigue behavior of Al-Al3Ni composites

Commercial purity Al-Al₃Ni eutectic composites have been prepared by directional solidification at growth rates ranging from 9.63 x 10^-3 to 1.0 mm/s. The composites were tested under rotating bending fatigue conditions and the results were analyzed using a model taking into consideration the difference in Poisson’s ratio of the phases, interfiber spacing and premature fracture of fibers. The calculated fatigue lives were shown to be in good agreement with the experimental results at all stress levels. Both the experimental results and calculations show that the composite fatigue life is not sensitive to large changes in the growth rate. The fatigue limit was also found to be insensitive to the mode of fatigue loading and notches. At high stress levels, however, smooth specimens tested under reversed bending exhibited longer lives than notched specimens tested under cyclic tension-tension conditions.     

Growth characteristics of directionally solidified Al203/YAG/ZrO2 ternary hypereutectic in situ composites under ultra-high temperature gradient

Oxide eutectic ceramic in situ composites have attracted significant interest in the application of high-temperature structural materials because of their excellent high-temperature strength,oxidation and creep resistance,as well as outstanding microstructural stability.The directionally solidified ternary Al_2O_3/YAG/ZrO_2 hypereutectic in situ composite was successfully prepared by a laser zone remelting method,aiming to investigate the growth characteristic under ultra-high temperature gradient.The microstrucnrres and phase composition of the as-solidified hypereutectic were characterized by using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and X-ray diffraction(XRD).The results show that the composite presents a typical hypereutectic lamellar microstructure consisting of fine A1203 and YAG phases,and the enriched ZrO_2 phases with smaller sizes are randomly distributed at the Al2O3/YAG interface and in Al_2O_3 phases.Laser power and scanning rate strongly affect the sample quality and microstructure characteristic.Additionally,coarse colony microstructures were also observed,and their formation and the effect of temperature gradient on the microstructure were discussed.     

Structure of directionally solidified two phase ternary alloys

Simple theories developed in a series of recent papers, describing solid/liquid interface stability and solute redistribution in three phase ternary alloys are summarized and extended to apply to ternary alloys that are two phase on equilibrium solidification. Calculated results compare well with experiments on alloys from the Al-rich corner of the Al-Cu-Ni ternary phase diagram. These alloys represent the type currently under development for gas turbine applications. At sufficiently high values ofG/R (thermal gradient divided by growth rate) the alloys grow with a fully plane front. At moderate values ofG/r they exhibit a unique structure of single phase cells and two phase intercellular root material which grows with a “plane front”. Further decreases inG/R produce more typical ternary cellular structures with regions of one, two and three phases. Formerly Graduate Student, Massachusetts Institute of Technology, Cambridge, Mass. Formerly Research Associate, Massachusetts Institute of Technology, Cambridge, Mass.     

Structure of directionally solidified two phase ternary alloys

Simple theories developed in a series of recent papers, describing solid/liquid interface stability and solute redistribution in three phase ternary alloys are summarized and extended to apply to ternary alloys that are two phase on equilibrium solidification. Calculated results compare well with experiments on alloys from the Al-rich corner of the Al-Cu-Ni ternary phase diagram. These alloys represent the type currently under development for gas turbine applications. At sufficiently high values ofG/R (thermal gradient divided by growth rate) the alloys grow with a fully plane front. At moderate values ofG/r they exhibit a unique structure of single phase cells and two phase intercellular root material which grows with a “plane front”. Further decreases inG/R produce more typical ternary cellular structures with regions of one, two and three phases.     

稀土元素Ce对Cu-Al2O3复合材料组织及性能的影响

采用机械合金化与放电等离子烧结的方法制备了不同质量分数的Cu-Al₂O₃-Ce复合材料,研究了稀土元素Ce对Cu-Al₂O₃复合材料显微组织形貌及硬度、抗拉强度、导电率、摩擦磨损等物理性能的影响。结果表明:Cu-Al₂O₃-Ce复合材料中的陶瓷颗粒更加均匀弥散的分布在基体中;加入稀土元素Ce后的Cu-Al₂O₃-Ce复合材料硬度为HV 108.2、拉伸强度为301 MPa、断面伸长率为19.6%、导电率为54.51 MS·m-1,与Cu-Al₂O₃相比有明显提升;Cu-Al₂O₃的磨损机理主要为磨粒磨损,Cu-Al₂O₃-Ce主要为黏着磨损,当摩擦速率较大时,Cu-Al₂O₃-Ce的摩擦系数和体积磨损率更小,耐磨性能优于Cu-Al₂O₃。     

Effect of shape variations on the structure of directionally solidified Al-Al3Ni composites

The effect on structure of some of the possible changes in shape of directionally solidified Al-Al₃Ni eutectic composites has been studied for two growth conditions. The shape changes investigated included both contraction and divergence in cross-section of the grown part, as well as 90 deg bends in the center-line of the composites. The experimental results showed that contractions in the solidifying cross section do not seriously affect the growth of composites, apart from some coarsening of the structure near the surface. Fiber branching took place in the case of gradual divergence in the solidifying cross section with the fibers deviating from the general growth direction by an angle determined by the shape of equitemperature contours during solidification. Sharp changes in growth direction, 90 deg bend, gave rise to nucleation of new grains of considerable misorientation and hypoeutectic alloys nucleated primary aluminum phase before the eutectic structure was established. The relatively large under cooling needed for nucleation gave rise to high local growth rates in the 90 deg bend area. As the Al₃Ni fibers grow at right angles to equitemperature contours during solidification, it is concluded that control of the composite structure can be achieved by controlling the growth conditions and mold design. Molten alloy-graphite reactions resulted in the formation of aluminum carbides which were more extensive at higher temperatures and longer exposure times.     

Structure and mechanical properties of the directionally solidified Al-Cu-Mg eutectic

The ternary eutectic is located at Al-33.1 pct Cu-6.25 pet Mg and consists of an aluminum-rich phase, CuAl₂, and CuMgAl₂. In this work its morphology, crystallography, interfacial dislocation arrangements, and mechanical properties have been studied. The scale of the phases is proportional to the negative one-half power of the growth rate. The phases have preferred growth directions and, in many cases, preferred interfacial planes. Interfaces between the aluminum-rich and the CuMgAl₂ lamellar phases are semicoherent, having a/2 〈110〉 {111} aluminum misfit dislocations spaced 300Å apart. The observed dislocations are imaged only in the aluminum and appear to be aluminum-phase slip dislocations. The deformation of the composite to failure in tension is macroscopically elastic, and the failure strength depends on growth rate. It is shown that Griffith brittle fracture theory may be applied to the failure process, with cracks developed in the CuMgAl₂ phase during loading acting as the required Griffith cracks. A specific failure mechanism is proposed and related to observations of the fracture surface.     

Creep regimes for directionally solidified Al-Al 3 Ni eutectic composite

Creep characteristics of Al-Al₃Ni eutectic composites directionally solidified at 2.2 × 10^-2 mm/s were determined over a wide range of stress and temperature. Four distinct regions of creep were observed. The rate controlling mechanisms for the four regions appear to be high-temperature dislocation climb in the Al matrix, low-temperature climb in the Al matrix, boundary sliding, and a mechanism involving deformation of the Al₃Ni fibers. Creep rates of the Al-Al₃Ni composite are several orders of magnitude smaller than for pure Al, and apparently, in the regions where deformation of the Al matrix is rate controlling, only a very small fraction of the matrix is deforming during creep of the composite. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Manitoba     

Influence of Cr and W alloying on the fiber-matrix interfacial shear strength in cast and directionally solidified sapphire nial composites

Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys reinforced with sapphire fibers. R. TIWARI, formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

Fiber strength and fiber/matrix bond strength in single crystal Al2O3 fiber reinforced Ni3Al based composites

A series of single-crystal A1₂O₃ fiber (Saphikon), reinforced Ni₃Al-based composites were fabricated by a liquid metal infiltration technique, pressure casting. Tensile testing and indentation techniques have been employed to measure fiber strength and fiber/matrix interfacial debond shear stress. The Weibull mean strength of the fiber has been observed to decrease drastically upon handling, exposure to high temperature, and casting. Alloying of Ni₃Al with Ti has resulted in a further decrease in fiber strength. Thermal expansion mismatch between the fiber and matrix led to the formation of compressive twins in the fiber. These twins, forming on planes, produced cracks at their intersections, which were parallel to the fiber axis,c-axis. Twin-induced fiber cracking was observed in all cases, but most predominantly when Cr was present. While addition of Cr at the 1 at. pct level had no appreciable effect on the interfacial debond shear stress, addition of 0.5 at. pct Cr resulted in an approximately threefold increase in debond stress, from 19 MPa to about 54.5 MPa. Alloying of Ni₃Al with Cr has also resulted in partial dissolution of the A1₂O₃ fiber. Addition of Ti had a moderate effect on increasing the fiber/matrix bond strength.     

B4C增强相颗粒粒度对Al基复合板材强度的影响

在颗粒增强相B4C质量分数相同的情况下,研究颗粒尺寸对B4C/6061Al复合材料轧制板材抗拉强度的影响。利用光学显微镜、扫描电镜、X射线衍射、能谱分析等对B4C/6061Al复合材料板材的组织和断口形貌进行观察和分析。实验结果表明,随B4C颗粒尺寸的减小,板材相对密度降低,抗拉强度提高,当加入颗粒尺寸不一的混合型B4C颗粒时,试件中的缺陷明显减少,相对密度显著提高,抗拉强度相对单一B4C粒度制备的复合材料提高18%;复合材料板材轧制方向和横向方向的试件拉伸试验表明,试样的纵向抗拉强度比横向提高15%。并探讨了B4C颗粒尺寸对复合材料板材强度的影响机理。     

新型高强抗热腐蚀定向凝固高温合金DZ409

在高温合金设计理论的指导下,综合考虑重型燃机叶片用耐热腐蚀定向凝固高温合金对持久强度、热腐蚀抗力、长期组织稳定性等多方面的要求,设计出 7 种成分的合金.通过热力学平衡相和电子空位数计算,优选3 种成分合金开展力学性能、抗热腐蚀、组织稳定性等研究,进行试验筛选工作,研制出新型抗腐蚀定向凝固高温合金 DZ409 合金.DZ409 合金具有良好的抗腐蚀性能、组织稳定性以及高的力学性能,可用作重型燃机的叶片材料.