The influence of grain size and composition on 1000 to 1400K slow plastic flow properties of NiAl

The compressive slow plastic flow behaviour of several B2 crystal structure NiAl intermetallics has been studied in air between 1000 and 1400 K ( 0.53 to 0.74T _M). Small grain-sized Ni-48.25 at % Al ( 5 and 10 µm) was found to be stronger than the previously studied 17 µm diameter material. While grain refinement improved the strength at all test temperatures, the exact mechanism is not clear. Experiments at lower temperature revealed that composition as well as grain size can be an important factor, since Ni-49.2Al was weaker than Ni-48.25 Al. Pronounced yield points were found during slow strain-rate testing at 1000 K; however, continued deformation appears to take place by the same mechanism(s) as found at high temperatures. Small changes in thermomechanical processing (TMP) schedules to fabricate Ni-49.2 Al indicated that basic deformation characteristics (stress exponent and activation energy) are not affected; however, the pre-exponential term could be modified if TMP alters the grain structure.     

Effect of composition and grain size on slow plastic flow properties of NiAl between 1200 and 1400 K

A series of 15 m diameter, polycrystalline B2 crystal structure NiAl alloys ranging in composition from 43.9 to 52.7 Al (at%) have been compression tested at constant velocities in air between 1200 and 1400 K. All materials were fabricated via powder metallurgy techniques with hot extrustion as the densification process. Seven intermediate compositions were produced by blending various amounts of two master heats of prealloyed powder; in addition a tenth alloy of identical composition, 48.25 AI, as one of the blended materials was produced from a third master heat. Comparison of the flow stress-strain rate behaviour for the two 48.25 AI alloys revealed that their properties were identical. The creep strength of materials for AI/Ni 1.03 was essentially equal, and deformation could be described by a single stress exponent and activation energy. Creep at low temperatures and faster strain rates is independent of grain sizes and appears to be controlled by a subgrain mechanism. However, at higher temperatures and slower strain rates, diffusional creep seems to contribute to the overall deformation rate.     

BDT and Creep Behaviors of NiAl-25 at. pct Cr Alloy at Various Temperatures

1.IntroductionIntermetallic compound of NiAl with B2crystal struc-ture is regarded as a potential candidate of high tem-perature structural materials because it offers attractivechemical and physical properties,such as high meltingpoint,low density,good thermal conductivity,high resis-tance to oxidation and high stiffness[1,2].Unfortunately,this kind of intermetallic compound shows limited tough-ness at ambient temperatures and poor strength at hightemperatures.An efficient way to prepare int…     

Effect of NiO and/or TiO2 mullite formation and microstructure from gels

Polymeric and colloidal gels with a constant molar ratio of (Al+Ni and/or Ti)/Si=3/1 and various (Al/Ni and/or Ti) ratios (up to 21.42 mol% NiO+TiO2) were prepared and used to study the effect of the precursor chemical homogeneity on mullite formation processes and the resulting microstructure. Both kinds of gel precursors were preheated at 750°C for 3 h in order to obtain appropriate gel-derived glasses for further thermal processing. After annealing for several time periods at temperatures between 750 and 1500°C, differences in crystallization pathways were observed. Polymeric gels crystallized Al–Si and NiAl2O4 spinels from the amorphous form at temperatures in the range between 900 and 1000°C, depending on the amount of aluminium substitution. Mullite formation was initiated at temperatures between 1100 and 1200°C, except for the higher substituted 3:2 mullite in which it was produced at 1000°C. In constrast, -Al2O3 and NiAl2O4 spinel were the first crystalline phases identified at 750°C in specimens from colloidal gels, whereas mullite was formed at temperatures higher than 1200°C. In specimens with high substitution, mullite was observed at lower temperatures. Although the sequences of reaction from either kind of gel were rather different, mainly at low temperatures (as could be inferred from the chemical homogeneity attained in both gel-derived glasses), the final set of crystalline phases after long annealing at 1400°C was quite similar. Differences in the microstructure of specimens from either type of gel precursor after annealing at 1400°C concerned the size of mullite particles and the presence of secondary phases in specimens derived from colloidal precursors. © 1998 Kluwer Academic Publishers     

Discontinuously reinforced intermetallic matrix composites via XD synthesis

Abstract

A review is given of recent results obtained for discontinuously reinforced intermetallic matrix composites produced using the XD process. Intermetallic matrixes investigated include NiAl, multiphase NiAl+Ni₂AlTi, CoAl, near γ titanium aluminides (i.e. TiAl+Ti₃Al), and L1₂ trialuminides containing minor amounts of second phase. A judicious match of the matrix and reinforcement can enhance both physical and mechanical properties. Particulate reinforcements can affect properties directly by enhancing modulus and strength or indirectly by refining grain size, enhancing microstructural homogeneity in a casting, or providing additional nucleation sites which improve phase transformation kinetics. Such mechanical properties as low and high temperature strength, compressive and tensile creep, elastic modulus, ambient ductility, and fracture toughness are discussed as functions of reinforcement size, shape, and volume fraction. Microstructures before and after deformation are examined and correlated with measured properties. An observation of interest in many of the systems examined is ‘dispersion weakening’ at high temperatures and high strain rates. This behaviour is not specific to the XD process; rather similar observations have been reported in other discontinuous composites. Proposed mechanisms for this behaviour are presented.

MST/1561     

Nanoscale Grain Growth Behaviour of CoAl Intermetallic Synthesized by Mechanical Alloying

Grain growth behaviour of the nanocrystalline CoAl intermetallic compound synthesized by mechanical alloying has been studied by isothermal annealing at different temperatures and durations. X-ray diffraction method was employed to investigate structural evolutions during mechanical alloying and annealing processes. The disordered CoAl phase with the grain size of about 6 nm was formed via a gradual reaction during mechanical alloying. The results of isothermal annealing showed that the grain growth behaviour can be explained by the parabolic grain growth law. The grains were at nanometric scale after isothermal annealing up to 0·7 T _m. The grain growth exponent remained constant above 873 K indicating that grain growth mechanism does not change at high temperatures. The calculated activation energy indicated that the grain growth mechanism in the disordered CoAl phase at high temperatures was diffusing Co and Al atoms in two separate sublattices. Furthermore, an equation has been suggested to describe the grain growth kinetics of nanocrystalline CoAl under isothermal annealing at temperatures above 873 K (T/T _m ≥ 0·5).     

氧化铝陶瓷的高温磨损与自润滑机理研究

研究了Al₂0₃陶瓷从室温至12000℃在于摩擦条件下的高温摩擦磨损行为。结果表明：在600℃以后的摩擦磨损随温度上升而逐渐减小,在1200℃的摩擦系数仅为室温的60％磨损则降低了两个数量级,表现出良好的高温自润滑特征。在不同温度下,存在三种显著不同的磨损机理．从室温至600℃,主要是磨粒磨损和微断裂,磨损随温度上升而略有增加．在600～1000℃,磨损机理逐渐由脆性断裂过渡到塑性变形和再结晶,在表面形成一个厚度为5～10μm的、类似于纳米材料结构的特殊表面层．随着这种特殊表面层的形成,磨损显著下降．在1200℃,摩擦表面由塑性变形发展到软化状态,出现流体动力润滑,使摩擦磨损进一步降低．     

Low-Temperature Internal Friction and Thermal Conductivity of Plastically Deformed, High-Purity Monocrystalline Niobium

The low-temperature internal friction Q ^–1 and thermal conductivity of plastically deformed, high-purity niobium monocrystals have been investigated and compared with measurements on an amorphous SiO₂ (a-SiO₂) specimen. After plastic deformation at intermediate temperatures, an approximately temperature independent internal friction Q ^–1 was observed with a magnitude comparable to that of the a-SiO₂ specimen. Plastic deformation at low temperatures leads to an internal friction Q ^–1 with a considerably smaller magnitude. In the temperature range between about 0.3 and 1.5K, the lattice thermal conductivity k of the deformed specimens decreases with increasing deformation. It is, however, nearly independent of the amount of deformation at the lowest temperatures investigated. In this temperature regime, the lattice thermal conductivity of the specimens varies proportional to T ³ and has a magnitude as would be expected for an undeformed sample. Additional heat release experiments on an undeformed sample clearly show no long-time energy relaxation effects. We conclude that the defects introduced by plastic deformation cannot be described with the tunneling model which had been proposed to describe the low temperature elastic and thermal properties of amorphous solids. The phonon scattering mechanisms observed in deformed niobium are tentatively related to the dynamic interaction of phonons with geometrical kinks in dislocations.     

Creep of hot-pressed silicon nitride

Creep tests were undertaken on hot-pressed silicon nitride in the temperature range 1200 to 1400° C. The activation energy for creep was determined to be 140 kcal mol^?1 and the stress exponent of creep rate was 1.7. The creep behaviour is ascribed to grain-boundary sliding accommodated by void deformation at triple points and by limited local plastic deformation. Electron microscopic evidence supporting this mechanism is presented.     

Structural defects in β phase FeAl

The density of B₂ type FeAl ordered alloys has been measured at room temperature. When the Al concentration is higher than 49 at % it is observed that the experimental values are lower than the calculated ones assuming two atoms per unit cell. Such a behavior is very similar to what happens in well known NiAl and CoAl β phases, and could be connected with the presence of structural vacancies.     

微量磷(P)对等原子比NiAl的微观组织与力学性能的影响

研究了微量P对挤压态等原子比NiAl的微观组织与高温力学性能的影响.结果表明:微量P的添加对NiAl的晶格常数有一定的影响,P偏聚于NiAl晶界处;并对其高温延伸率有重要影响.P偏聚于晶界阻碍了合金变形过程中的动态回复和再结晶,加剧了晶界处孔洞的形成,造成了NiAl-P合金与二元NiAl合金高温力学性能的显著差异,主要表现在:应力-应变曲线经历了较长的加工硬化阶段;最大延伸率明显下降;变形激活能升高,应变速率敏感指数下降.NiAl-P合金的高温变形机制为变形过程中位错的滑移与攀移共同作用.     

Tensile properties and microstructure of in situ NiAl–Cr (Zr) eutectic composite

An in situ NiAl–33.5Cr–0.5Zr composite was prepared by vacuum induction melting followed by hot isostatical pressing (HIP) at 1573 K and 100 MPa for 3 h. The microstructure of the composite is composed of NiAl matrix and Cr phase with small amount of Ni₂AlZr and Zr-rich phases distributing at the interfaces of NiAl and Cr phases. It is found that the composite is stronger and more ductile than that of some NiAl-based alloys at high temperature. However the composite is still brittle at room temperature and becomes ductile with the increase of testing temperature. The brittle-to-ductile transition temperature (BDTT) is the function of initial strain rate. Under the creep deformation at 1273 K and 100 MPa, the minimum creep rate of the composite is three orders lower than that of the polycrystalline NiAl and single crystalline NiAl [0 0 1]. TEM observations on the deformed samples of both tension and creep show that dynamic recrystallization has occurred during high temperature deformation process.     

Shock experiments on quartz targets pre-cooled to 77 K

The influence of temperature on the development of shock deformation effects in quartz single-crystal targets has been investigated in this parameter study. Quartz targets were pre-cooled to 77 K and shocked parallel to their (101^;−0) orientation in shock reverberation experiments of microsecond shock duration. Shock deformation effects in quartz were characterized by optical and spectroscopic methods. The degree of shock metamorphism in pre-cooled quartz targets has then been compared with literature data from μs-shock reverberation experiments on quartz targets shocked parallel to the same crystallographic orientation but at initial temperatures of 293-903 K.The degree of shock metamorphic overprint in quartz was found to be rather insensitive to temperature variation in the low-temperature region (<548 K), but sensitive to temperature variation in the high-temperature region up to 903 K. This temperature dependence allows to distinguish between different mechanisms that had been proposed to explain the formation of shock deformation effects in quartz. The identified temperature dependence points toward a simple mechanism in which the crystal lattice collapses due to overpressure at low to moderate temperatures. Very high temperatures result in weakening of the silicon-oxygen bonds that support the crystal lattice, leading to a decrease of the onset pressure for structural collapse.     

Low-temperature deformation mechanisms in an Fe-Si-Ti alloy

The temperature and strain-rate dependence of the yield and flow stresses of an age-hardened Fe-Si-Ti alloy, and a binary Fe-Si alloy, have been investigated in the temperature range 77 to 330 K. The parameters of activated deformation have been determined as a function of the precipitate dispersion and it is concluded that these parameters are unaffected by precipitation even when particles are sheared by dislocations, implying that bcc iron alloys cannot be dispersion strengthened at room temperature without making them even stronger, and hence more brittle, at lower temperatures. Work-hardening of all alloys is due to increases in the athermal component of the applied stress. The activation enthalpy at low effective stresses is larger than that for pure iron, indicating that silicon exerts a higher lattice friction stress due to its effect on the core asymmetry of screw dislocations in iron.     

High-temperature deformation and fracture behaviour of Cu-SiO2 bicrystals

Bicrystals of CU-SiO₂ dispersion-hardened alloys and of pure copper were tensile tested at various temperatures between 450 and 1050 K at a strain rate of 1.5 x 10^–4 sec^–1. In the case of pure copper bicrystals, elongation to fracture did not depend significantly on temperature and the fracture mode was invariably transgranular up to 850 K. On the other hand, the ductility of CU-SiO₂ bicrystals decreased with increase in temperature and the transition in the fracture mode from transgranular to intergranular occurred at around 450 K. SiO₂ particles on grain boundaries play an important role on intergranular fracture by suppressing grain-boundary sliding and also on the retardation of recrystallization during deformation. Two types of Cu-SiO₂ bicrystals having different crystal orientation relationships show quite different deformation and fracture behaviour. This can be explained in terms of the contribution of lattice dislocations to the grain-boundary sliding.     

High temperature dispersion strengthening of NiAl

A potential high temperature strengthening mechanism for alloys based on the intermetallic compound NiAl has been investigated. This study forms part of an overall program at NASA Lewis Research Center for exploring the potential of alloys based on NiAl for high temperature applications. An alloy containing 2.26 at% Nb and produced by hot extrusion of blended powders has been examined in detail using optical and electron microscopy. Interdiffusion between the blended niobium and NiAl powders results in the formation of intermediate phases. A fine dispersion of precipitates of a hexagonal, ordered NiAINb phase in a matrix of NiAl can be produced and this results in strengthening of the alloy by interfering with dislocation motion at high temperature. These precipitates are, however, found to coarsen during the high temperature (1300 K) deformation at slow strain rates and this may impose some limitations on the use of this strengthening mechanism.     

Chemical stability of titanium diboride reinforcement in nickel aluminide matrices

Chemical stability of titanium diboride (TiB₂) reinforcement in NiAl (45 at% Al) and Ni₃Al (24 at% Al) matrices has been theoretically and experimentally investigated. Calculations were made using thermodynamic properties of the systems to predict behaviour at temperatures between 1173 and 1573 K. Experimental investigation of hot-press consolidated TiB₂ particulate/prealloyed matrix powder blends were conducted using energy dispersive X-ray analysis, X-ray diffraction analysis, Auger electron spectroscopy and transmission electron microscopy. The theoretical and experimental analyses suggest that TiB₂ is chemically stable in both matrices up to 1573 K, however, TiB₂ was found to be less active in NiAl than in Ni₃Al due to lower nickel activity in NiAl.     

Deformation of Ni-Al at high temperature

Nickel-rich Ni-Al was deformed in the temperature range 720 K to 1200 K. The deformation mechanism was found to be independent of the composition. An anomalous behaviour of the activation volume is described in the stoichiometric compound and is attributed to the easy activation of [1 10] slip in the temperature range 850 K to 950 K.     

烧结温度对TiO2压敏陶瓷非线性和介电性质的影响

基于一次烧结工艺,通过改变烧结温度,制备5种组分相同、(Sr,Bi,Si,Ta)掺杂的TiO2陶瓷试样.借助于伏安特性、介电频率特性、损耗频率特性及非线性系数的测定,研究烧结温度对TiO2基压敏陶瓷压敏和介电性质的影响.结果表明,在1200～1400℃范围内,随着烧结温度的降低,陶瓷的压敏电压降低、介电常数增大,同时非线性系数有所减小.兼顾陶瓷压敏和介电特性,烧结温度选择1350℃为宜.     

Oxidation behaviour of a pressureless sintered AlN-SiC composite

The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y₂O₃. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300–1450°C for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400°C for 100 h, in relationship with the microstructure of the exposed surfaces.The material starts to oxidize notably at temperatures higher than 1300°C. The oxidation kinetics is parabolic in the temperature range 1350–1450°C, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina.The surface modification induced by long term oxidation does not affect mechanical strength until 1200°C, while after oxidation at 1400°C, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation.