Fe Ni Al Alloy Research Articles

Microstructure control in two-phase (B2+L1 2) Ni–Al–Fe alloys by addition of carbon

Carbon and titanium were added in two-phase (B2+L1 2) Ni–Al–Fe alloys for carbides precipitation, and the additions of 0.2 wt.% C and 3 at.% Ti have been found to produce the most adequate high temperature structural material. Carbon doped alloys showed more refined microstructure than carbon free alloys. The carbides, which were formed in the as-cast microstructure showing equiaxed dendrites, appeared to play a role in suppressing grain growth and coarsening of the second phases. The NAF29-10 alloy, including no carbides, showed the lamellar type microstructure, while the NAF29-10-C alloy, including carbides, showed a much more refined mesh type microstructure. When the carbon free alloys were quenched into water, cracks occurred at the grain boundaries probably due to martensitic transformation, however, these cracks were not observed in the carbon doped alloys. In order to suppress quenching cracks in the carbon free alloys, the solutionizing treatment time was reduced. However, cracks still formed as a severe intergranular mode at an early stage of elastic deformation. The refined microstructure of carbon doped alloys could compensate for elongation loss due to solution hardening and precipitation hardening. As a result, the carbon doped alloys showed good room temperature ductility and also showed much higher yield strength than other alloys including no carbides over the entire temperature range.

High temperature oxidation of mechanically alloyed NiAl–Fe–AlN–Al 2O 3

A new NiAl–Fe alloy containing uniformly distributed ultra-fine AlN and Al 2O 3 dispersoids was produced by mechanical alloying in a controlled atmosphere and by subsequent hot extrusion process. The new NiAl–Fe–AlN–Al 2O 3 alloy was oxidized in the temperature range between 1073 and 1473 K in air. The alloy displayed similar oxidation behavior with conventional NiAl alloys. As the oxidation temperature increased, the oxidation rate, the scale thickness and scale spallation increased. The scales formed during isothermal and cyclic oxidation consisted primarily of α-Al 2O 3. No transient, metastable aluminas were found. The presence of not nitrogen but iron and nickel within the alumina scales was observed.

Stability of the Two-Way Shape-Memory Effect of an Aged NiAl-Fe Alloy

Stability of the Two-Way Shape-Memory Effect of an Aged NiAl-Fe Alloy

Modeling of the site preference in ternary B2-ordered Ni–Al–Fe alloys

The underlying equilibrium structure, site substitution behavior, and lattice parameter of ternary Ni–Fe–Al alloys are determined via Monte Carlo–Metropolis computer simulations and analytical calculations using the BFS method for alloys for the energetics. As a result of the theoretical calculations presented, a simple approach based on the energetics of small atomic clusters is introduced to explain the observed site preference schemes.

Microstructure, mechanical properties and wear of Ni–Al–Fe alloys

The purpose of this paper is to describe the microstructure, mechanical properties and wear behavior of NiAl-based alloys both with and without a ductile b.c.c. phase. The materials studied had equiatomic amounts of Ni and Al, with four different additions of iron – 10, 20, 30 and 44 at.%. The microstructures were examined using both optical microscopy and transmission electron microscopy. The alloys containing 10 and 20% iron were single phase with an ordered b.c.c. (or B2 structure), whilst those containing 30 and 44% Fe were two-phase with a B2 matrix containing fine b.c.c. precipitates. The addition of iron did not improve the room temperature tensile ductility significantly, although some increase in both the compressive yield strength and the hardness was noted as the iron content increased. Wear tests, which were conducted in an argon atmosphere to minimize environmental effects, showed a higher wear rate for the two-phase alloys despite their slightly higher hardnesses, presumably due to shear of the ductile phases present in these alloys.

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni 5Al 3 phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.

Effect of ageing on the phase transformation temperatures and shape memory effect of a NiAl-Fe alloy

Effect of ageing at temperatures of 200–600°C on the phase transformation temperatures and shape memory effect of a NiAl-Fe alloy were investigated. It was found that the martensitic transformation temperature and the reverse temperature of Ni-24.1 at% Al–18.2 at% Fe specimens decreased gradually as the ageing temperature increased from 200 to 400°C, and then increased as the ageing temperature increased higher than 400°C. The amount of one-way, without load, shape recovery of Ni-24.1 at%Al–18.2 at% Fe specimens, aged, increased as the ageing temperature increased from 200 to 400°C for 1 h, and decreased dramatically as the ageing temperature increased to 600°C.

The properties of two-phase Ni–Al–Fe shape memory alloys in the virgin and shape-memory-cycled states

Abstract Five polycrystalline Ni–Al–Fe alloys with 14–26 (at.)% Al and 16–26% Fe were made and heat-treated to have β and γ dual phases with from 87% to less than 1% of β phase and to exhibit M s temperatures at about 300 K. Depending on the β phase fraction and prestraining temperature, the alloys showed pseudoelastic strain recovery ratio from 1–2 up to 35% and shape memory strain recovery ratio from over 10–90% after prestraining by up to 1%. In the alloy with 87% β phase, the latter ratio reached 100% against 0.8% prestrain after a few cycles of shape memory training. Observed results were analyzed based on a model that assumed a simple linear mixture of the properties of the β and γ phases. The results of the analysis suggest that the γ phase after the training in certain conditions is almost elastic and thus acts as a bias spring.

The effects of the point defects on precipitation in NiAlFe alloys

The high thermal conductivity, high melting point and low density of NiAl alloys compared to Ni-based superalloys make attractive materials for high temperature structural applications. The lack of high temperature strength, however, has been one of the barriers which limits the use of NiAl alloys. The use of these alloys in structures will be severely hampered, unless this problem can be overcome. Consequently, a large amount of effort has been devoted to the improvement of the various measures of high temperature strength, e.g., precipitation strengthening. In order to obtain better understanding of the effects of iron on the properties of NiAl, a systematic study of its effects on both physical and mechanical properties of NiAl has ben undertaken. Of specific interest are the correlation of point defects (thermal vacancies, constitutional vacancies and anti-site atoms) and Fe precipitates and the variation of the concentration of point defects along with iron content, substitution scheme (for Ni, for Al and for both Ni and Al) and temperature. The effects of vacancies on Fe precipitation in NiAlFe have been studied by using magnetic susceptibility measurements and transmission electron microscopy.

Effect of rare earth element Nd on the ductility and fracture behavior of a Ni-rich NiAl alloy

NiAl alloy is a promising high temperature shape memory alloy (SMA), but its ductility is low at room temperature. Some alloying elements such as Fe, Mn were found useful in improving the ductility of NiAl alloy. However, a {gamma} second phase with no shape memory effect (SME) formed in NiAl-Fe(Mn) alloy and the shape recovery becomes incomplete. The shape recovery was usually {le}70%. It is necessary to look for other alloying elements which improve the ductility without causing the formation of unwanted second phase. Microalloying element B had been tried, but the effect was small. In the present study the rare earth element neodymium (Nd) is used as a microalloying element in NiAl. The results reveal that Nd is a promising alloying element for improving the ductility of NiAl based SMA.

Two-way shape memory effect in a NiAl-Fe alloy

The dual phase ({beta} + {gamma}) NiAl-Fe shape memory alloys have recently been considered for their good ductility at room temperature. It was reported that the NiAl-Fe alloys quenched from high temperature show a shape recovery of 60%--80% or 90% after training, though they possess a dual phase microstructure, in which the {gamma}-phase can not contribute any SME. Factors affecting the one way SME, such as the quenching temperature, the amount of {gamma}-phase and the grain size of the {beta} phase matrix, have been studied in rather detail. However, two-way SME in the NiAl-Fe alloys has not been noticed yet. The present work is devoted to investigate the two-way SME in a dual phase NiAl-Fe alloy.

Effect of Ti addition on microstructure of Ni-Al-Fe system alloy

The NiAl intermetallic compound has a superior strength to density ratio and fairly good oxidation resistance for rotating high-temperature structural materials. However, its low ductility and toughness at ambient and intermediate temperatures hinders its full utilization. Therefore, the improvement of ductility is a main problem for the development of new heat resistant materials. Recent attempts to improve the ductility through alloying and microstructural modification by heat treatment have shown encouraging results. For example, for the NiAl base and Ni{sub 3}Al base alloys, the additions of Fe and Mn improve the ductility at ambient temperature, Cr, Fe and Hf additions increase the ductility at intermediate temperatures and Si, Ti, V, Zr, etc. additions improve their high temperature strength. As for the alloying effect mechanisms, there are only a few published works. The effect of Fe on the ductility of the NiAl alloy has been studied by Zhao et Gaha, who showed that the Ni-20Al-30Fe(at%) alloy possessed the highest ductility and Ti addition increased the high-temperature strength and strongly decreased the ductility of the NiAlFe alloy. However, the information concerning the microstructural evolution and the embrittling mechanism of the Ti addition to NiAlFe alloys have not been reported. In the present work,more » the effect of the Ti addition on the microstructural evolution and the embrittlement of NiAlFe alloy is studied.« less

Three stage yielding phenomenon in a NiAlFe alloy

The binary NiAl alloy is extremely brittle at ambient temperatures, so it is withdrawn from practical applications as structural materials or shape memory alloys. It has been found that iron is one of the effective elements in improving the ductility of the NiAl alloy. The NiAl-Fe alloys with ([gamma]+[beta]) dual-phase microstructures have good ductility and also good hot-working properties. These results provide the possibility for using the NiAl-Fe alloys for commercial use. However, there are still many experiments to be performed before these alloys can be practically used. The present paper reports a part of the authors work on the deformation behavior of a NiAl-Fe alloy with a ([gamma]+[beta]) microstructure.

Training and aging effects on shape memory behavior in a two-phase NiAlFe alloy

Training and aging effects on the shape memory behavior in a two-phase Ni-25 Al-15 Fe alloy have been investigated by tensile tests. In comparison with previous results, the mechanisms of training and aging effects are discussed in terms of the original microstructures and their changes arising from training and aging. The mechanical property changes are also reported in consideration of the formation of Ni 5Al 3 and its deleterious effects.

The order-disorder transformation in Ni 3Al and Ni 3AlFe alloys—II. Phase transformations and microstructures

From a combination of dilatometric and micrographie evidence, the precipitation of the β′ phase at temperatures above the metastable ordering transition, T tm , is demonstrated for those NiAlFe alloys containing at least 19 at.% Al. Ni-AlFe alloys which have been heated above T tm and then slow-cooled are found to contain a network of disordered γ phase which outlines the boundaries of antiphase domains in the ordered γ′ phase. The implication of these observations is that the formation of equilibrium γ phase from off-stoichiometric γ′ requires antiphase domain boundaries as nucleation sites: the T tm determined by dilatometry represents an upper absolute instability limit at which the ordered phase is converted into disordered phase of the same composition, whereas the equilibrium disordered phase would have a different composition. The microstructure of a series of binary Ni-Al alloys are examined: at 22–23 at.%Al, the γ′ phase consists of grains which partly contain fine antiphase domains (with γ at the APD boundaries) and partly very coarse APD's; microanalysis established that this is due to microségregation, and that the coarse APD's are in regions with excess Al so that T tm is locally above the freezing temperature. The structural distinction between directly and sequentially ordered alloys is thereby made clear: fine APD's form only in alloys which freeze in disordered form.

A Study of Spinodal Decomposition in Fe–Ni–Al Alloys by Mössbauer Effect

Using the Mössbauer effect of 57Fe, the process of phase separation in Fe–Ni–Al alloys was studied with an attempt to clarify whether the separation is made by the spinodal decomposition or the nucleation and growth type decomposition. Two compositions, Fe1⁄2(NiAl)1⁄2 and Fe1⁄3(NiAl)2⁄3, were chosen. The specimens were aged for various periods of time at constant temperatures after quenching from the one-phase region, and the Mössbauer spectra were taken. The hyperfine field of 57Fe in the Fe-rich phase gradually increased with the aging time until the final value was attained, while the isomer shift of 57Fe in the paramagnetic NiAl-rich phase shifted gradually to that of 57Fe surrounded by eight Al atoms. Spreading of the width of each ferromagnetic absorption peak indicated that the Fe-rich phase had no definite composition but a wide composition distribution. These results directly showed that the Fe–Ni–Al alloy system was decomposed by the spinodal decomposition mechanism.