Near-stoichiometric NiAl Research Articles

Size effects in the plastic deformation of NiAl thin films

Abstract This paper presents the first study of the plastic behavior of NiAl thin films with thicknesses in the micron and sub-micron range. The influence of geometrical and microstructural constraints was studied for different Al contents (45 to 50 at.% Al) and film thicknesses (0.2 to 3.1 μm) in the temperature range from 20 to 700 °C. The films were deposited on Si substrates and subjected to temperature changes resulting in thermal strains of up to 0.84%. The stress evolution in the films was measured by the substrate curvature method. The residual stress at room temperature of nearstoichiometric NiAl films significantly increased with decreasing film thickness indicating an increase of the film strength. At temperatures above 400 °C strong stress relaxation was found, which was more pronounced in thinner films suggesting the contribution of diffusional creep processes. The existence of a critical film thickness is suggested at which a transition from diffusion-controlled to dislocation-controlled plasticity occurs. The results have potential implications for the reliability of NiAl films as protective coatings in thermal engines.

Physically motivated model for creep of directionally solidified eutectics evaluated for the intermetallic NiAl–9Mo

Directional solidification (DS) of near-stoichiometric NiAl with a reinforcing refractory metal is of great interest for high-temperature structural applications. A three-dimensional model based on single-crystal plasticity theory is introduced for the description of the creep behavior of DS NiAl–9Mo (at.%) with a well-aligned fibrous microstructure. A hardening model considering also the transition from theoretical to bulk strength is motivated. To evaluate the model, NiAl–9Mo samples were directionally solidified using various growth rates. With the DS samples, creep experiments were performed at 900 °C and 1000 °C for different applied stresses. The model reproduces correctly a change of the applied stress, the temperature as well as a change in the fiber diameter. It is found that creep of the composite is mainly controlled by the plastic behavior of the fibers. A closer insight into the interactions between the fiber and the matrix is obtained by the simulation. Finally, by revealing the impact of each phase on the composite's behavior, the shape of the creep curve could be explained.

NiAl Behavior at Plasma Spray Deposition

The behavior of stoichiometric and near-stoichiometric NiAl at plasma spray deposition, without and with a bond coat, for coating layers realization on a low alloyed steel substrate has been investigated and is presented. In all variants, NiAl particle melting and their welding at the impact with substrate were observed. Furthermore, a relatively compact and adherent coating layer was formed and the NiAl was found to maintain its stability. These are all important factors for assuring the coating layer oxidation and corrosion resistance. Good results from the coating structure and adherence to the substrate points of view were obtained for the 45:55 Ni:Al composition, without a bond coat but adopting an Ar protective surrounding of plasma jet. The high resistance to corrosion of 45:55 NiAl composition was further validated through corrosion tests.

Tensile creep fracture of polycrystalline near-stoichiometric NiAl

Tensile creep fracture behavior of polycrystalline near-stoichiometric NiAl has been studied between 700 and 1200 K under initial applied stresses varying between 10 and 200 MPa. The stress exponent for fracture varied between 5.0 and 10.7 while the activation energy for fracture was 250 +/- 22 kJ/mol. The fracture life was inversely proportional to the secondary creep rate in accordance with the Monkman-Grant relation although there was extensive scatter in the data. This observation suggests that the fracture life for near-stoichiometric NiAl was influenced by creep under these stress and temperature conditions. Several different fracture morphologies were observed. Transgranular ductile cleavage fracture occurs at 700 K and at the higher stresses at 800 K. The fracture mode transitions to transgranular creep fracture at 900 and 1000 K and at lower stresses at 800 K, while plastic rupture and grain boundary cavitation occur at 1100 and 1200 K. An experimental fracture mechanism map is constructed for near-stoichiometric NiAl.

Size effects in the plastic deformation of NiAl thin films

This paper presents the first study of the plastic behavior of NiAl thin films with thicknesses in the micron and sub-micron range. The influence of geometrical and microstructural constraints was studied for different Al contents (45 to 50 at.% Al) and film thicknesses (0.2 to 3.1 μm) in the temperature range from 20 to 700 °C. The films were deposited on Si substrates and subjected to temperature changes resulting in thermal strains of up to 0.84 %. The stress evolution in the films was measured by the substrate curvature method. The residual stress at room temperature of near-stoichiometric NiAl films significantly increased with decreasing film thickness indicating an increase of the film strength. At temperatures above 400 °C strong stress relaxation was found, which was more pronounced in thinner films suggesting the contribution of diffusional creep processes. The existence of a critical film thickness is suggested at which a transition from diffusion-controlled to dislocation-controlled plasticity occurs. The results have potential implications for the reliability of NiAl films as protective coatings in thermal engines.

Tensile creep of polycrystalline near-stoichiometric NiAl

Long term tensile creep studies were conducted on binary NiAl in the temperature range 700–1200 K with the objectives of characterizing and understanding the creep mechanisms. Inverse and normal primary creep curves were observed depending on stress and temperature. It is concluded that the primary creep of NiAl is limited by dislocation mobility. The stress exponent for creep, n, decreased from 13.9 at 700 K to 5.5 at 1200 K. The true activation energy for creep, Q c, was constant and equal to about 400 kJ mol −1 between 20 and 50 MPa but decreased to a constant value of 250 kJ mol −1 between 50 and 110 MPa. The activation energy was observed to be stress dependent above 110 MPa. The tensile creep results reported in this investigation were compared with compression creep data reported in the literature. A detailed discussion of the probable dislocation creep mechanisms governing compressive and tensile creep of NiAl is presented. It is concluded that the non-conservative motion of jogs on screw dislocations influenced the nature of the primary creep curves, where the climb of these jogs involves either the next nearest neighbor or the six-jump cycle vacancy diffusion mechanism. A phenomenological model discusses the nature of the atom–vacancy exchange process likely to lead to the climb of these jogs.

Mechanisms of compatible deformation in NiAl-bicrystals

The compatibility of deformation in near-stoichiometric NiAl bicrystals was analysed above and below the brittle-to-ductile transition (BDT) of NiAl polycrystals and correlated with the active deformation mechanisms. The bicrystal configuration consisted of two specially arranged [110]-orientations and was chosen in such a way that the bicrystal had to deform incompatibly during uniaxial compression below the BDT due to the sole activity of 〈100〉-slip. However, above the BDT the bicrystal deformed compatibly by activation of a deformation mechanism in addition to 〈100〉-slip. In the bicrystal boundary region of the compatibly deformed bicrystals an adaptive deformation was clearly observed and correlated with pronounced orientation gradients as determined by single grain orientation measurements in a SEM. These were not observed during unconstrained deformation of a single crystal or in the bicrystals far away from the boundary. A comparison with simulated orientation changes led to the conclusion that the additional deformation mechanism activated at the BDT consists of 〈110〉{110}-slip, while 〈100〉-climb, which was discussed alternatively, had to be ruled out in this temperature regime. However, a transition from 〈110〉-slip to 〈100〉-climb most probably occurs at higher temperatures. In addition the analysis reveals a strong tendency for latent hardening of 〈100〉{100}-slip systems and also of 〈110〉{110}-slip systems which decreases with increasing deformation temperature.

Microstructural development in NiAl/Ni-Si-B/Ni transient liquid phase bonds

A transmission electron microscopy (TEM) based investigation of microstructural development during transient liquid phase bonding of near-stoichiometric NiAl to commercial purity nickel is presented in this article. The work described employed Ni-4.5 wt pct Si-3.2 wt pct B (BNi-3) melt-spun interlayers. The precipitation of both Ni-Al based phases and borides within the joint and adjacent substrate regions is discussed. The article considers martensite formation (within the NiAl substrate) and the precipitation of Ll2 type phases (both within the joint and at the interface with the NiAl substrate). The relative roles of the two substrate materials (NiAl and Ni) in the isothermal resolidification process are identified. The preferential formation of Ni3B boride phases in the Ni substrate near the original location of the Ni substrate-joint interface is discussed and contrasted with the absence of similar events in the NiAl substrate.

Microstructures and room temperature mechanical properties of NiAl prepared by high pressure reaction sintering

The microstructures and room temperature compressive mechanical properties of a nearstoichiometric NiAl manufactured by a new high pressure reaction sintering (HPRS) process are investigated. Applying a very high uniaxial pressure (2 GPa) leads to considerable lowering of the sintering temperature and reducing the hold time gives NiAl with a good sintering density. The HPRS NiAl consists of β-NiAl with a B2 structure containing a high density of dislocations, 3.5×109 cm−2, and very fine Al2O3 particles. The NiAl exhibits quite high true compressive strain, 14.5%, and a reasonably high yield strength, 526 MPa. The effects of employing the high pressure in the HPRS process on the reaction sintering, microstructures and mechanical properties of the NiAl are studied.

A microstructural investigation of NiAl/Ni-Si-B/NiAl transient liquid phase bonds

Microstructural development during transient liquid-phase bonding of cast polycrystalline near-stoichiometric NiAl (B2 compound, β-phase) was investigated using cross-sectional transmission electron microscopy. Bonds made using Ni-4.5 wt % Si-3.2 wt % B (AWS designation BNi-3) interlayers in the form of melt-spun foils were studied. Microstructural changes in the joint region and surrounding substrates were examined as a function of holding time at temperatures of 1065 and 1150°C. The transformation of the substrate β-phase to L10-type martensite due to aluminium transfer to the joint has been discussed, and the formation of L12 type (γ′ structure) layers at the joint-substrate interfaces considered. The nature of the isothermal solidification process has also been discussed and the changes, with holding time at the bonding temperature, in the character of the residual eutectic examined. The formation of M23X6-type borides within both the joint and surrounding substrates was also considered.

Effect of refractory alloying additions on mechanical properties of near-stoichiometric NiAl

Alloying effects have been investigated in near-stoichiometric NiAl for the purpose of improving it mechanical properties. NiAl alloys containing up to 3% of Mo, W, Nb, Ta, Zr and Hf were prepared by are melting and drop casting, followed by hot extrusion at temperatures to 1000 °C. The mechanical properties of the NiAl alloys were characterized by tensile and creep testing at temperatures to 1000 °C. Among the alloying elements, Mo is found to be most effective in improving the room temperature ductility of NiAl alloys in stress-relieved conditions. Alloying with 1.0 ± 0.6% Mo almost doubles the room temperature tensile ductility of NiAl and triples its yield strength at 1000 °C. The creep properties of Mo-modified NiAl alloys can be further improved by alloying with up to 1% Nb, Ta and Zr. Alloying with 1% MoZr reduced the creep rate and extended the rupture life of NiAl by more than 5 orders of magnitude. Transmission electron microscope and electron microprobe studies show that the solubility of Mo, Nb and Zr is low, less than 0.1 at.%. Both coarse and fine precipitates enriched with refractory elements are observed, with fine particles about (0.1 μm) distributed within grains. The mechanical properties of these NiAl alloys are discussed in terms of both particle strengthening and solid solution hardening.

Monitoring of Deformation Induced Microcracking in Polycrystalline NiAl

AbstractMicrocracking in polycrystalline near-stoichiometric NiAl produced by room temperature plastic deformation under uniaxial compression was investigated by means of optical microscopy, velocity of sound measurements, and acoustic emission monitoring. Results show that strains greater than 2 % are required to produce microcrack populations which can be evaluated by microscopical investigation or velocity of sound measurements. However, acoustic emission monitoring during compression testing indicates that microcracking starts at about 0.7 % compressive plastic strain which is identical with the typical tensile fracture strain for NiAl. Thus it is concluded that there is little or no stable microcracking prior to failure in tension. Acoustic emission results show also that the process of microcracking does not primarily occur during the applied compressive deformation. A considerable fraction of the microcracking takes place during the quasi-elastic unloading following deformation.

Formation of voids and dislocation loops in near-stoichiometric nial by aging at 700° to 900°c, and some effects on alloy properties

Dislocation loops and voids form spontaneously in stoichiometric NiAl annealed between 700° to 900°C. The former probably are formed by a punching mechanism from impurity particles, and the latter are frequently seen to have nucleated on particles. However, it is possible that both loops and voids are nucleated by vacancy/impurity complexes. Some of the loops act as dislocation sources during plastic deformation so that, for example, NiAl annealed at 700° to 900°C is softer and has greater ductility at room temperature, or has poorer creep resistance at higher temperatures, than material annealed at 1300°C. It is probable that small precipitate particles, possibly carbides and/or aluminum oxide form at 700° to 900°C but are in solution at higher temperatures. Their effect on mechanical properties is a form of precipitation softening.