Oriented NiAl Single Crystals Research Articles

Deformation mechanisms of hard oriented NiAl single crystals

Hard oriented NiAl single crystals (<100> deformation axis) have been compressed at temperatures between 296K and 963K. At about 600K the yield stress changes from a plateau to a steep fall. Slip line investigations show that this coincides with a transition from {112}<111> to {110}<110> slip. At low temperatures slip of <111> dislocations is determined by the Peierls mechanism while in the plateau region the deformation mechanism is still not understood. For the deformation by {110}<110> slip the activation enthalpy measured supports the deformation mechanism proposed by Mills et al. [1] based on the diffusion-assisted motion of macro-kinks in <110> edge dislocations which are found to be decomposed into <100> edge dislocations. The slip transition temperature is discussed with regard to the brittle-to-ductile transition temperature of polycrystalline NiAl.

Cyclic plasticity of soft oriented NiAl single crystals at 600 K

Cylindrical specimens of stoichiometric NiAl single crystals with axial orientations near [ 2 ¯ 2 3 ] in the stereographic unit triangle were cyclically loaded at 600 K, imposing low plastic strain amplitudes. A main aim was to study the slip phenomena, involving type and persistence of strain localisation, as well as the dislocation patterns occurring after cycling at ɛ pa = 2.5 × 10 −4. Single slip is dominant in the [0 0 1]( 1 ¯ 1 0 ) system on a macroscopic level. Broad cumulative slip bands appear after a sufficiently large number of cycles. Within these bands the strain is considerably concentrated in individual zones of localised slip, although the average strain carried by all of these slip zones is rather small. During a quarter cycle, the slip is highly localised in a small number of thin slip bands. Atomic force microscope (AFM) measurements show that there exists a broad spectrum of local strain amplitudes (LSAs) reaching up to 100%. The mean strain of the LSAs is more than a factor of 150 larger than the average strain of the cumulative zones of localised slip. The cumulative slip bands are not cyclically persistent. The dislocation patterns in a zone of localised slip (ZLS) characteristically differ from those formed in the surrounding matrix. Multiple slip has to be taken into account on a micrometer-scale in order to understand the ZLS dislocation substructure.

Substructure and texture formation during extrusion of NiAl single crystals

and oriented NiAl single crystals have been extruded at 1000 °C through a round die. The crystals subdivide into two types of deformation bands producing fibre texture components parallel and normal to the extrusion axis. These rotations as well as shape changes under partially constrained deformation conditions yield informations on the slip system activity. The relevance of deformation banding on texture formation in polycrystals is discussed.

The mechanics of slip transition at intermediate temperatures in &lt;001&gt;-oriented NiAl single crystals I. Experimental observations of the decomposition of α&lt;111&gt; dislocations in Ni–44 at.% Al

Abstract Ni–44 at.% Al single crystals were studied after compressive deformation along the hard orientation between 700 and 825 K. In order to understand the mechanism of the slip transition occurring at intermediate temperatures, compression was performed at various strain rates and to various strain levels. Local decomposition of α<111> dislocations was observed between 750 and 800 K. At 800 K (the slip transition temperature), complete decomposition occurs into α<101> and α<010> dislocations. Several unique features are associated with this decomposition event, such as the preference for near-edge line directions (for the α<111> dislocations), significant strain softening between 800 and 900 K, and the effect of thermal activation in determining the extent of the actual decomposition event.

The mechanics of slip transition at intermediate temperatures in -oriented NiAl single crystals II. A metastable state for α{110} dislocations in NiAl and its role in their decomposition

Abstract We show in this paper that there exists a metastable arrangement of products in the reaction α<111> → α<110> + α<001> in B2 NiAl. Atomistic calculations show that the metastable state can be formed spontaneously by a local shuffle or climb decomposition. Once formed, the ‘piggyback’ configuration can glide as a unit, with a restoring force helping to keep it intact. However, if the product lines become separated locally beyond a critical distance, the result is a decomposition loop. Small dislocation loops (less than 100 Å) collapse, large loops (greater than 3000 Å) evolve into dislocation dipoles, and medium loops can fail to evolve further. These results are consistent with detailed observations of the microstructure. The implications for this metastable state for the mechanical properties are examined.

The mechanics of slip transition at intermediate temperatures in &lt;001&gt;-oriented NiAl single crystals II. A metastable state for a &lt; 111 &gt;{110} dislocations in NiAl and its role in their decomposition

Abstract We show in this paper that there exists a metastable arrangement of products in the reaction α → α + α in B2 NiAl. Atomistic calculations show that the metastable state can be formed spontaneously by a local shuffle or climb decomposition. Once formed, the ‘piggyback’ configuration can glide as a unit, with a restoring force helping to keep it intact. However, if the product lines become separated locally beyond a critical distance, the result is a decomposition loop. Small dislocation loops (less than 100 A) collapse, large loops (greater than 3000 A) evolve into dislocation dipoles, and medium loops can fail to evolve further. These results are consistent with detailed observations of the microstructure. The implications for this metastable state for the mechanical properties are examined.

Study of the fracture behavior in soft and hard oriented NiAl single crystals by AFM

In order to understand the fracture behavior and room temperature brittleness of NiAl single crystals, atomic force microscopy (AFM) investigations were performed on in-situ loaded 4-point-bend specimens of hard and soft oriented crystals. A significant amount of dislocations are emitted from the crack tip on different slip systems. {1 0 0} slip planes are activated in soft crystals, whereas in hard crystals also slip planes, which are identified as {1 1 2} or {1 2 3} are observed. From evaluations of the AFM images, dislocation distributions ahead of crack tips and the plastic crack opening displacement (COD) from blunted crack tips are examined. The fracture toughness is calculated from the measured plastic COD and compared with the results obtained from load-displacement curves. First in-situ experiments on NiAl specimens heated up to 400 K are described, where a significant increase in plasticity is observed.

Influence of surface treatment on creep behaviour of NiAl single crystals

Abstract Stoichiometric [110] oriented NiAl single crystals were deformed by compression creep tests at 300 °C up to 3.6%. To ascertain the role of surface conditions, deformation samples with mechanically polished, rough and oxidised surfaces were used. The polished samples were most easy to deform. An oxide layer of about 600 nm thickness did not give rise to softening due to poor oxide adherence. The dislocation substructure in the bulk and near the surfaces was investigated by TEM. The different arrangements were discussed with regard to dislocation generation at the surfaces, cross-slip and mutual annihilation of screw dislocations and edge dislocation trapping to form dipoles.

Effects of strain-rate and ageing at 573 and 973 K on the mechanical (compressive) behaviour of 〈110〉 oriented (‘soft’) NiAl single crystals with and without microalloying additions of Ti, Zr, and Hf. II

The effects of impurity gettering on the mechanical behaviour of NiAl have received extensive attention recently. The present work was directed at static and dynamic strain ageing phenomena and strain-rate sensitivity minima, which are supposed to be related to the dislocation-impurity interactions, for 〈110〉 oriented NiAl single crystals with microalloying additions of Ti, Hf, and Zr as getters (0.1 at.%). The influence of ageing at 573 and at 973 K on compressive properties and microhardness was also investigated. The gettering effects were found to be less pronounced for NiAl + Ti and more pronounced for NiAl + Hf and NiAl + Zr, and the gettering effects lead to marked changes in the strength and ductility behaviour after ageing at 973 K as well as static strain-ageing at 573 K, and to strain-rate sensitivity. The observed effects are believed to be due to the formation of precipitates containing Ti, Hf, or Zr, their coarsening during ageing, and interactions of dislocations with either precipitates and/or solute atoms and interstitials.

Effects of ageing at 673 K on the compressive behaviour of 〈110〉 oriented (‘soft’) NiAl single crystals and polycrystals with and without Ti additions

Since the remarkable increase in room temperature (RT) ductility of 〈110〉 oriented NiAl single crystals by microalloying with Fe, Ga or Mo the effects of microalloying and of impurity gettering on the mechanical behaviour of NiAl are still under discussion. Thus the compressive behaviour of 〈110〉 oriented (‘soft’) NiAl single crystals and polycrystals with and without Ti (up to 0.1 at%), which has long been used as a getter in steel metallurgy, has been studied in detail. In particular, the effects of ageing at 673 K on RT mechanical properties, and static strain-ageing at 673 K, have been investigated. It was found that dislocation-impurity interactions play an important role in the mechanical behaviour of NiAl, and Ti additions may indeed serve as a getter for solute atoms. However, this effect does not lead to improved RT ductility of NiAl. The motion of 〈100〉 dislocations at RT was found to be restricted in single crystals with Ti compared with crystals without Ti. Ageing at 673 K decreases the RT ductility drastically and this is due to the formation of Ti-containing precipitates rather than pinning of mobile dislocations by interstitials. Likewise, the Ti-containing precipitates on dislocations lead to strain-ageing effects at 673 K.

A dislocation mechanics-based crystallographic model of a B2-type intermetallic alloy

Abstract A crystallographic slip based model for cubic oriented NiAl single crystals is derived from an idealization of the dislocation network observed in the active slip systems, viz. {110} 〈110〉. The crystallographic model successfully accounts for the cyclic steady-state behaviour of crystals subjected to strain histories within the range ϵ〈100〉 = ϵm ± 0.5%, for ϵm = 0 and 35%, at 750 and 850°C. It accurately predicts the flow stress dependence on temperature, strain rate and dislocation density arising from the lattice resistance to dislocation motion and from discrete obstacle resistance due to dislocation interactions. The kinematic and isotropic hardening modes associated with defect trails left behind by gliding dislocations and dislocation storage, respectively, are properly represented. The average distance that dislocations have to glide for their density to increase beyond the level needed to balance dynamic recovery processes was predicted to be approximately 260 times the random forest dislocation spacing. Measured dislocation densities at different mean strains were found to be consistent with the predictions of the theoretical model.

Dislocation mobility in 〈100〉-oriented NiAl single crystals containing iron and hafnium additions

Abstract The dislocation mobility produced by deformation at room temperature has been studied in 〈100〉-oriented single crystals of NiAl and of those containing hafnium and iron additions. Comparison of the dislocation microstructures produced by compression with those observed after hardness indentation of thin foils and during in-situ tensile tests in the transmission electron microscope have shown that only 〈100〉 dislocations are sufficiently mobile in the three alloys. The iron-containing alloy shows a sufficient density of 〈111〉 dislocation segments activated during compression while the binary and the hafnium-containing alloys only deform by 〈100〉 dislocations because of kinking. The latter glide preferentially on {110} planes while they are more mobile on {010} planes in the iron-containing material. These differences have been interpreted as being due to a change in kink density or structure produced by the presence of the iron atoms that cause a lower kink mobility on {110} planes in that alloy.

Influence of Dislocation Mobility on Room and High Temperature Ductility of &lt;100&gt;-Oriented Nial Single Crystals

AbstractThe dislocation configurations produced by room and high temperature compression of &lt;100&gt; oriented single crystals of binary NiAl and in those containing iron and hafnium additions have been analysed and compared to those obtained by hardness indentation and TEM insitu tensile tests. Kinking occurs during room temperature compression such that &lt;100&gt; dislocations are activated in all cases but the iron-containing alloy also exhibited a large density of &lt;111&gt; screw dislocations. The latter however, appear immobile when they are created by hardness indentations of thin foils, while only pile-ups of &lt;100&gt; segments can propagate. Similarly, although different slip systems are present after high temperature compression, only &lt;100&gt; dislocation segments have been confirmed to be mobile after room temperature hardness indentation of these predeformed thin foils. The improvement in ductility observed at room temperature in the predeformed specimens of the binary and the iron containing alloys has been attributed to the increased production of these mobile &lt;100&gt; dislocations.

The effects of alloy purity on the mechanical behavior of soft oriented NiAl single crystals

Preliminary results of the effects of alloy purity on the mechanical properties of NiAl single crystals are presented. Two stoichiometric NiAl single crystals with different impurity contents were studied. It is concluded that reductions in the interstitial and substitutional levels cause reduced yield strengths in NiAl. Heat treatment also results in reduced yield and flow stresses in both CP-NiAl and Hp-NiAl which are considered to be due to a reduction in the concentration of thermal vacancies due to vacancy coalescence during heat treatment.

Slip systems in 〈001〉 oriented NiAl single crystals

An investigation of the change in slip behavior in NiAl with temperature has been conducted, with special emphasis on the 〈001〉 “hard” orientation. Single crystal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations and extensive dislocation analysis performed in the TEM on the 〈001〉 oriented specimens. It was found that, although 〈111〉 slip can occur in RT compression of 〈001〉 oriented specimens, the increased tensile ductility observed at higher temperatures is due to the glide of b = 〈110〉 dislocations. The debris left behind by these dislocations consists of b = 〈100〉 dislocations, making identification of the operative Burgers vector difficult after any appreciable plastic strain. A mechanism for the formation of the b = 〈100〉 debris is presented.

The effect of alloying on slip systems in 〈001〉 oriented NiAl single crystals

An investigation of the effect of alloying on slip behavior in NiAl as a function of temperature has been conducted. Single Cyrstal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations. It was found that the addition of Cr and other alloying additions promote the activation of 〈111〉 slip over deformation by kinking in NiAl based alloys. This is believed to result from differential proportional hardening of the 〈100〉 vs 〈111〉 slip systems. No increase in RT tensile elongation is observed in these alloys. Increased tensile ductility observed at higher temperatures is due to the movement of b = 〈110〉 dislocations.