Dislocation density evolution upon plastic deformation of Al-Pd-Mn single quasicrystals

Dislocation density studies have been performed on icosahedral Al-Pd-Mn single quasicrystals after plastic deformation and after subsequent heat treatment. The deformation tests were carried out at a constant strain rate of 10- 5 s-1 at temperatures between 695 and 820 C. The heat treatments were performed at 730 C, corresponding to one of the deformation temperatures. The development of the dislocation density during heat treatment and that during plastic deformation are compared. The experimental data are interpreted using a kinetic equation, which describes the evolution of the dislocation density during deformation. Numerical values for the dislocation multiplication constant and the annihilation rate for icosahedral Al-Pd-Mn are presented.     

Dynamic precipitation at elevated temperatures in a dual-phase AlCoCrFeNi high-entropy alloy: an in situ study

ABSTRACT

The effect of a possible phase transformation or precipitation of the face-centred cubic (FCC) phase on intermediate-temperature deformation of a dual-phase AlCoCrFeNi high-entropy alloy has been studied using in situ tensile testing at 550°C. Electron backscatter diffraction (EBSD) results showed localised precipitation of the FCC phase during the intermediate-temperature deformation. The overall fracture behaviour and crack propagation of the material was not altered much compared to the room-temperature behaviour, namely brittle trans-granular fracture. Deformation at higher temperatures (above 750°C) is suggested as a way to enhance the dynamic FCC phase precipitation, in order to improve the ductility or deformability of the alloy.     

New insights on tensile plasticity of ultrafine-grained metallic materials

ABSTRACT

The tensile properties of TiNi_43.5Fe_6.5 alloy samples having different grain sizes (0.16, 0.35, 1.7, 2.3, and 3.9?μm) and fabricated by severe plastic deformation and annealing were investigated. It was observed that both the strength and the elongation of the alloy increase with a decrease in the grain size until the average size reaches 1.7?μm. However, for average grain sizes smaller than 1.7?μm, the elongation decreases continuously with further grain refinement. On the other hand, the strain-hardening rate does not decrease with the decrease in plasticity but instead increases slightly. The poor ductility of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is accompanied by a high degree of strain hardening. This newly observed ductility behaviour of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is elucidated by characterising the intragranular and grain boundaries.     

Severe-to-mild wear transition at higher temperatures in Al-17Si-5Cu alloy after short-duration isothermal heat treatment

ABSTRACT

The wear behaviour of hypereutectic Al–Si alloys, which are suitable for use in automobile engines as a replacement for cast iron, has not been explored at higher temperatures. In the present study, the wear behaviour of Al-17Si-5Cu alloy (AR alloy) has been studied in as-received condition as well as after applying a short-duration isothermal heat treatment developed by the present research team. It was found that AR alloy exhibits severe wear, whereas heat-treated alloy (HT alloy) shows mild wear at higher temperatures (up to 300°C) at all applied loads. This severe-to-mild wear transition is addressed here.     

Planar faults in MoSi2 single crystals deformed at high temperatures

Abstract

Dislocation structure and planar faults have been examined in MoSi₂ single crystals deformed at high temperatures. Pure stacking faults were found in a crystal deformed at 900°C. The formation of the stacking fault is closely related to the phase stability of the C11_b and C40 ordered structures. Profuse stacking faults with increasing deformation temperature assist the ductility improvement of the MoSi₂ above about 1200°C. The critical resolved shear stress for {110}(331) and {013}(331) slip is determined in the temperature range 1000 to 1500°C.     

Origin of the initial rapid age hardening in an Al-1.7 at.% Mg-1.1 at.% Cu alloy

The origin of the rapid hardening which occurs during the initial ageing stage of an Al-1.7 at.% Mg-1.1 at.% Cu alloy has been investigated by the threedimensional atom-probe (3DAP) and transmission electron microscopy techniques. Although the rapid hardening reaction occurs within 1 min at 150 degrees C, no evidence for solute clusters or Guinier-Preston-Bagaryatsky (GPB) zones was detected using the 3DAP after this ageing time. When a short-term aged specimen is deformed and then aged at 150 degrees C, further rapid hardening occurs. This suggests that the rapid hardening is associated with a solutedislocation interaction. Uniform dispersions of Cu-Mg co-clusters do not occur until closer to the end of the hardness plateau and it is thought that these evolve into GPB zones during the second stage of the age-hardening process.     

Microstructure of icosahedral Al-Pd-Mn quasicrystals deformed at room temperature in an anisotropic confining medium

Plastic deformation of Al-Pd-Mn icosahedral quasicrystals has been achieved at room temperature using a high-confining-pressure medium. The deformation microstructure, investigated by transmission electron microscopy, is characterized by long straight bands of dislocations. A detailed analysis of the dislocation configurations indicates that the plastic deformation is controlled by dislocation glide.     

Accelerating heterogeneous nucleation to increase hardness and electrical conductivity by deformation prior to ageing for Cu-4 at.% Ti alloy

ABSTRACT

Deformation bands, especially shear bands were intentionally formed in supersaturated Cu-4 at.% Ti alloy by groove-rolling with 75% area reduction. Cold deformation prior to the ageing process changed the precipitation behaviour of the alloy. The deformed structure with shear bands was maintained, without showing recovery and recrystallisation, even after prolonged ageing at 450°C for more than one day. The thermally stable shear bands act as nucleation initiation sites and prevent the expansion of discontinuous cellular precipitates. The hardness reached a value of 305?Hv and an electrical conductivity of 15% IACS (IACS?=?International Annealed Copper Standard. 100% IACS is defined as the conductivity corresponding to a volume resistivity at 20°C of 17.241?nΩ), compared to 283?Hv and 13% IACS for conventional solid-solutioned and peak-hardened alloy. In addition, the ageing time to reach the highest hardness was shortened from 720 to 180?min at 450°C.     

Bending experiments on the xi'-(Al-Pd-Mn) quasicrystal approximant

Single crystals of the quasicrystal approximant phase xi'1-(Al-Pd-Mn) were deformed at a high temperature in three-point bending geometry. Two different mechanisms of plastic deformation were observed in this phase: one based on the motion of phason lines and the other based on dislocations. Line directions and Burgers vector directions of the dislocations were determined. The relative importances of the two mechanisms are discussed as a function of the sample orientation with respect to the bending geometry.     

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.     

Formation of icosahedral quasicrystals in an annealed Zr65Al7.5Ni10Cu12.5Ag5 metallic glass

The formation and thermal stability of an icosahedral quasicrystalline phase in an annealed Zr₆₅Al_7.5Ni₁₀Cu_12.5Ag₅ metallic glass have been investigated by X-ray diffraction and transmission electron microscopy analyses. It was found that the quasicrystalline phase can precipitate from the glassy state and the supercooled liquid of the alloy over a wide range of annealing temperatures. After optimizing the heat-treatment conditions, the volume fraction of the quasicrystalline phase in the alloy can reach as high as about 80%. Investigation of the thermal stability of the quasicrystalline phase demonstrates that it is very stable when the annealing temperature is below the glass transformation temperature T _g of the alloy.     

Stress anomaly in Al-rich Ti-Al single crystals deformed by the motion of 1/2«110] ordinary dislocations

The microstructure and plastic deformation behaviour of Al-rich Ti-Al single crystals containing 54.7 and 58.0 at.% Al have been examined, focusing on the effect of chemical ordering of a Al5Ti3 superstructure on anomalous strengthening. Fine precipitates with the Al5Ti3 superstructure were developed in the L10 matrix of the Ti-58.0 at.% Al alloy. The size and volume fraction of the precipitates varied depending on temperature. An anomalous increase in the yield stress of the two alloys appeared at around 800oC. This strengthening mechanism is discussed on the basis of the difference in antiphase-boundary energies on (111), (110) and (001) planes in the Al5Ti3 phase.     

Observation of planar stacking faults in a Ti-rich two-phase Ti-Al alloy after deformation at elevated temperatures

It is a common observation that in two-phase Ti-Al-based binary alloys, deformation of the gamma phase occurs by 1/2<110]-type ordinary dis-location activity and twinning associated with 1/6<112] type partials. In the present study the microstructure of a new Ti-Al-based alloy (Ti-47at.% Al-2at.%Mn-2at.%Nb+ 0.8 vol.% TiB₂) with a duplex microstructure consisting of primary equiaxed gamma grains and lamellar alpha₂+ gamma colonies was studied by transmission electron microscopy (TEM) after deformation at elevated temperatures. Planar stacking faults were found in the gamma laths. A detailed contrast analysis by TEM shows that these planar stacking faults lying on {111} planes are bound by all the fcc variants of the Shockley partial dislocations of type 1/6<121>, in contrast with the observations in stoichiometric binary TiAl alloys, where only 1/6<112]-type Shockley partials are found to be associated with the true twins. It is proposed that the addition of ternary and quaternary elements such as Mn and Nb promotes the other variants of the fcc-like dissociations (not common in L1₀structure) in the present alloy.     

An explanation of the indentation size effect in ceramics

Abstract

A quantitative model is proposed to explain the indentation size effect (ISE) often observed in the hardness response of hard brittle materials, namely that hardness is observed to increase with decreasing indentation size. The model is based on a mixed elastic/plastic materials deformation response whereby plastic deformation occurs in a discrete manner progressively to relieve stresses created by elastic flexure of the surface at the edges of the indentation. During unloading of the indenter, recovery of the elastic increment of deformation, which precedes each new band of plastic deformation, results in the indentation appearing smaller than expected, particularly as the indentation sizes decrease to approach the scale of the plastic deformation band spacing. The model fits observed experimental data well and analysis of hardness/size data in this way is shown to allow both a bulk hardness value and a characteristic deformation band scale to be calculated for a given sample.     

Strain localization in nickel single crystals cyclically deformed at 77 K

Nickel monocrystals oriented for single slip have been cyclically deformed at 77 K at plastic strain amplitude between 5 x 10^-4 and 1 x 10^-2 up to saturation of the stress amplitude. After unloading from maximum compression, the slip markings on the surface of the specimens were removed and the deformation continued for one half cycle in tension. As previously observed for room-temperature deformation, the plastic strain was found to be localized in narrow slip bands (SBs). Using atomic force microscopy at a given imposed strain amplitude, a wide spectrum of local plastic strains was found. The averaged resolved shear strain of the SBs was found to be independent of the imposed plastic strain amplitude and turned out to be a factor of three larger than the upper plateau strain limit of the cyclic stress-strain curve.     

The yield point of GaAs:Zn pre-deformed in the Peierls regime

In previous work by the present authors, the influence of pre-deformation at 600°C on the strain-rate and temperature dependence of the yield point of GaAs:Zn was investigated. Marked deviations in comparison with the behaviour of as-grown material were found in subsequent deformation at lower temperatures. In the present study the specimens were pre-deformed at 420°C, and the second tests were performed at temperatures between 270 and 390°C. This is the range in which the dislocation motion governing plasticity is dominated by a Peierls mechanism. Again, marked deviations from the properties of as-grown material were observed. They are mainly characterized by a distinctly smaller strain-rate dependence of the yield stress. Only close to the ductile-brittle transition in a rather limited temperature and strain-rate range does the behaviour of pre-deformed crystals approximate that of as-grown material.     

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800^oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.     

A study of the restitution coefficient in elastic-plastic impact

The aim of this work is to provide useful quantitative support for theories of the restitution coefficient for normal impacts involving plastic deformation, especially in the region close to the threshold at which plastic deformation begins. The impacts were from spheres of 5 mm diameter of aluminium oxide (which deformed elastically), dropped on to thick plates of mild steel or aluminium alloy. Very accurate measurements of impact and rebound velocities were made for drop heights from 1.75 m down to 0.6 mm, covering a range of velocities down to the yield threshold. Most of these results are in the elastic-plastic velocity range over which the indentation pressure varies from its value at initial yield of about 1.1sigma_y to the value when full plasticity is established of about 2.8sigma_y. The variation in the restitution coefficient with velocity in this range closely fits the rebound model of Tabor, which allows the indentation pressure to vary through the Meyer index n. The fitted values of n take into account these elastic-plastic effects as well as work hardening, demonstrating that a power-law compliance relationship is a very good model for the increase in indentation pressure as plasticity develops. Full plasticity only becomes established at impact velocities which are of the order of 100 times the velocity at which initial yield takes place. Models which assume a constant indentation pressure (perfect plasticity) are not a good match for the data in the elastic-plastic regime.     

Identification of crystal nucleation and growth in an amorphous FeZr2 alloy by means of electrical resistance measurements

A polymorphous crystallization process in an amorphous FeZr₂ alloy has been investigated by means of accurate electrical resistance measurements (ERMs) at elevated temperatures. It was found that, upon crystallization of the amorphous alloy, the electrical resistance increased with increasing temperature, exhibiting three distinct stages. Quantitative microscopy observations revealed that the three stages originated from crystal nucleation, from subsequent growth of crystal nuclei and from coarsening of the crystallites respectively. The activation energies for the crystal nucleation and growth determined from the ERM data agree satisfactorily with the data in the literature. The success in identification of the crystal nucleation and growth processes by means of ERMs may originate from differences in electrical resistance changes due to the crystal formation and the crystalline-amorphous interface formation processes from the amorphous phase.