The mechanical response of the Ni−Ni3Nb eutectic composite: Part I. Monotonic behavior

To understand the mechanical behavior of the Ni?Ni₃Nb eutectic composite, it was necessary to determine the operative deformation and fracture mechanisms in the Ni₃Nb intermetallic phase. It was found that Ni₃Nb deforms primarily by twinning along {112} planes and {011} planes when tension and compression, respectively, are applied parallel to the [100] growth direction. The {112} twins were observed to serve as crack nucleation sites with cracks forming along the twin boundaries. The monotonic response of the Ni?Ni₃Nb eutectic composite was investigated with tension and compression tests, metallography, and electron fractography. Room temperature tensile testing of the Ni?Ni₃Nb composite revealed this material to be capable of sustaining tensile strains in excess of 11 pct. This large composite ductility was associated with extensive {112} twinning of the Ni₃Nb lamellae and subsequent twin boundary cracking. When amassed in sufficient numbers in a given cross-section, these {112} twin boundary fissures initiated composite rupture. The room temperature ultimate tensile and compressive strengths of the alloy were found to be 109 and 235 ksi, respectively.     

Deformation,twinning and thermo-mechanical strengthening of Ti50Ni47Fe3

The dislocation Burgers vectors for the B2 intermetallic compound, Ti₅₀Ni₄₇Fe₃, are shown to be the 〈010〉 type, which provide this alloy with only three independent slip systems. However, concurrent dislocation slip and mechanical twinning on {114} planes affords the polycrystalline material with greater than 50% room temperature ductility. Individual twins do not grow greater than 50–150 nm in width, and the twinning density increases with the extent of cold working. Annealing a cold worked structure, comprised of mechanical twins and dislocations, results in the formation of subgrains with a size limited by the width of the twins. Greater cold working produces a finer twin spacing and, subsequently after annealing, a refined subgrain size which in turn brings about an improved combination of yield strength and ductility.     

Fatigue life and twinning in alpha-zirconium

Two grades of polycrystalline α-Zr containing ~0.003 and ~0.0775 wt pct O have been fatigued in tension-compression about a zero mean load at a frequency of 100 cps at room-temperature. A combined metallographic and microbeam X-ray pole-loci analysis has revealed the presence of {10•12}, {11−21}, and {11•22} twins and {1O−10} slip. In grains completely enclosed by surrounding material fatigue damage has been found in association with 11•21 twins and very occasionally with {11−22} twins but not with 10•12 twins. A further quantitative interpretation has shown that there is a critical resolved shear stress for {11−21} twinning of ~0.5 kg mm⁻² in zirconium containing 0.003 wt pct O. During observations confined to internal grains it was found that increasing grain size and decreasing oxygen content favored the formation of fatigue damage in association with these {11−21} twins. In external grains the fatigue damage was mainly in the form of edge cracks emanating from the specimen surface. The most appropriate criterion for the formation of damage in {11−2l} twin-matrix interfaces appears to be the attainment of a critical resolved shear stress in the {11−21} twinning plane in the (11−26) twinning direction. Formerly with the Department of Physical Metallurgy and Science of Materials, The University, Birmingham, England     

The mechanical response of the Ni−Ni3Nb eutectic composite: Part II. Cyclic behavior

In an effort to more fully understand fatigue crack propagation in composite materials, tension-tension fatigue studies of the Ni?Ni₃Nb eutectic composite were conducted. This composite displayed excellent fatigue resistance in a notched configuration by exhibiting an endurance limit of 55 pct of the 108.7 ksi smooth bar tensile strength. Under high stress-low cycle fatigue conditions, the fatigue resistance of the composite was controlled by the {112} twinning and subsequent twin boundary fracture of the Ni₃Nb phase. However, under low stress-high cycle fatigue conditions, Stage I crack propagation was observed in the nickel matrix. It was concluded that the nickel matrix controls the high cycle fatigue resistance of the composite. The fatigue behavior of the Ni?Ni₃Nb eutectic composite was compared with that of the Al?Al₃Ni composite. The similarity in the operative crack growth mechanisms in these two composites suggests a trend which may apply to all composites.     

Microstructure and mechanical properties of ductile Ni3AI-type compound in Fe-(Ni,Mn)-AI-C systems rapidly quenched from melts

By the rapid quenching technique, nonequilibrium Ni₃Al-type compounds with high strength and hardness as well as large elongation have been found in Fe-Ni-Al-C and Fe-Mn-Al-C systems. This formation region is limited to about 7 to 55 wt pct Ni, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Ni-Al-C and to about 7 to 65 wt pct Mn, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Mn-Al-C. The Ni₃Al-type compound has fine grains of about 1 to 10 μm in diam. Their Vickers hardness and yield strength increase with increase in the amounts of carbon, aluminum or nickel and the highest values attain about 665 DPN and 1690 MPa for Fe-Ni-Al-C and 600 DPN and 1740 MPa for Fe-Mn-Al-C. Elongation increases with decrease in carbon or aluminum and attains about 11 pct for Fe-20 wt pct Ni-6 wt pct Al-1.2 wt pct C and 28 pct for Fe-20 wt pct Mn-8 wt pct Al-1.6 wt pct C. The good strength and ductility of the Ni₃Al-type compounds remain unchanged on tempering for 1 h until heated to about 750 K. Further, it has been found that the addition of chromium, molybdenum or cobalt is effective for the improvement of mechanical properties and thermal stability of the compounds. Thus, the use of materials containing Ni₃Al-type compounds may be attractive for fine gage high-strength wire or plate applications. Formerly Graduate Student of Tohoku University.     

The role of twinning in the cyclic stress-strain behavior of directionally solidified γ/γ′-δ

Under cyclic straining of the γ/γ’-δ directionally solidified alloy a special type of hysteresis loop with an inflection in the compressive loading region was obtained. Observations gave direct evidence that the inflection was due to a process in which the 5 twins formed during tension untwinned under the subsequent compression. Thin foil electron microscopy established the 5 twins to be two specific variants of the four {211} variants, the selectivity being imposed by the crystallographic relation of the 5 phase with the γ/γ’ matrix. Characteristic distribution of (010) 5 faults was also noted in the foils and they were found associated with the {211} untwinning events. The compressive {011} 5 twinning mode reported in literature was not observed; the forward plastic strain in tension was reversed by {211} untwinning mode at a compressive stress level much lower than that required for the onset of the {011} twins.     

Mechanical twinning in Ti50Ni47Fe3 and Ti49Ni51 alloys

Pseudo-twinning and mechanical twinning have been observed in a transmission electron microscopy study of Ti₅₀Ni₄₇Fe₃, and Ti₄₉Ni₅₁ alloys which have the B2(CsCl) structure. Observation of twinning in ordered alloys is rare and this is the first observation of twinning reported in a B2 structure. The twin planes are the {112} and {114} planes. For {112} pseudo-twins, the composition plane is not the twin plane and the pseudo-twin does not have the B2 structure. For {114} mechanical twins, the composition plane is the twin plane and the twin does have the B2 structure. It is shown that a shear on the {114} plane plus a shuffle of the atoms results in the ordered B2 structure in the twinned region.     

Deformation behavior of a Ni3Al(B,Zr) alloy during cold rolling: Part II. Microstructural and textural changes

Part I of this study described the changes in order and of structure during cold rolling of a Ni₃Al(B,Zr) alloy. The textural and microstructural changes that occur during deformation are reported in this part. In addition to other features, a high density of shear bands start forming in this alloy from a rather early stage of deformation. The cold rolling texture of the material, which is basically of pure metal type at low strain levels, changes into alloy type after rolling between 35 and 45 pct. The maximum pole density of the alloy type texture is obtained at the {168}〈211〉 location. Transmission electron microscopy (TEM) micrographs show the presence of twins in the material from a deformation level of 35 pct onward, their density increasing with increase in deformation level. As has been proposed earlier, a structural transformation from L1₂ to DO₂₂ appears to take place in the γ′ phase during rolling. This will change the deformation mode from primarily slip to twinning and this could be responsible for the observed textural change with rolling. The γ phase deforms by slip in a manner similar to a fcc material with high stacking fault energy. The final texture of the material actually reflects an aggregate of the components developed in the two phases.     

Deformation in wrought Mg−9Wt Pct Y

Considerable tensile-compressive yield strength anisotropy is normally associated with textured wrought magnesium alloys.¹ The ease of {1012} twinning is responsible for the lower compressive yield strengths of these materials. In Mg-9 wt pct Y, however, approximately equal tensile and compressive yield strengths of about 50 ksi have been previously reported.² This investigation was performed to study the deformation of wrought Mg-9 wt pct Y with the purpose of understanding its unusual isotropic behavior. It was found that almost no {1012} twinning occurs in this alloy, thus accounting for the absence of anisotropy. Initial plastic deformation both in tension and compression occurs almost entirely by slip, primarily on the basal plane. Subsequent deformation occurs by a combination of slip and {1121} twinning with short {1012} twins appearing only occasionally.     

Basal slip and twinning in α-titanium single crystals

Single crystals of α-titanium, with small Schmid factors for prismatic slip, have been deformed in tension between 78 and 1120 K. At low temperatures, {1012} twinning has been observed in specimens having the angle between the basal plane and the tensile axis,x _B , close to 90 deg, whereas at intermediate orientations withx _B = 60 deg and 47 deg twinning occurs on the {1121} planes. A critical resolved shear stress law is not obeyed for either twinning mode. First order prismatic slip in the microstrain region appears to be responsible for the nucleation of {1121 twins. Slip is unlikely a pre-requisite for {1012} twinning. Basal slip without interference from twinning is observed in a variety of orientations at temperatures above 500 K. Plastic flow above 900 K may be described by an equation of the form:γ=Aτ ⁿ e^-Q/kT The relative ease of basal and prismatic slip in Ti and Zr is discussed in terms of the hcp ⇆ bcc allotropic transformation.     

Room-temperature mechanical properties of cold-rolled thin foils of binary,stoichiometric Ni3Al

In our previous works, thin foils of boron-free stoichiometric Ni₃Al, with thicknesses ranging from 57 to 315 μm, were fabricated by cold rolling of single-crystalline sheets which were sectioned from directionally solidified ingots. In this article, the room-temperature mechanical properties of the 83 and 95 pct cold-rolled foils were examined. Depending on the initial rolling direction, the foils exhibited two types of deformation microstructures: a banded structure with dual {110} textures and a band-free structure with a single {110} texture. The 83 pct cold-rolled foils showed very high Vickers hardness numbers: 649 and 604 for the banded and band-free structures, respectively. The foils possessed very high tensile fracture stress (1.7 to 2.0 GPa), with no appreciable plastic elongation along the rolling direction. The fracture stress of the 95 pct cold-rolled foils was slightly higher than that of the 83 pct cold-rolled foils. The banded-structure foils showed slightly higher fracture stress than the band-free-structure foils at the 83 pct reduction, but there was no difference between both the structures at the 95 pct reduction. Although there was no appreciable tensile elongation, slip traces were clearly observed on the surfaces of the foil specimens after the tensile test, indicating traces of some plastic deformation. The 95 pct cold-rolled foils possessed bending ductility, which was estimated as about 12 pct elongation on the tension-side surface of the bent specimen.     

A study on the subgrain superplasticity of extruded Al-Al3Ni eutectic alloy

A directionally solidified Al-Al₃Ni eutectic alloy was extruded to obtain micron-size subgrains with [111] fiber texture. The extrusion temperature was varied to have different distributions of the Al₃Ni eutectic particles. Choosing the fiber axis as the loading axis, the tensile test results at 500 °C indicate that the elongation is concave downward and strain-rate dependent. Reducing the number of intragranular particles increases the maximum elongation as well as the strain rate of maximum elongation. With the particles residing only intergranularly in the as-extruded state, the maximum elongation, which occurs under the initial strain rate of 6.3×10⁻³ s⁻¹, is about 300 pct. This subgrain superplasticity is associated with low strain-rate sensitivity but high resistance against strain softening. The fiber texture is always retained, and the microstructure reveals slip of long parallel dislocations. If intragranular particles are also present in the as-extruded state, the occurrence of dislocation tangling and dynamic recovery will give rise to early onset of strain softening and inferior ductility.     

Twin Nucleation by Slip Transfer across Grain Boundaries in Commercial Purity Titanium

The role of strain transfer in the activation of deformation twinning at grain boundaries has been characterized in commercially pure titanium deformed in bending. Two different orientations of a textured polycrystal were deformed in bending and were analyzed using electron backscattered diffraction (EBSD) to determine the active slip and twinning systems in the surface tensile region. Prismatic slip and { 10[`1]2 } á [`1]011 ñ \left\{ {10\bar{1}2} \right\}\left\langle {\bar{1}011} \right\rangle twinning were the most widely observed deformation modes in both orientations. Nonprismatic slip systems were also activated, most likely to accommodate local strain heterogeneities. A slip-stimulated twin nucleation mechanism was identified for soft/hard grain pairs: dislocation slip in a soft-oriented grain can stimulate twin nucleation in the neighboring hard grain when the slip system is well aligned with the twinning system. This alignment was described by a slip-transfer parameter m′.[24] Twins activated by this mechanism always had the highest m′ value among the six available { 10[`1]2 } á [`1]011 ñ \left\{ {10\bar{1}2} \right\}\left\langle {\bar{1}011} \right\rangle twinning systems, while the Schmid factor, based on the global (uniaxial tensile) stress state, was a less significant indicator of twin activity. Through slip transfer, deformation twins sometimes formed despite having a very low global Schmid factor. The frequency of slip-stimulated twin nucleation depends strongly on the texture and loading direction in the material. For grain pairs having one grain with a large Schmid factor for twinning, nonparametric statistical analysis confirms that those with a larger m′ are more likely to display slip-stimulated twinning.     

Effect of heat treatment on the structure and properties of extruded P/M alloy 718

The structure and mechanical properties of alloy 718 prepared from argon atomized powder have been investigated for a wide range of extrusion ratios and temperatures. The tensile and stress-rupture properties of extruded bar are sensitive to heat treatment. Notch ductility can be conferred through appropriate combination of solution anneal and intermediate temperature age. Structural evaluation shows that such treatment provides a uniform grain structure together with a coarseγ′’ precipitate dispersion and small amounts of δ Ni₃Cb phase at grain boundaries.     

Twinning-induced sluggish evolution of texture during recrystallization in AISI 316L stainless steel after cold rolling

This paper deals with the evolution of texture in AISI 316L austenitic stainless steel during annealing after 95 pct cold rolling. After 95 pct cold rolling, the texture is mainly of the brass type {110}〈112〉, along with a scatter toward the S orientation {123}〈634〉 and Goss orientation {011}〈100〉. Weak evidence of Cu component is observed at this high deformation level. During annealing, recovery is observed before any detectable recrystallization. Recrystallization proceeds through nucleation of subgrain by twinning within the deformed matrix and, later, preferential growth of those to consume the deformed matrix. After recrystallization, the overall texture intensity was weak; however, there are some discernible texture components. There was no existence of the brass component at this stage. Major components are centered on Goss orientation and Cu component {112}〈111〉 as well as the BR component {236}〈385〉. Also, a few orientations come up after recrystallization (i.e., {142}〈2−11〉 and {012}〈221〉). With increase in annealing temperature, the textural evolution shows emergence of weak texture with another new component, {197}〈211〉. The evolution of texture was correlated with the deformation texture through twin chain reaction.     

Deformation behavior in quenched and aged beta Ti-V alloys

The deformation characteristics of quenched and aged Ti-V alloys in the composition range 20 to 40 wt pct vanadium have been examined by optical metallography and transmission electron microscopy. A coarse lenticular deformation product similar in appearance to previously reported strain induced “martensites” was found to be associated with the occurrence of the omega phase. These features proved to be {112}〈111〉 twins. In the omega bearing alloys prolonged aging resulted in a transition of the deformation mode from twinning to slip at a point which corresponded either to the onset of embrittlement or alpha phase precipitation.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

Improved ductility of Ni3Si by microalloying with boron or carbon

The effects of boron and carbon additions on the tendency for intergranular fracture in trinickel silicide intermetallics are reported. Melt spinning of Ni₇₇Si₂₃ alloyed with 0.1 at. pet boron results in full bend ductility and complete transgranular fracture compared with brittle intergranular fracture for the unmodified compound. Alloying with 0.1 at. pet carbon also produces full bend ductility but a mixed mode failure (≈30 pct transgranular). For both carbon and boron additions, reducing the Ni concentration of the base compound results in a greater percentage of intergranular fracture. The boron solubility limit depends on the Ni concentration of the base compound. For Ni₇₇Si₂₃, the solubility limit is between 0.1 and 0.2 at. pet boron. For compounds with silicon concentrations of 23.5 and 24.0 at. pct, the solubility limit is less than 0.1 at. pct boron. Boron additions above the solubility limit result in Ni₃B precipitates which degrade the bend ductility and increase the percentage of integranular fracture. Alloying with carbon above the solubility limit (<0.1 at. pct) produces graphite precipitation. For Ni₇₇Si₂₃, increasing the carbon concentration from 0.1 to 1.0 at. pct resulted in no change in the ductility. Auger examination of the grain boundary composition showed strong segregation of both boron and carbon. Enrichment in silicon concentration was also observed.     

Crystallography of Martensite in TiAu Shape Memory Alloy

The twin structure, habit plane orientation, and morphology of B19 martensite in TiAu, which is a candidate shape memory alloy (SMA) for high-temperature and biomedical applications, were investigated by conventional transmission electron microscopy. Almost all internal twins were {111} type I twins as lattice-invariant deformation (LID). The <211> type II twin was scarcely observed in TiAu, unlike in TiPd and TiPt SMAs. The habit plane roughly corresponded to the twinning plane (K ₁ plane) of the <211> type II twin because of the superb lattice parameter ratio of TiAu. As a result, an energy-minimizing microstructure referred to as “twins within twins” appears as the major microstructure. The selection rules for the twinning of LID are also discussed considering the results of extensive studies on LID in SMAs.