Criteria for tensile plasticity in Cu–Zr–Al bulk metallic glasses

(Cu_0.5Zr_0.5)_100?xAl_x (x = 5, 6, 8) bulk metallic glasses (BMGs) were deformed in tension. Besides ductility up to 0.5%, the material shows work-hardening behaviour. Both effects are attributed to the deformation-induced precipitation of B2 CuZr nanocrystals and the formation of twins in the nanocrystals larger than 20 nm. The precipitation of the nanocrystals alters the stress field in the matrix and is expected to retard shear band propagation, which in turn allows stresses in the nanocrystals to rise. This stress build-up is more severe in the larger grains and might be responsible for the subsequent twinning. Both deformation-induced nanocrystallization and twinning consume energy and avoid crack formation and with it premature failure.     

Double-peak age strengthening of cold-worked 2024 aluminum alloy

The microstructure and mechanical properties of a 2024 Al alloy subjected to different levels of cold-rolling at room temperature and their evolution upon ageing at 453 K were investigated by means of microhardness measurements, tensile tests and transmission electron microscopy. The cold-worked 2024 Al alloy showed double-peak age strengthening behavior. After ageing for 120 min, the samples reached the first peak strength with quite low ductility. However, simultaneous high strength and ductility were achieved by prolonged ageing of 720 min. The first strengthening peak is due to the precipitation of fine S′ precipitates. The optimized mechanical properties of high strength and suitable ductility are attributed mainly to the precipitation of Ω-phase particles at the expense of S′ precipitates after ageing for 720 min. The Ω precipitates are effective in dislocation pinning and accumulation, and they can undergo plastic deformation to some extent, leading to simultaneously improved tensile strength, work-hardening ability and ductility. The present finding sheds light on the development of processing techniques to optimize the mechanical properties of 2024 Al alloy.     

Oxidation-induced embrittlement of TiAl alloys

The ductility of oxidised TiAl-based alloys is reduced even when the oxygen-rich region is of the order 100 nm thickness; this loss in ductility is smallest in lamellar samples. Removal of this oxidised region restores ductility. Acoustic events are observed during tensile tests at stresses above 300 MPa and cracks at about 250 MPa. In-situ tensile tests on samples, with part of the oxygen-enriched region removed, have shown that cracks are formed only in regions where the oxygen-rich region is present. X-ray diffraction measurements have shown that the oxygen-rich surface generates a tensile stress in the top 1 or 2 μm of the alloy of about 250 MPa corresponding to a compressive stress in the oxygen-rich layer of 2000 MPa. It is concluded that embrittlement is caused by (i) the tensile stress induced by the oxygen-rich region and (ii) the corresponding ease of crack nucleation in this region. Subsequent propagation is controlled by the fracture toughness.     

Effects of stress ratio on fatigue crack propagation properties of submicron-thick free-standing copper films

The dominant mechanics and mechanisms of fatigue crack propagation in ca. 500 nm thick free-standing copper films were evaluated at the submicron level using fatigue crack propagation experiments at three stress ratios, R = 0.1, 0.5 and 0.8. Fatigue cracking initiated at the notch root and propagated stably under cyclic loading. The fatigue crack propagation rate (da/dN) vs. stress intensity factor range (ΔK) relation was dependent on the stress ratio R;da/dN, increases with increasing R. Plots of da/dN vs. the maximum stress intensity factor (K_max) exhibited coincident features in the high-K_max region (K_max ? 4.5 MPa m^1/2) irrespective of R, indicating that K_max is the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode irrespective of the R value. The region ahead of the fatigue crack tip is plastically stretched by tensile deformation, causing necking deformation in the thickness direction and consequent chisel-point fracture. In contrast, in the low-K_max region (K_max < 4.5 MPa m^1/2), the da/dN vs. K_max function assumes higher values with decreasing R; in this region, the fracture mechanism depends on R. At the higher R value (R = 0.8), the fatigue crack propagates in the tensile fracture mode similar to that in the high-K_max region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appears: within several grains, intrusions/extrusions form ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagates preferentially through the intrusions/extrusions.     

Bulk amorphous FC20 (Fe–C–Si) alloys with small amounts of B and their crystallized structure and mechanical properties

The addition of a small amount (0.4 mass%) of B to a commercial FC20 cast iron was found to cause the formation of an amorphous phase in melt-spun ribbon and cast cylinders with a diameter of up to 0.5 mm. The structure of a melt-spun B-free FC20 alloy consisted of α-Fe, γ-Fe and Fe₃C. The effectiveness of additional B is presumably due to the generation of attractive bonding nature among the constituent elements. The amorphous alloy ribbon exhibits a high tensile strength of 3480 MPa and good bending ductility. The annealing causes the formation of an amorphous phase containing α-Fe particles with a size of about 30 nm. The mixed phase alloy exhibits an improved tensile strength of 3800 MPa without detriment to good ductility. With further increasing temperature, the mixed amorphous and α-Fe structure changes to α-Fe+Fe₃C+graphite through the metastable structure of α-Fe+Fe₃C. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe₃C and 1 μm for graphite. The graphite-containing alloy exhibits high tensile strength of 1200–2000 MPa and large elongation of 5–13%. The high tensile strength and good ductility were also obtained for the 0.5 mm cylinder annealed at 1200 K. The good mechanical properties are due to the combination of fine subdivision of crack initiation sites by the homogeneous dispersion of small graphite particles and the dispersion strengthening of Fe₃C particles against the deformation of the α-Fe phase. The synthesis of the finely mixed α-Fe+Fe₃C+graphite alloys having good mechanical properties by crystallization of the new amorphous alloy in the melt-spun ribbon and cast cylinder forms is encouraging for the future development of a new Fe-based high-strength and high-ductility material.     

The role and effect of residual stress on pore generation during anodization of aluminium thin films

The role and effect of residual stress on pore generation of anodized aluminium oxide (AAO) have been investigated into anodizing the various-residual-stresses aluminium films. The plane stresses were characterised by X-ray diffraction with sin²ψ method. The pore density roughly linearly increased with residual stress from 64.6 (−132.5 MPa) to 90.5 pores/μm² (135.9 MPa). However, the average pore size around 40 nm was not changed significantly except for the rougher film. The tensile residual stress lessened the compressive oxide growth stress to reduce AAO plastic deformation for higher pore density. The findings provide new foundations for realizing AAO films on silicon.     

Microstructure and mechanical properties of MoSi2/TaSi2 two-phase alloys

Effects of pre-strain on the compressive mechanical behavior were investigated on the alloys with an aligned lamellar microstructure consisting of C11_b MoSi₂ and C40 TaSi₂ prepared through optical floating zone (OFZ) method and annealing in the two-phase MoSi₂/TaSi₂ region. Single crystals of C40 TaSi₂ were successfully grown at solidification rate of 5 or 10 mm/h, and well-aligned lamellar microstructure can be achieved by selecting an MoSi₂–17 mol% TaSi₂ alloy. Firstly, compression tests were conducted to examine the effect of the angle ϕ between the aligned lamellae and the loading axis on strength and ductility. Results indicate that ϕ = 0, lamellae parallel to the axis, is a hard-orientation and ϕ = 54 and 40 (identically 45) are soft orientations. The alloy with ϕ = 0 shows higher strength but lower ductility than those with ϕ = 54 and 40 where ductility is evaluated in terms of brittle-to-ductile transition temperature (BDTT). Then effects of pre-straining at 1773 K on the mechanical behavior at 1573 K were investigated using soft-oriented lamellar alloys with ϕ = 40 whose BDTT is determined at least lower than 1673 K. Pre-straining to a few to several percent at 1773 K improves ductility of the alloy at 1573 K and also raises 0.2% flow stress, compared with the absence of the pre-strain. We believe that dislocations can be generated and stored in the alloy at 1773 K, and these dislocations are mobile even at 1573 K, although the analyses of operative slip systems and characteristics of dislocations remain as future works.     

Influence of boron and oxygen on the microstructure and mechanical properties of high-strength Ti66Nb13Cu8Ni6.8Al6.2 alloys

The influence of boron additions and different oxygen contamination levels on the microstructure and the mechanical properties in the Ti_66?xNb₁₃Cu₈Ni_6.8Al_6.2B_x (0 ? x ? 1) system were investigated. The alloys were prepared by levitation copper mould casting as rods with a diameter of 5 mm using different grades of starting elements. The alloy without boron exhibits a maximum compressive stress of more than 2500 MPa, associated with a compressive strain of more than 30%. The ultimate tensile stress is ～1075 MPa with a maximum elongation of 1.6%. Increased oxygen content leads to a rise of yield strength due to solid solution hardening. Boron additions promote grain refinement and reinforce the interdendritic phase compound by forming needle-like TiB precipitates. This change in microstructure increases the yield stress and the Young’s modulus and lowers the plastic strain. The microstructure was analysed in terms of the boron content by means of scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy. The presented mechanical properties are compared with the compression and tensile properties of the commercially available Ti6Al4V ELI (ELI = extra low interstitial) alloy.     

Microstructure and mechanical properties of Ni3Al base alloy reinforced with Cr particles produced by powder metallurgy

The microstructural evolution of a powder metallurgy (PM) Ni₃Al–8Cr (at.%) alloy reinforced with Cr particles has been correlated with its mechanical properties. The material was synthesised using rapidly solidified Ni₃Al–8Cr powders which were mixed with a Cr volume fraction of 10% and milled for 20 h. Consolidation by HIP was carried out at 150 MPa for 2 h at 1250 °C. For comparative purposes the unreinforced Ni₃Al–8Cr alloy was processed following the same route. After consolidation by HIP both materials show a bimodal microstructure consisting of coarse and fine grain regions in which fine particles are heterogeneously distributed. Besides Cr reinforcement, the difference between the two materials is the presence of β phase and higher volume fractions of γ+γ′ regions and α-phase precipitates in the reinforced material. The reinforced material presents the highest hardness, yield stress and the ultimate tensile strength values. The yield stress and ultimate tensile strength of the reinforced material at room temperature is 1286 and 1335 MPa, respectively. The strength of the composite is determined by the strength of the Cr particles and the good bonding between the matrix and Cr reinforcement. Although the ductility loss as the temperature increases is not suppressed, an improvement in ductility is obtained at temperatures above 500 °C compared with the unreinforced material.     

On-chip laboratory suite for testing of free-standing metal film mechanical properties,Part II – Experiments

In this paper, we demonstrate the fabrication of electrostatically loaded, free-standing Al–0.5 wt.%Cu thin-film samples, realizing a near-zero compliance support post. We measure Young’s modulus E = 74 GPa using cantilevers, in good agreement with grain texture measurements. We measure residual stress σ_R ranging from 30 to 60 MPa using fixed–fixed beams and find that processing induces significant plastic straining, which leads to residual stress values significantly less than the as-deposited value. Strength of this alloy is at least 172 MPa if the film is not severely strained, and the material exhibits no room-temperature fatigue up to 1 billion cycles at this stress level. Notched devices that have been subjected to process-induced plastic straining of ∼4% are weaker and fatigue logarithmically with the number of cycles. We compare deformation processes on the samples using ex situ TEM. The mechanism for the high strength value is attributed to the grain size and the thin surface oxide which constrain dislocation glide, while fatigue of the highly strained material is associated with the appearance of persistent slip bands.     

Effect of annealing temperature on microstructural modification and tensile properties in 0.35 C–3.5 Mn–5.8 Al lightweight steel

The microstructural modifications occurring during annealing treatment of an Fe–0.35 C–3.5 Mn–5.8 Al ferrite-based lightweight steel and its effects on the tensile properties were investigated with respect to (α + γ) duplex microstructures. Steels annealed above the dissolution finishing temperature of κ-carbides (795 °C) were basically composed of ferrite band and austenite band in a layered structure. As the annealing temperature was increased the tensile strength increased, while the yield strength and elongation decreased. This could be explained by a decrease in the mechanical as well as thermal stability of austenite with increasing size and austenite volume fraction. In the 980 °C annealed steel in particular, whose mechanical stability due to austenite was lowest, cracks were readily formed at ferrite/austenite (or martensite) interfaces with little deformation, thereby leading to the least tensile elongation. In order to obtain the best combination of strength and ductility the formation of austenite having an appropriate mechanical stability was essentially needed, and could be achieved when 22–24 vol.% fine austenite was homogeneously distributed in the ferrite matrix, as in the 830 °C or 880 °C annealed steels.     

Effect of milling and reinforcement on mechanical properties of nanostructured magnesium composite

Recycled Mg chips were used to synthesize nanostructured Mg composite of Mg–5 wt%Al reinforced with x wt% (x = 1, 2 and 5) in-situ formed AlN powder. Mechanical milling was employed to produce the composite powder of crystalline size 30–43 nm. The mechanically milled (MMed) powders were subjected to uniaxial pressing, sintering and hot extrusion processes to produce bulk solid samples. After sintering at 400 °C and hot extrusion at 350 °C, the crystalline size of the composite samples still remains in nanometer range from 52 to 84 nm. The effect of milling and the percentage of reinforced AlN on the mechanical properties such as tensile strength and ductility were discussed with the general explanation of deformation mechanisms involved.     

Bimodally grained high-strength Fe fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline iron containing a certain fraction of coarse grains with nanosized oxide dispersoids has been processed by mechanically milling Fe powder and subsequent spark plasma sintering. Sintered samples exhibited a high tensile strength of 2100 MPa with 5% ductility; by optimizing the sintering conditions, it was possible to tune the strength–ductility balance. The optimally sintered material showed a tensile strength of 1500 MPa and 15% elongation. The microstructure consists of nanograined (<100 nm) as well as coarse-grained regions (>1 μm) with uniform dispersion of nanosized chromium oxide particles (～10 nm). The strength and elongation show strong dependence on the volume fraction of the coarse grains, and the high strength can be attributed to the ultrafine grain size of the nanograined regions and precipitation hardening by the oxide dispersoids. The ductility is considered to be due to the presence of coarse grains.     

Ductile tensile failure in metals through initiation and growth of nanosized voids

We here reveal the initiation of ductile failure in metals at the nanometer scale by molecular dynamics simulations coupled with a novel analytical model. This proceeds by the emission of a special type of dislocation shear loop, which can expand as a partial or perfect dislocation, evolve into a prismatic loop through reaction, or develop into twins. Molecular dynamics (MD) simulations predict a strong dependence of the stress required for the initiation of plastic flow at the surface of the void for both Cu (a model fcc metal) and Ta (a model bcc metal). The decrease in stress with increasing void size is also analyzed in terms of a new analytical approach based on the energetics of dislocation loop emission. For both fcc (copper) and bcc (tantalum) metals initiation of plastic flow in MD simulations takes place at voids as small as a tri-vacancy (radius R ≈ 0.1 nm). Extensive calculations for tantalum combined with the analytical model, which tracks the simulations, enable extrapolation to R ≈ 300 nm, in the realm of second phase particles and inclusions. Thus we conclude that this is a general mechanism of tensile failure in pure monocrystalline metals where other initiation sites are absent.     

Texture,microstructure and mechanical properties of equiaxed ultrafine-grained Zr fabricated by accumulative roll bonding

The texture, microstructure and mechanical behavior of bulk ultrafine-grained (ufg) Zr fabricated by accumulative roll bonding (ARB) is investigated by electron backscatter diffraction, transmission electron microscopy and mechanical testing. A reasonably homogeneous and equiaxed ufg structure, with a large fraction of high angle boundaries (HABs, ∼70%), can be obtained in Zr after only two ARB cycles. The average grain size, counting only HABs (θ > 15°), is 400 nm. (Sub)grain size is equal to 320 nm. The yield stress and UTS values are nearly double those from conventionally processed Zr with only a slight loss of ductility. Optimum processing conditions include large thickness reductions per pass (ε ∼ 75%), which enhance grain refinement, and a rolling temperature (T ∼ 0.3T_m) at which a sufficient number of slip modes are activated, with an absence of significant grain growth. Grain refinement takes place by geometrical thinning and grain subdivision by the formation of geometrically necessary boundaries. The formation of equiaxed grains by geometric dynamic recrystallization is facilitated by enhanced diffusion due to adiabatic heating.     

Ductilisation of tungsten (W): On the increase of strength AND room-temperature tensile ductility through cold-rolling

Here we show that cold-rolling is a method to achieve room-temperature ductility in commercial purity, monolithic tungsten (W). Furthermore, we show that a decrease in rolling temperature concomitantly increases the strength and ductility of tungsten. So cold-rolling is a way to overcome the strength–ductility trade-off.In this work, we assess three different cold-rolled microstructures obtained from rolling at (i) 1000 °C (1273 K), (ii) 800 °C (1073 K), and (iii) 600 °C (873 K). Benchmark experiments were performed on a sintered ingot as well as on a hot-rolled plate. From these plates tensile test specimens were cut by spark erosion and tested at room temperature. The results show an increase of total uniform elongation, A_ut, ranging from 1.38% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 1.47% (cold-rolled at 600 °C (873 K)) and an increase of the total elongation to fracture, A_t, ranging from approximately 3% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 4.19% (cold-rolled at 600 °C (873 K)) with decreasing rolling temperature.The microstructure of the plates is analysed by means of scanning electron microscopy (SEM) (grain size, subgrains, crystallographic texture) and transmission electron microscopy (TEM) (bright field imaging, scanning TEM). Furthermore, strain-rate jump tests have been performed at 400 °C (673 K) to determine the strain-rate sensitivity, m, (sintered ingot m = 0.088, cold-rolled at 600 °C (873 K) m = 0.011) and the activation volume, V, (hot-rolled W plate V = 191 b³, cold-rolled at 600 °C (873 K) V = 111 b³) of the tungsten sheets.The question of why cold-rolling increases both strength and ductility is discussed against the background of cold-rolling-induced lattice defects. We speculate that the increase of ductility is caused by the ordered glide of screw dislocations, that move with low deformation incompatibility along the high-angle grain boundary (HAGB) channels (confined plastic slip).     

Deformation in metals after low-temperature irradiation: Part II – Irradiation hardening,strain hardening,and stress ratios

Effects of irradiation at temperatures ⩽200 °C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000 GPa/dpa, at the lowest dose of <0.0001 dpa and decrease with dose to a few tens of MPa/dpa or less at about 10 dpa. However, average irradiation-hardening rates over the dose range of 0 dpa−D_C (the dose to plastic instability at yield) are considerably lower for stainless steels due to their high uniform ductility. It is shown that whereas low-temperature irradiation increases the yield stress, it does not significantly change the strain-hardening rate of metallic materials; it decreases the fracture stress only when non-ductile failure occurs. Such dose independence in strain-hardening behavior results in strong linear relationships between the true stress parameters. Average ratios of plastic instability stress to unirradiated yield stress are about 1.4, 3.9, and 1.3 for bcc metals (and precipitation hardened IN718 alloy), annealed fcc metals (and pure Zr), and Zr-4 alloy, respectively. Ratios of fracture stress to plastic instability stress are calculated to be 2.2, 1.7, and 2.1, respectively. Comparison of these values confirms that the annealed fcc metals and other soft metals have larger uniform ductility but smaller necking ductility when compared to other materials.     

Deformation mechanisms of a 20Mn TWIP steel investigated by in situ neutron diffraction and TEM

The deformation mechanisms and associated microstructure changes during tensile loading of an annealed twinning-induced plasticity steel with chemical composition Fe–20Mn–3Si–3Al–0.045C (wt.%) were systematically investigated using in situ time-of-flight neutron diffraction in combination with post mortem transmission electron microscopy (TEM). The initial microstructure of the investigated alloy consists of equiaxed γ grains with the initial α′-phase of ～7% in volume. In addition to dislocation slip, twinning and two types of martensitic transformations from the austenite to α′- and ε-martensites were observed as the main deformation modes during the tensile deformation. In situ neutron diffraction provides a powerful tool for establishing the deformation mode map for elucidating the role of different deformation modes in different strain regions. The critical stress is 520 MPa for the martensitic transformation from austenite to α′-martensite, whereas a higher stress (>600 MPa) is required for actuating the deformation twin and/or the martensitic transformation from austenite to ε-martensite. Both ε- and α′-martensites act as hard phases, whereas mechanical twinning contributes to both the strength and the ductility of the studied steel. TEM observations confirmed that the twinning process was facilitated by the parent grains oriented with 〈1 1 1〉 or 〈1 1 0〉 parallel to the loading direction. The nucleation and growth of twins are attributed to the pole and self-generation formation mechanisms, as well as the stair-rod cross-slip mechanism.     

Determination of slip systems and their relation to the high ductility and fracture toughness of the B2 DyCu intermetallic compound

DyCu single crystals with CsCl-type B2 structure were tensile tested at room temperature. Slip trace analysis shows that the primary slip system in DyCu with a tensile axis orientation of 〈1 1 0〉 is {1 1 0}〈0 0 1〉 and the critical resolved shear stress for {1 1 0}〈0 0 1〉 slip is 18 MPa. Slip traces were also observed from a secondary slip system, {1 1 0}〈1 1 1〉, and this slip system appears to be a key contributor to the previously reported high ductility and high fracture toughness of polycrystalline DyCu. Transmission electron microscopy determinations of the Burgers vectors of dislocations in tensile tested specimens revealed 〈1 0 0〉 and 〈1 1 1〉 dislocations, with 〈1 0 0〉-type dislocations being more abundant. The implications of these findings for the understanding of the mechanical properties of DyCu and the large family of ductile rare earth B2 intermetallics are discussed.     

High temperature deformation behaviors of a high Nb containing TiAl alloy

In the present paper, high temperature tensile and creep behaviors of Ti–45Al–9(Nb,W,B,Y) alloy with duplex (DP) microstructure were investigated. In addition to tensile tests at 815 °C and a strain rate range of 1 × 10⁻⁴ s⁻¹−1 × 10⁻³ s⁻¹ and tensile, creep tests at 760 °C and 815 °C under the stress of 180 MPa, the microstructure evolutions during tensile and creep tests were studied. The results show that high temperature high Nb containing TiAl alloy with DP microstructure has a good balance between ductility and strength and intermediate creep resistance. The tensile properties have the strain rate dependence, and ultimate tensile strength (UTS) and yield strength (YS) vs. strain rate obey a single-logarithm linear relationship. Minimum creep rate is affected by the test temperature and stress. Using loading change experiment a stress exponent of 4.3 is determined. DP microstructure is unstable after long-term exposure at high temperatures, and the spheroidization of lamella and recrystallization along grain boundaries occur during the high temperature deformation. It is assumed that the diffusion-assisted climb of dislocations might be the controlling mechanism at the minimum creep rate stage.