Ni3Al-based Intermetallic Alloy Research Articles

Influence of aging heat treatment on microstructure and mechanical properties of a novel polycrystalline Ni3Al-based intermetallic alloy

A novel polycrystalline Ni3Al-based intermetallic alloy was prepared using the vacuum induction melting process. Subsequently, the alloy was subjected to the solution-treatment underwent two different aging processes: 870 °C for 24 h in air cooling, referred to as AT 1, and 1060 °C for 4 h in air cooling followed by 870 °C for 24 h in air cooling, referred to as AT 2. The effects of different aging processes on the microstructure and mechanical properties of the polycrystalline Ni3Al-based intermetallic alloys were analyzed. Results indicated the cubicity of the primary γ′ phase within the dendritic γ+γ′ structure of the AT 2 sample was significantly enhanced, yielding the best overall mechanical properties for the alloy. The percentage elongation increased from less than 3 % for the AT 1 sample to about 4.5 %, highlighting a notable enhancement in room temperature tensile plasticity. The creep rupture time at 1100 °C/30 MPa reached 113 h. The α-Cr particles in the As-cast and solid solution alloy gradually transformed into Cr23C6 during the aging process, exhibiting the orientation relationships of (020)Cr23C6||(010)γ' and [103]Cr23C6||[103]γ'. High-temperature aging treatments resulted in Cr23C6 particles that provided enhanced precipitation strengthening effects, thereby improving the creep rupture performance of the Ni3Al-based intermetallic alloys. Following high-temperature aging, a four-layer interface structure formed around the blocky γ′ matrix phase, which altered the alloy's fracture mechanism from a brittle intergranular fracture to a ductile transgranular fracture.

P phase precipitation and strengthening behavior of a novel polycrystalline Ni3Al-based intermetallic alloy at 1100 °C

The microstructural evolution and strengthening behavior of a polycrystalline Ni3Al-based intermetallic alloy were investigated during varying durations of thermal exposure at 1100 °C. The study focused on the topologically closed-packed orthorhombic P phase precipitate, which was found to form in the interdendritic γ channel and grow in the γ'-matrix after thermal exposure. The morphology of the P phase evolved from needle-like to lath-like and then to blocky-like. The crystal structure and composition of the precipitates were analyzed using scanning transmission electron microscopy (STEM) and electron probe X-ray microanalysis (EPMA). The chemical composition of the P phase changed continuously during thermal exposure and the P phase was related to the γ'-matrix by three sets of orientation relationships (OR). The alloy's tensile properties and creep fracture time were enhanced during thermal exposure at 1100 °C for 2000 h due to P phase precipitation at grain boundaries or within the grain. Based on the experimental evidence of this study and theoretical strengthening behavior reported in the literature, a novel mechanism for in-situ precipitation strengthening was proposed.

Atomistic analysis of temperature-dependent dislocation dynamics in Ni3Al-based intermetallic alloys

In this study, we investigate the dynamics of superlattice edge dislocations composed of partial dislocations, complex stacking faults, and antiphase boundaries in binary and ternary Ni-based alloys (γ’ phase) with the L12 structure. In the case of Ni3Al, the dislocation velocity decreases with increasing temperature at low applied shear stress, mainly owing to the phonon drag effect. Meanwhile, in the case of ternary alloys constructed by exchanging Ni atoms with Co atoms in Ni3Al, the effects of temperature on dislocation motion are more complex. At low applied shear stress, the dislocation motion is disturbed by solute Co (i.e., solid-solution strengthening). In this case, the temperature promotes dislocation motion because the potential energy barrier dominates the thermally activated dislocation dynamics. In contrast, when the applied shear stress increases, the temperature suppresses dislocation motion owing to the phonon drag effect. The drag effect also increases with increasing Co concentration. The origin of the energy barrier due to solute atoms at low applied shear stress can be explained by the interaction between the partial dislocation and solute Co. Meanwhile, the origin of the drag effect on the superlattice dislocation dynamics is discussed on the basis of atomic vibrational motions and wavy dislocation behaviors. In this study, we propose and construct a map showing the dominant effects on complex superlattice dislocation dynamics. The map has energy-barrier-dominated, phonon-drag-dominated, and sound velocity-dominated regions, suggesting that the dominant factor changes depending on the applied shear stress, temperature, and strength of the solute–dislocation interaction.

Laser powder bed fusion of a Ni3Al-based intermetallic alloy with tailored microstructure and superior mechanical performance

Ni3Al-based alloys are excellent candidates for the structural materials used for turbine engines due to their excellent high-temperature properties. This study aims at laser powder bed fusion and post-hot isostatic pressing (HIP) treatment of Ni3Al-based IC-221 ​M alloy with a high γ′ volume fraction. The as-built samples exhibits unavoidable solidification cracking and ductility dip cracking, and the laser parameter optimization can reduce the crack density to 1.34 ​mm/mm2. Transmission electron microscope (TEM) analysis reveals ultra-fine nanoscale γ′ phases in the as-built samples due to the high cooling rate during rapid solidification. After HIP treatment, a fully dense structure without cracking defects is achieved, which exhibits an equiaxed structure with grain size ∼120–180 ​μm and irregularly shaped γ′ precipitates ∼1–3 ​μm with a prominently high fraction of 86%. The room-temperature tensile test of as-built samples shows a high ultimate tensile strength (σUTS) of 1039.7 ​MPa and low fracture elongation of 6.4%. After HIP treatment, a significant improvement in ductility (15.7%) and a slight loss of strength (σUTS of 831.7 ​MPa) are obtained by eliminating the crack defects. Both the as-built and HIP samples exhibit retained high σUTS values of 589.8 ​MPa and 786.2 ​MPa, respectively, at 900 ​°C. The HIP samples exhibita slight decrease in ductility to ∼12.9%, indicating excellent high-temperature mechanical performance. Moreover, the abnormal increase in strength and decrease in ductility suggest the critical role of a high γ′ fraction in cracking formation. The intrinsic heat treatment during repeating thermal cycles can induce brittleness and trigger cracking initiation in the heat-affected zone with notable deteriorating ductility. The results indicate that the combination of LPBF and HIP can effectively reduce the crack density and enhance the mechanical properties of Ni3Al-based alloy, making it a promising material for high-temperature applications.

Microstructure and mechanical properties of a novel polycrystalline Ni3Al-based intermetallic alloy

A polycrystalline Ni3Al-based intermetallic alloy was successfully prepared via vacuum induction melting processing. Next, the microstructural evolution and mechanical properties of the intermetallic alloy were investigated by solution treatments at temperatures ranging from 1230 to 1290 °C. The results showed that the as-cast polycrystalline Ni3Al-based intermetallic alloy consisted of dendrite γ+γ′ with interdendritic distribution, matrix γ′ phase, and eutectic γ–γ′, and the total volume fraction of γ′ phase was ∼83.2%. After solution treatment at 1270 °C, the size and morphology of the γ′ phase changed and the secondary γ′ phase precipitated in the γ channel. The maximum room temperature yield strength, tensile strength and elongation reached 488 MPa, 686 MPa, and 5.2%, respectively. At 1100 °C, the high-temperature tensile strength of sample subjected to solution treatment at 1250 °C reached 203 MPa. It was found that solution treatment did not eliminate the incipient γ′m, resulting in the weaker interdendritic boundary area in the alloy. Moreover, noteworthy, α-Cr particles were found in the γ′m of the polycrystalline Ni3Al-based intermetallic alloy, revealing an orientation relationship such as [123]α-Cr∥[123]γ' and (1–21)α-Cr∥(11–1)γ'. The α-Cr particles have the strengthening effect of second-phase particles through pinning dislocations, which can improve the high-temperature creep performance of the alloy.

Influences of laser remelting treatments on microstructure and tensile properties of multiphase Ni3Al-based intermetallic alloy

The microstructure control of multiphase Ni3Al-based intermetallic alloy by traditional heat treatment is still limited, especially size and volume fraction of interdendritic β phase cannot be effectively reduced, due to the limitation of interdendritic β phase dissolution temperature close to the melting point of the alloy. In present work, the temperature gradient and solidification rate of the alloy under different laser remelting parameters were calculated by the Rosenthal equation. Afterward, the as-cast multiphase Ni3Al-based intermetallic alloy (19.4 vol% of interdendritic β phase) was remelted under different laser parameters of 3000–4500 W laser power and 5–20 mm/s laser scanning rate. And the effects of different laser remelting parameters on the microstructure, room temperature, and 980 °C high-temperature tensile properties of multiphase Ni3Al-based intermetallic alloy were studied. The results show that laser remelting treatment with the appropriate parameters (4000 W laser power and 10 mm/s laser scanning rate) can effectively reduce the size and volume fraction of interdendritic β phase and the morphology and size of γ′ phase in the γ′+γ dendrite, and significantly refine the microstructure of alloy, thus effectively improving the room temperature and 980 °C tensile properties of multiphase Ni3Al-based intermetallic alloy. Under the optimum laser remelting parameters of 4000 W-10 mm/s, the tensile strength at room temperature and 980 °C of the corresponding laser remelting alloy can reach 1215 MPa and 280 MPa, respectively, and the elongations after fracture of the corresponding alloy are 14.09% and 6.41%, respectively.

Strain-modulated Ni3Al alloy promotes oxygen evolution reaction

Oxygen evolution reaction (OER) is crucial for electrochemical water splitting. However, corrosive issues in the alkaline environment impede its applications. Here, we explore the corrosion-resistant multiphase Ni3Al-based intermetallic alloy (Ni3Al alloy) as self-supported OER catalysts for the first time. A two-step treatment, annealing at high-temperature followed by acid oxidation is performed to improve the catalytic activity. Further investigations demonstrate that the annealing process can tune the size of γ′ precipitates, which will modify the strain effects caused by γ/γ′ lattice mismatch to boost the OER activity. The oxidation of Ni during the acid pretreatment can facilitate the formation of active high-valent Ni sites during the OER process, and further enhance the OER performance. The optimized electrode exhibits a low overpotential of 280 mV at 10 mA cm−2, a small Tafel slope of 73.5 mV dec−1 and long-term stability of 180 h in 1 M KOH solution. The findings in this paper will inspire the exploration of high stability OER catalysts and the functional application of Ni3Al alloy.

Creep behaviors of multiphase Ni3Al-based intermetallic alloy after 1000 °C-1000 h long-term aging at intermediate temperatures

The microstructure evolutions during 1000 °C-1000 h long-term aging and relevant 800 ~ 840 °C/220 ~ 300 MPa creep behaviors of a novelly designed multiphase Ni3Al-based intermetallic alloy were investigated. Results showed this alloy presented excellent intermediate temperature creep properties even after 1000 °C-1000 h long-term aging. A kind of bimodal R-type γ′ rafting structures were observed in the 800 °C/220 MPa creep process of the long-term aged alloy, and the creep resistance levels of three kinds of γ′ rafting structures were evaluated to be in the order of bimodal R-type γ′ rafting structures > R-type γ′ rafting structure > N-type γ′ rafting structure. The 1000 °C-1000 h long-term aging can promote the inhomogeneous precipitation of Cr4.6MoNi2.1 phase at the edge γ′+γ dendrite area, which can play the role of pinning dislocations and inhibiting the further rafting and coarsening of the R-type γ′ rafts during creep. Differed from Ni-based alloys and other Ni3Al-based alloys, this long-term aged alloy exhibited abnormal four-stage creep characteristics, which consisted of atypical short steady-state creep stage with step fluctuant feature and atypical long accelerated creep stage with creep rate increasing at a constant rate.

Precipitation of intersected plate-like γ′ phase in β and its effect on creep behavior of multiphase Ni3Al-based intermetallic alloy

The precipitation and evolution of intersected plate-like γ′ (γ′P) phase within interdendritic β during long-term aging and its relevant influence on the creep behavior of a multiphase Ni3Al-based intermetallic alloy are investigated. Results show that the precipitation and growth of the γ′P precipitates during 800 °C aging of original as-cast alloy are significantly rapid, which can be fully precipitated in 60 min to form the intersected morphology and continue to coarsen and grow during the extended aging and subsequent 800 °C creep processes. The γ′P precipitates have the Bain orientation relationship (OR) of (100)γ′P//(100)β and [0-11]γ′P//[001]β with the interdendritic β-matrix. The precipitation of intersected γ′P phase is sensitive to the elemental fluctuations in β-matrix, and the preceding high-temperature annealing treatment can significantly reduce the elemental inhomogenization and inhibit its generation within the interdendritic β. Precipitation of adequate intersected γ′P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic β during creep, resulted in the improved creep properties. Slightly rafted γ′ after long-term aging at 800 °C transitioned into R-type γ′-raft structure during creep at 800 °C/220 MPa, and can induce larger creep resistance than N-type γ′-raft structure.

Formation and widening mechanisms of envelope structure and its effect on creep behavior of a multiphase Ni3Al-based intermetallic alloy

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni3Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni3Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni3Al phase with ordered face-centered L12 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101]γ'-envelope//[01‾1]dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L12 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni3Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr23C6 carbides surrounded by the widened γ′-envelope.

Effect of Temperature on the Low-Cycle Fatigue Characteristics of Single Crystals Made of an Ni3Al-Based Rhenium-Containing Intermetallic Alloy

Abstract—The low-cycle fatigue (LCF) of the single crystals made of a γ'-Ni3Al-based VKNA-25 γ' + γ nickel superalloy is studied at 104 cycles and a temperature of 20, 750, and 900°C. A temperature anomaly in the LCF fatigue limit of the VKNA alloy, namely, its increase with temperature in the range from 20 to 750°C, is detected. This anomaly is related to a change from octahedral to cubic slip at the temperature of the peak in L12-ordered alloys and to a change in the properties of the γ' and γ phases. At normal temperature, the static tensile strength and the LCF characteristics of the [111] single crystals are higher than those of the single crystals with other crystallographic orientations (CGOs). The static tensile strengths of the [001] and [111] single crystals at 900°C and the strengths of the single crystals with all CGOs at 1000°C are almost the same, and the LCF fatigue limit of the [001] single crystals at 900°C is slightly higher than that of the [111] single crystals.

Influences of solution cooling rate on microstructural evolution of a multiphase Ni3Al-based intermetallic alloy

In present work, a multiphase Ni3Al-based intermetallic alloy with complex components of γ'+γ dendrite comprising extremely high volume fraction (81 vol %) of γ′ phase and interdendritic β phase is investigated, the microstructure evolution and phase transformation behavior tailored by different solution cooling rates are studied by applying different cooling methods of water cooling (WC, 138 °C/s), air cooling (AC, 72 °C/s), and furnace cooling (FC, 0.05 °C/s) after solution treatment at 1200 °C for 10 h. Results show that the cubic degree, size and volume fraction of cuboidal primary γ′ as well as size of spherical secondary γ′ particles in the γ'+γ dendrite are improved by decreasing cooling rates, and the average sizes of the primary and secondary γ′ particles are found to meet logarithmic relationships with cooling rates. There are three kinds of precipitates in the interdendritic β phase resulted from different cooling methods, i.e., the rod-like Cr3C2 carbides, the semi-spherical α-Cr particles, and the long acicular γ′ precipitates. The lower cooling rate decreases the number of rod-like Cr3C2 carbides, increases the number and size of the semi-spherical α-Cr particles, and promotes the formation of long acicular γ′ precipitates in the interdendritic β-matrix. The semi-spherical α-Cr particles play the role of pinning dislocations in the β-matrix and have the orientation relationship of [‾110]α-Cr//[‾110]β-matrix. In addition, the precipitation of semi-spherical α-Cr particles in β-matrix is observed to accomplish in two sequential stages and grow in an onion-like ordered way. Stacking faults existed in partial of α-Cr particles due to the nanotwins in β-matrix. Microtwins are observed in the interdendritic β-matrix after water cooling, which have a bimodal configuration of nanotwins at the microtwinning boundaries and interior stacking faults. Besides, γ′-envelope formed between the γ'+γ dendrite and interdendritic β phase dependent on the solution cooling processes.

Dynamic deformations tests of Ni3Al based intermetallic alloy by using the split Hopkinson pressure bar technique

In this work, the Ni3Al-based intermetallic alloy was subjected to room temperature dynamic plastic deformation tests by using a split Hopkinson pressure bar technique. The dynamic compression processes were carried out at strain rates in the range of =(1.9x102 / 1x104 s-1). A strong impact of applied deformation conditions on microstructure and mechanical properties evolution in the examined Ni3Al intermetallic, was documented. Generally, very high maximum compressive stress values were obtained, reaching 5500 MPa for the sample deformed at the highest strain rate (i.e. ??=1x104 s-1). The results of performed SEM/EBSD evaluation point towards an occurrence of dynamic recovery and recrystallization phenomena in Ni3Al samples deformed at high strain rates.

Structural and Phase Characteristics of the Ni3Al-Based Intermetallic Alloy Fabricated by High-Gradient Directional Solidification and Selective Laser Melting

The microstructure and the phase composition of an experimental Ni3Al-based intermetallic alloy containing (wt %) 7.8 Al, 14.5 Ta, 0.9 Mo, 0.5 W, 0.5 Re, 0.15 C, and Ni for balance, which was prepared by slow (6 mm/h) high-gradient (15°C/mm) directional solidification and selective laser melting, are studied, and the lattice parameters of phases and solidus and liquidus temperatures are determined. The phase compositions of the intermetallic alloy samples prepared by these technologies are found to be identical and consist of a γ'-phase matrix and TaC carbide particles.

Structure and Properties of the Solid-Phase Joints of a Wrought EP975 Nickel Alloy and a Single-Crystal Intermetallic VKNA-25 Alloy Formed by Pressure Welding at a Strain of 24% under the Superplasticity of the EP975 Alloy

Experiments on solid-phase pressure welding (PW) of a wrought EP975 nickel superalloy with a Ni3Al-based VKNA-25mono intermetallic alloy in a single-crystal state are performed under the superplasticity of the EP975 alloy, where its deformation was 24%. The structure and the phase composition of the interaction zone that forms between the materials to be joined after PW and PW followed by heat treatment are studied. The mechanical properties of the welded samples with a solid-phase joint are investigated. The ultimate tensile strength of the welded samples is shown to be 0.76 of the strength of the VKNA-25 alloy.

ПЕРСПЕКТИВЫ ПРИМЕНЕНИЯ СПЛАВА НА ОСНОВЕ ИНТЕРМЕТАЛЛИДА Ni3Al ТИПА ВКНА-4УР В АДДИТИВНЫХ ТЕХНОЛОГИЯХ

ПЕРСПЕКТИВЫ ПРИМЕНЕНИЯ СПЛАВА НА ОСНОВЕ ИНТЕРМЕТАЛЛИДА Ni3Al ТИПА ВКНА-4УР В АДДИТИВНЫХ ТЕХНОЛОГИЯХ

Differential speed rolling of Ni3Al based intermetallic alloy – Effect of applied processing on structure and mechanical properties anisotropy

Results of the first principles study on the effect of differential speed rolling (DSR) and a post-deformation annealing on a microstructure and microtexture evolution as well related mechanical properties anisotropy and formability of Ni3Al-based intermetallic alloy, have been shown in the present paper. Results of the EBSD structure analysis and mechanical properties evaluation indicate that the application of the proposed DSR process leads to a higher stored energy of deformation (and thus a higher strain hardening) than in the case of the conventional rolling process. Consequently, a subsequent post-deformation annealing results with a greater extend of the structure restoration processes than in the case of the normal cold rolling. The formability of the investigated intermetallic was evaluated by both a calculation of Lankford parameter (the r values) and Erichsen cupping tests. It is found that the susceptibility to deep drawing of the investigated Ni3Al intermetallic alloy is much lower than that of conventional deep drawn materials (e.g. aluminum alloys or low carbon steels). Nonetheless, the Ni3Al alloy exhibited a very low planar anisotropy and the yield strength anisotropy (and thus, a low tendency to strain localization upon drawing) independently on its structure state. This behavior should be mainly attributed to its simple phase composition (the lack of “a second phase” precipitation).

Dendrite segregation in Ni3Al-based intermetallic single crystals alloyed with Cr, Mo, W, Ti, Co, and Re

The character of dendrite segregation in Ni3Al-based intermetallic VKNA-type alloy single crystals with a dendritic–cellular structure is studied. Distribution coefficient kd of an alloying element (AE) in the alloy during solidification kd = cd.a.I/c0 (c0 is the AE content in the alloy (liquid phase composition), cd.a.I is the AE content in primary dendrite arms of the alloy (in the solid phase)) and segregation coefficient ks = cd.a.I/ci.d (ci.d is the AE content in the interdendritic space) have been found. A comparative study of the dendrite segregation parameters in VKNA-nype Ni3Al-based intermetallic alloys and the well-known ZhS36-type nickel superalloy shows that the intermetallic alloys satisfy to the rule deduced for two- and three-component nickel-based superalloys: if an introduced AE increases the melting temperature of the basic metal, we have kd > 1 (Co, W, Re); if it decreases the melting temperature, we have kd < 1 (Al, Ti, Cr, Mo). Dendrite segregation coefficients ks are dependent on the proportion of the AE contents in the alloys. In nickel superalloys, the dendrite segregation of aluminum, tungsten, and rhenium is higher than that in the intermetallic alloys. The dendrite segregation coefficients of tungsten and rhenium is higher by a factor of 1.5–2 than that in the VKNA-type intermetallic alloys with a low content of refractory metals. This can be due to the retardation of diffusion of refractory metals in the solid phase of a nickel superalloy highly alloyed with these elements.

Analysis of the Effect of the Wearing Type on Surface Structural Changes of Ni&lt;sub&gt;3&lt;/sub&gt;Al-Based Intermetallic Alloy

Results of an analysis of effect of wearing type on surface structural changes of a Ni3Al intermetallic alloy, are shown in the present paper. A microstructure evaluation was carried out by Quanta 3D FEG field emission gun scanning electron microscope (FEG SEM) equipped with an integrated EDS/WDS/EBSD system. The Ni3Al-based intermetallic alloy with an addition of boron, zirconium and chromium was examined. The investigated material had γ’ single-phase, ordered solid solution structure with 20 μm grain size. An electron backscatter diffraction (EBSD) method was applied to visualize surface structural changes upon an abrasive, a cavitational and a tribological wearing of the material.An observation of surface layer after the abrasive wear was carried out on samples examined in loose abradant by T-07 tester and according to GOST 23.2008-79 norm. An analysis of cavitational wear on changes in the near surface area of Ni3Al-based alloy was performed on an impact-jet stand. Observed structural changes were described based on results of the SEM/EBSD complex structural examination and hardness measurements. It was found, that the EBSD is an effective and sensitive method that allows estimating surface strain introduced during analyzed wearing types.

Differential speed rolling of Ni3Al based intermetallic alloy—Analysis of the deformation process

Results of the first principles study on applying of a differential speed rolling (DSR) to a processing of the Ni3Al-based intermetallic alloy have been shown in the present paper. Obtained samples were free of cracks and shape defects, what indicates that selected parameters of the DSR method are suitable for the processing of Ni3Al intermetallics. It was also found that, the differentiation of rolling speed results with a prominent reduction and stabilization of the rolling force value during a rolling pass, and causes an increase of the imposed strain and a linear rise of the strain rate. Additionally, the DSR processed samples were characterized by a higher mechanical strength than the material deformed with an equal speed of both rolls.