Tensile Uniform Elongation Research Articles

Revealing the high strength and high thermal stability of a nano-lamellar Cu-0.1 at.% Zr alloy

Nano-lamellar structured Cu-0.1 at.% Zr alloys are prepared by the combination of equal channel angular pressing and subsequent cryogenic rolling. The advanced processing route enables a systematic reduction of the lamellae boundary spacing down to 45 nm and a concurrent increase of yield strength up to 626 MPa. Subsequent annealing at temperatures up to 673 K induced recovery processes and uniform coarsening through triple junction motion, avoiding a catastrophic degradation of the microstructure and performance and preserving the yield strength at a level comparable to ultrafine-grained copper. Simultaneously, the tensile uniform elongation experienced a substantial recovery up to 11.9 %. The high thermal stability is mainly attributed to the low mobility of grain boundary and the relaxation of non-equilibrium grain boundaries, both of which are facilitated by the microalloying of Cu with Zr. Based on the theory of dislocation pile-up at boundaries, the Hall-Petch equation is adopted and extended to the lamellar structured Cu-0.1 at.% Zr alloy, revealing a lower-critical bound of 90 nm for lamellae boundary spacing, corresponding to an effective boundary spacing of about 150 nm.

TiZrHfNb refractory high-entropy alloys with twinning-induced plasticity

The twinning-induced plasticity (TWIP) effect has been widely utilized in austenite steels, β-Ti alloys, and recently developed face-centered-cubic (FCC) high-entropy alloys (HEAs) to simultaneously enhance the tensile strength and ductility. In this study, for the first time, the TWIP effect through hierarchical {332<113> mechanical twinning has been successfully engineered into the TiZrHfNb refractory HEAs by decreasing temperature and tailoring the content of Nb to destabilize the BCC phase. At cryogenic temperature, the comprehensive hierarchical {332}<113> mechanical twins are activated in TiZrHfNb0.5 alloy from micro- to nanoscale and progressively segment the BCC grains into nano-scale islands during deformation, ensuring sustainable strain hardening capability and eventually achieving a substantial 35% uniform tensile elongation. In addition, at 77 K, the tensile strength and uniform elongation of TiZrHfNbx alloys can further be adjusted by a moderate increase of Nb. The results above shed new light on the development of high-performance refractory HEAs with a high and adjustable strength and ductility combination down to the cryogenic temperature.

Simultaneously increasing mechanical and corrosion properties in CoCrFeNiCu high entropy alloy via friction stir processing with an improved hemispherical convex tool

It generally shows the trade-off for the mechanical properties and corrosion resistance of high entropy alloys (HEAs). In this study, friction stir processing (FSP) with an improved hemispherical convex tool was applied to process the as-cast CoCrFeNiCu HEA, which simultaneously increased the mechanical properties and corrosion resistance of the HEA by a large degree. After FSP, the grains were significantly refined from 6 μm to ∼300 nm, with 91% of high angle grain boundaries and 18% of nanometer twins, leading to a remarkable increase in the yield strength from ∼194 MPa to ∼935 MPa. More importantly, the FSP HEA showed an excellent strength-ductility synergy with yield strength of ∼695 MPa and a uniform tensile elongation of ∼17.5% after annealing, which was mainly attributed to the comprehensive role of equiaxed ultrafine microstructure, second phase and the high fraction of annealing twins (27%). In addition, the FSP HEA showed higher corrosion resistance than the base material by electrochemical test, which was mainly attributed to the improvement of the self-corrosion potential and the stability of the passivation film caused by the mitigation of Cu segregation and grain refinement. This study provides a new way to simultaneously increase the mechanical properties and corrosion resistance in HEAs.

Hierarchical precipitates facilitate the excellent strength-ductility synergy in a CoCrNi-based medium-entropy alloy

An interstitial carbon-doped CoCrNiSi0.3 medium-entropy alloy (MEA) has been developed, which exhibits excellent strength-ductility synergy. This has been achieved through the implementation of a three-level hierarchical-precipitate-microstructure design. The CoCrNiSi0.3C0.048 MEA displays an ultra-high yield strength of 1.34 GPa, accompanied by a uniform tensile elongation of 8.62%, surpassing the performance of most carbon/silicon-doped MEAs. The three-level hierarchical precipitate structure comprises the primary Cr23C6 carbides (2–10 μm), the secondary SiC precipitates (200–500 nm) surrounding the grain boundaries, and the tertiary SiC precipitates (∼50 nm) within the grains. The strengthening and toughening behavior of the current MEAs are attributed to multiple mechanisms, including crack branching and blunting in Cr23C6 carbides, dislocation-bypass mechanism around the non-shearable SiC precipitates, hetero-deformation-induced strengthening caused by the interface between matrix/three-level hierarchical precipitates, as well as stacking-fault networks and dense nanotwins activated by precipitates in the matrix. These findings provide valuable insights into the development of high-performance alloys for engineering applications in the future.

Origin of the excellent ductility in super high oxygen doped titanium

Here, we prepare a novel Ti-doped ultra-high oxygen content (0.79 wt%) material with outstanding ductility. Meanwhile, through a micropillar compression test, the remarkable uniform tensile elongation and high work hardening rate shown in Ti-0.79 wt%O are attributed to the multi-slipping systems, which are then often cut and tangled with one another.

In situ observation of the effect of the twin boundary orientation on the mechanical properties of single crystalline Ni

The construction of twin boundaries (TBs) in materials is a remarkable way of promoting their strength and ductility. However, the effects of TB orientation on the mechanical properties have not been reported experimentally so far. Using a state-of-the-art in situ tensile stage equipped in a transmission electron microscope, uniaxial tensile tests were performed on three single-crystalline Ni samples with TB parallel and perpendicular to the tensile direction and no TB. The results showed that the uniform tensile elongation strongly depends on TB, 120% for the perpendicular TB sample, 99% for the parallel TB sample, and only 55% for the no TB sample. In addition, dislocation interaction before reaching the perpendicular CTB contribute to cross-slip and dynamic formation of dislocation jogs, thereby improving strain hardening and resulting in a large uniform tensile elongation.

Mechanical property comparisons between CrCoNi medium-entropy alloy and 316 stainless steels

• The CrCoNi medium-entropy alloy shows a superior strength and ductility synergy, Charpy impact energy ( A K ), and fracture toughness at cryogenic temperatures as compared to both 316L and 316LN stainless steels. • These three alloys all rely heavily on shear transformations upon plastic straining, with deformation twinning dominating in CrCoNi while martensitic transformation involved in stainless steels. We systematically compared the mechanical properties of CrCoNi, a recently emerged prototypical medium-entropy alloy (MEA) with face-centered-cubic (FCC) structure, with hallmark FCC alloys, in particular, the well-known austenitic 316L and 316LN stainless steels, which are also concentrated single-phase FCC solid solutions and arguably next-of-kin to the MEAs. The tensile and impact properties, across the temperatures range from 373 K to 4.2 K, as well as fracture toughness at 298 K and 77 K, were documented. From room temperature to cryogenic temperature, all three alloys exhibited similarly good mechanical properties; CrCoNi increased its tensile uniform elongation and fracture toughness, which was different from the decreasing trend of the 316L and 316LN. On the other hand, the stainless steels showed higher fracture toughness than CrCoNi at all temperatures. To explain the differences in macroscopic mechanical properties of the three alloys, microstructural hardening mechanisms were surveyed. CrCoNi MEA relied on abundant mechanical twinning on the nanoscale, while martensitic transformation was dominant in 316L at low temperatures. The deformation mechanisms in the plastic zone ahead of the propagating crack in impact and fracture toughness tests were also analyzed and compared for the three alloys.

Enhanced tensile ductility of tungsten microwires via high-density dislocations and reduced grain boundaries

• W microwires with designed microstructure were microfabricated under EBSD guidance. • High-density dislocations were introduced in W with reduced grain boundaries. • Ultrahigh strength and ductility were achieved in engineered W microwires. • The motion of pre-existing high-density dislocations produces high ductility of W. Despite being strong with many outstanding physical properties, tungsten is inherently brittle at room temperature, restricting its structural and functional applications at small scales. Here, a facile strategy has been adopted, to introduce high-density dislocations while reducing grain boundaries, through electron backscatter diffraction (EBSD)-guided microfabrication of cold-drawn bulk tungsten wires. The designed tungsten microwire attains an ultralarge uniform tensile elongation of ~10.6%, while retains a high yield strength of ~2.4 GPa. in situ TEM tensile testing reveals that the large uniform elongation of tungsten microwires originates from the motion of pre-existing high-density dislocations, while the subsequent ductile fracture is attributed to crack-tip plasticity and the inhibition of grain boundary cracking. This work demonstrates the application potential of tungsten microcomponents with superior ductility and workability for micro/nanoscale mechanical, electronic, and energy systems.

Improved Mechanical Property and Corrosion Resistance of SAF2507 Duplex Stainless Steel with Heterogeneous Lamella Structure

Severe cold rolling and subsequent annealing are implemented on ferrite and austenite duplex stainless steel (DSS). The microstructure and mechanical properties of the processed sample are investigated systematically. The results show that the heterogeneous lamella (HL) structure can be generated in the cold‐rolled DSS by an appropriate annealing strategy. After annealing at 900 °C for 2 min, the HL‐structured sample with an alternative distribution of micrograined lamellae in ferrite and ultrafine‐grained lamellae in austenite obtains a unique combination of high strength and good ductility; the corresponding yield strength and uniform tensile elongation are ≈730 MPa and ≈23%, respectively. In addition, the corrosion behavior of the DSS in different processing states is investigated by the electrochemical method. The HL‐structured DSS possesses a superior corrosion resistance compared with other counterparts, which is attributed to the formation of a more stable passive layer on the surface. The precipitation of the sigma phase after long‐term annealing leads to deterioration of corrosion performance and mechanical properties.

Hierarchical crack buffering triples ductility in eutectic herringbone high-entropy alloys.

In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strain-hardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength.

Enhanced strengthening and hardening via self-stabilized dislocation network in additively manufactured metals

The advent of additive manufacturing (AM) offers the possibility of creating high-performance metallic materials with unique microstructure. Ultrafine dislocation cell structure in AM metals is believed to play a critical role in strengthening and hardening. However, its behavior is typically considered to be associated with alloying elements. Here we report that dislocations in AM metallic materials are self-stabilized even without the alloying effect. The heating–cooling cycles that are inherent to laser power-bed-fusion processes can stabilize dislocation network in situ by forming Lomer locks and a complex dislocation network. This unique dislocation assembly blocks and accumulates dislocations for strengthening and steady strain hardening, thereby rendering better material strength but several folds improvements in uniform tensile elongation compared to those made by traditional methods. The principles of dislocation manipulation and self-assembly are applicable to metals/alloys obtained by conventional routes in turn, through a simple post-cyclic deformation processing that mimics the micromechanics of AM. This work demonstrates the capability of AM to locally tune dislocation structures and achieve high-performance metallic materials.

Hierarchical grain size and nanotwin gradient microstructure for improved mechanical properties of a non-equiatomic CoCrFeMnNi high-entropy alloy

This study explored a multi-mechanism approach to improving the mechanical properties of a CoCrFeMnNi high-entropy alloy through non-equiatomic alloy design and processing. The alloy design ensures a single-phase face-centered cubic structure while lowering the stacking fault energy to encourage the formation of deformation twins and stacking faults by altering the equiatomic composition of the alloy. The processing strategy applied helped create a hierarchical grain size gradient microstructure with a high nanotwins population. This was achieved by means of rotationally accelerated shot peening (RASP). The non-equiatomic CoCrFeMnNi high-entropy alloy achieved a yield strength of 750 MPa, a tensile strength of 1050 MPa, and tensile uniform elongation of 27.5%. The toughness of the alloy was 2.53 × 1010 kJ/m3, which is about 2 times that of the same alloy without the RASP treatment. The strength increase is attributed to the effects of grain boundary strengthening, dislocation strengthening, twin strengthening, and hetero-deformation strengthening associated with the heterogeneous microstructure of the alloy. The concurrent occurrence of the multiple deformation mechanisms, i.e., dislocation deformation, twining deformation and microband deformation, contributes to achieving a suitable strain hardening of the alloy that helps to prevent early necking and to assure steady plastic deformation for high toughness.

Understanding material behaviour of ultrafine-grained aluminium nano-composite sheets with emphasis on stretch and bending deformation

Abstract This study investigates the forming limits of cryo-rolled Ultrafine Grained (UFG) aluminium sheet for loading conditions relevant to sheet metal forming. It is shown that a combination of nano-particle strengthening with an annealing heat treatment enables the production of cryo-rolled UFG aluminium with up to 12 % higher material strength and 20 % higher formability compared with the unreinforced UFG condition. While nano-particle strengthening does not significantly influence the tensile ductility, it reduces the bend fracture limit by over 50 % due to an increased dislocation density and a diffuse sub-grain structure. The level of the forming limit curves (FLD) of all UFG materials was up to 5 times higher than their uniform tensile elongation and hardening index; this is attributed to a high strain-rate sensitivity, which increased with annealing heat treatment. In roll forming, the UFG aluminium showed similar material behaviour and shape defects as observed for conventional sheet metals in previous studies. However, given the reduced bend fracture limits observed with nano-particle addition, this study suggests that roll forming may not be a suitable manufacturing solution for nano-particle strengthened UFG sheet metal.

Additive manufacturing of fine-grained and dislocation-populated CrMnFeCoNi high entropy alloy by laser engineered net shaping

The equiatomic CrMnFeCoNi high entropy alloy is additively manufactured by the laser engineered net shaping (LENSTM) process, and the solidification conditions, phase formation, as-deposited microstructures, and tensile behavior are investigated. The LENSTM-deposited CrMnFeCoNi alloy exhibits a single-phase disordered face centered cubic (FCC) structure, as evidenced by X-ray diffraction (XRD), and rationalized by Scheil's solidification simulation. Furthermore, microstructures at multiple length scales, i.e. columnar grains, solidification substructures, and dislocation substructures, are formed. The tensile deformation process is mainly accommodated by dislocation activities with the assistance of deformation twinning. The tensile yield strength of the LENSTM-deposited CrMnFeCoNi alloy is comparable to that of finer-grained wrought-annealed counterparts, due to the additional initial-dislocation strengthening. However, the uniform tensile elongation, by contrast, is lowered, which is attributed to the increased dynamic dislocation recovery rate and hence the weakened work hardening capability of the LENSTM-deposited CrMnFeCoNi. This study demonstrates the capability of the LENSTM process for manufacturing the CrMnFeCoNi alloy, with high performance, for engineering applications.

Significant strengthening in superlight Al-Mg alloy with an exceptionally large amount of Mg (13 wt%) after cold rolling

Cold rolling could be successfully applied to an Al-Mg alloy with an exceptionally high amount of Mg (13 wt%) by effectively removing the Al3Mg2 phases; this removal was performed before cold rolling using solid solution treatment before extrusion and re-solid solution treatment after extrusion. The microstructures and the tensile properties of the Al-13Mg alloy were compared with those of the Al-Mg alloys with lower amounts of Mg (5–10 wt%), which were prepared following the same processing route. As the Mg content increased, the density of shear bands crossing each other increased during cold rolling, leading to more effective breakup of the initial coarse grains and development of nanosized cell structures and ultrafine grains, and a higher accumulation of dislocation density in the matrix. This result occurred because dynamic recovery was suppressed by the decrease of stacking fault energy and dislocation slip mobility due to the addition of Mg amount. The cold rolled Al-13Mg alloy, which is lighter than pure aluminum by 8.2%, exhibited a yield stress of 653 MPa, a maximum strength of 733 MPa and a total (uniform) tensile elongation of 5.1%. These strength values are superior to those of the conventional Al-Mg alloys with lower Mg contents (≤7 wt%) processed by severe plastic deformation via equal channel angular pressing. The analysis of the strengthening mechanisms of the cold rolled Al-13Mg alloy indicates that the strengthening effect by generation and arrangement of dislocations is more significant that solid solution strengthening.

New non-contact measurement method of deformation at tensile test of thin film via digital image correlation technique

Digital image correlation technique has been widely and effectively applied in the measurement of material properties. This paper presents a new method named Stride-Crawl Approaching Method by combining the steepest descent method and the Levenberg-Marquardt method. It is no longer necessary to provide initial guess of deformation by applying this method. And, the result can always converge to the absolute optimum. Moreover, regardless of the deformations are large or small, this method can always be applied directly. Therefore, this method is more suitable and has great advantages compared to other conventional methods. The proposed method was tested two different simple deformation forms including the rigid body translation, and the uniform tensile elongation by comparing the experimental values with the theoretical values. The images for the verification were generated by numerical simulation. And then the errors were analyzed and it turned out that the error of the displacements was about pixels and the error of displacement gradients was usually limited in 3%. Finally, the stress-strain curves of BeCu was drawn using the output data of the proposed method. And then the tensile properties of BeCu including the elastic modulus, 0.2% yield strength, ultimate tensile strength, and fracture strain were obtained from the stress-strain curves.

Tensile elongation of lean-alloy austenitic stainless steels: transformation-induced plasticity versus planar glide

ABSTRACTAn Fe–13Cr–3.4Mn–0.47C lean-alloy stainless steel was made austenitic by solution annealing at 1250°C. Tensile tests between 20 and 200°C indicated enhancement of ductility at higher temperatures. At 200°C where planar glide, manifested as deformation twinning, was the dominant deformation mechanism, a uniform tensile elongation of 102% was achieved. At 20°C where deformation-induced α′-martensitic transformation replaced deformation twinning as the dominant deformation mechanism, tensile elongation was significantly impaired. The tensile elongation contribution by the planar glide was estimated to be at least four times that of the α′-TRIP (transformation-induced plasticity) mechanism. The results indicate that inexpensive lean-alloy austenitic stainless steels exhibiting pronounced α′-formation at room temperature could become highly formable at higher temperatures.

Mechanical behavior and impact toughness of the ultrafine-grained Grade 5 Ti alloy processed by ECAP

This paper reports on a study of the relationship between microstructure, mechanical behavior and impact toughness of the UFG Grade 5 Ti alloy. The mechanical behavior and impact toughness of the Grade 5 Ti alloy in a coarse-grained state, and in an ultrafine-grained (UFG) state produced by equal-channel angular pressing (ECAP) with subsequent deformation-and-thermal treatments via extrusion and warm upsetting in isothermal conditions, were studied extensively. It is shown that a strong refinement of α-grains (less than 250nm) in the alloy by ECAP and extrusion leads to high strength but with low values of the uniform elongation and lower impact toughness. It is demonstrated that, in order to increase the impact toughness of UFG Ti alloys, it is possible to use approaches realizing ductility enhancement associated with an increase of the strain hardening capacity. An enhancement in the impact toughness of the UFG alloy through an increase in the uniform tensile elongation of the sample is achieved by the preservation of the ultrafine size of α-grains (about 800nm) with predominantly high-angle boundaries and a decrease in the dislocation density due to recovery and dynamic recrystallization during warm upsetting.

Significant enhancement of tensile properties through combination of severe plastic deformation and reverse transformation in an ultrafine/nano grain lath martensitic steel

In this paper, microstructural evolution together mechanical properties variation in a Fe-24Ni (wt%) lath martensitic steel processed by accumulative roll bonding (ARB) and reverse transformation was investigated. The microstructural analysis indicated that after 6-cycle of the ARB process ultrafine/nano structure having mean grain size below 200nm is obtained. Furthermore, the typical lath martensitic structure transformed to complicated microstructure consisting of ultrafine equiaxed ferrite (α-Fe) as well as ultrafine grains of austenite (γ-Fe). The results also showed that by increasing the number of ARB cycle martensite starting (Ms) temperature decreased. As well, tensile test results showed that both the yield strength and uniform tensile elongation improved considerably by 6-cycle of the ARB process. Increasing uniform elongation can be attributed to stabilization of austenite during the ARB process.

Towards engineering of mechanical properties through stabilization of austenite in ultrafine grained martensite–austenite dual phase steel processed by accumulative roll bonding

In this study, microstructure development with respect to mechanical properties in ultrafine grained dual phase (martensite/austenite) by chemical composition of Fe-24Ni-0.1C (wt%) steel processed by 1-cycle and 6-cycle of accumulative roll bonding (ARB) process was investigated. Microstructure characterization by electron backscatter diffraction (EBSD) system showed that significant grain refinement occurred after 6 cycles ARB process and an average grain size smaller than 200nm was obtained. Moreover, phase analysis map obtained from EBSD system showed that the microstructure of starting material is mostly constituted of martensite phase (more than 70%) and transforms to retained austenite by 6 cycles of ARB process. This phenomenon has proved that retained austenite is stabilized during the ARB process through grain refinement. Uniaxial tensile tests exhibited that yield strength did not significantly improve by 1-cycle reverse transformation compared to the starting material. In contrast, further continuation of the ARB process caused gradual increase in yield and ultimate tensile strengths. The significant improvement in yield strength should be originated from significant grain regiment introduced by 6 cycles of the ARB process. Surprisingly, the uniform tensile elongation for the starting material and the 6 cycles ARB processed specimen are almost the same, though yield and ultimate tensile strengths are considerably improved by employing 6 cycles of ARB process.