Increase In Uniform Elongation Research Articles

Revisit the Efficacy of Transformation‐Induced Plasticity Mechanism and Martensite‐Austenite Composite Fiber Effect on the Increase of Uniform Elongation in Steel

The ductility of a retained austenite (RA)‐bearing steel is conventionally correlated with the gradual transformation of RA into martensite during straining, i.e., transformation‐induced plasticity (TRIP) mechanism. Nevertheless, current experiments and mathematical calculations illustrate that most of the RA transformed into martensite at quite an early stage of tensile straining. The result shows that only 4% of leftover RA exists after the initial tensile loading. It looks like enough RA is not available to operate TRIP to obtain successive ductility in the steel. Inexplicably, current work exhibits the maximum uniform tensile elongation after the major martensite transformation, which contradicts the conventional TRIP‐driven ductility theory. Thus, only 4% RA could be one of the prime factors that operate unfailingly to impart ductility in the steel. This work has created an understanding of the possible factors responsible for the observed ductility in the current steel while TRIP is not operating.

In-situ alloying of maraging steel with enhanced mechanical properties and corrosion resistance by laser directed energy deposition

Maraging steels are known for their exceptional strength derived from the martensite matrix and nano-precipitate strengthening. However, the trade-off to the high strength is often a loss of ductility and minimal strain hardening. Introducing metastable austenite to the matrix to activate transformation-induced plasticity (TRIP) is an effective approach to achieve high strength and ductility synergy. Existing methods to introduce TRIP into additively manufactured maraging steels are limited by the compositions of pre-alloyed powder or additional heat treatment steps. In-situ alloying via the laser directed energy deposition (L-DED) process allows flexibility in tailoring the alloying composition to achieve an austenite–martensite microstructure. Herein, we develop a TRIP–maraging steel by in-situ alloying of M789 with 316L (4–8 wt%) during the L-DED process. The addition of 316L facilitates austenite reversion in solution-treated maraging steel. As a result, the TRIP–maraging steel exhibits a 76 % increase in uniform elongation at the expense of a minimal sacrifice of strength. Furthermore, the corrosion resistance of the TRIP–maraging steel is enhanced. This work showcases a pathway to developing superior alloys by in-situ alloying via additive manufacturing.

Enhanced strength-ductility synergy in a gradient pseudo-precipitates heterostructured Al-2.5%Mg alloy: Design, fabrication, and deformation mechanism

Heterostructures of alloyed composites, comprising heterogeneous domains with dramatically different constitutive properties, hold remarkable potential to expand the realm of material design systems and resolve the trade-off between strength and ductility. This study introduces an innovative materials design method for synthesizing gradient pseudo-precipitates heterostructure (GPHS) in non-heat-treatable Al-2.5%Mg alloys. Utilizing cost-effective mild steel as both the diffusion source and protective layer, this heterostructure is achieved through pin-less friction stir-assisted cyclic localized deformation process. Exogenous Fe atoms diffuse across the interface by friction stir-induced heat conduction, forming Fe-Al second-phase particles in the Al alloy matrix. A rapid inter-diffusion mechanism is activated in conjunction with dense dislocation walls, grain boundaries, and sub-structures, resulting in the formation of pseudo-precipitates. These pseudo-precipitates are ultimately dispersed in a gradient distribution throughout the entire thickness of the Al alloy matrix induced by localized incremental deformation. The GPHSed Al-2.5%Mg alloy exhibits an enhanced synergy of strength and ductility, with a uniform elongation increase from 11 % to 21.2 %, while maintaining the strength. Multiple strengthening and hardening mechanisms, such as solid solution strengthening, dislocation hardening, and second phase strengthening, work synergistically to promote mechanical performance. Notably, the hetero-deformation between hard pseudo-precipitates and soft Al alloy matrix induces additional strain hardening, leading to high ductility. This work provides a fresh perspective on the design and fabrication of high-performance alloys with advanced heterostructures, especially for non-heat-treatable alloys.

Achieving high strength and high ductility of dual-phase steel via alternating lamellar microstructure

Overcoming the strength-plasticity trade-off in GPa-grade dual-phase steel while simultaneously enhancing both properties poses a significant challenge. In this study, we propose an innovative strategy to break this inherent dilemma by designing an alternately arranged ferrite and martensite lamellar microstructure with appreciable deformation coordination in dual-phase steel. Compared to traditional dual-phase steel (DP steel) with a block microstructure and the same phase components, the newly developed heterogeneous lamellar dual-phase steel (L-DP steel) prepared by a cyclic intercritical quenching process demonstrates a remarkable 90 % increase in uniform elongation (from 9.1 ± 0.4 % in DP steel to 17.3 ± 0.3 % in L-DP steel). The ultimate tensile strength increases slightly from 1002 ± 10 MPa (DP steel) to 1034 ± 11 MPa (L-DP steel). The lamellar martensite and lamellar ferrite with a certain hardness difference lead to high back stress during plastic deformation, thereby ensuring the ultra-high tensile strength of the L-DP steel. The improved ductility of the L-DP steel is primarily attributed to the appreciable coordinated deformation ability of the lamellar microstructure and the considerable plastic deformation ability of lamellar martensite. These two outstanding capabilities allow this unique microstructure to elongate continuously along the tensile direction, facilitating persistent work hardening without premature cracking. Furthermore, the heterogeneous hardening triggered by the mutual constraints of the lamellar microstructure during the plastic deformation also contributes additional work hardening to enhance the ductility. The design of lamellar microstructure under a cyclic intercritical quenching process presents an efficient and feasible pathway for producing GPa-grade dual-phase steel with considerable ductility.

Influence of thermal process parameters on the properties of material jetted CuSn8 components

The material jetting process (MJT) is a well-known additive manufacturing (AM) process for polymers. In recent years, research has also been conducted on the processing of metals using the MJT process. However, most studies were limited to the processing of metals with comparatively low melting points (< 1000°C), such as tin and aluminum. In order to investigate the processability of Cu-based alloys using the material jetting process, the fabrication of components from a copper–tin–bronze (CuSn8) is investigated in this work. For this purpose, first a suitable system consisting of a print head and heated platform was developed and cuboid test specimens were produced at three different droplet temperatures and three different platform temperatures. This is followed by the optical examination of the component surface and the material structure. In addition, the mechanical properties are characterized by uniaxial tensile tests. The resulting fracture surfaces are analyzed using scanning electron microscope images. With increasing temperature of the platform, an improvement in droplet connectivity can be determined. In addition, an increase in grain size can be observed. Increasing the platform temperature from 650°C to 850°C results in an increase in uniform elongation from 0.5% to 54 %. The ultimate tensile strength increases from 103 MPa to a maximum of 330 MPa for the parameter combination of the highest droplet temperature of 1190°C and the highest platform temperature of 850°C. When fracture surfaces are compared, the lower platform temperatures show areas where the newly deposited droplets have not fused with the substrate droplet, which is the reason for the lower values for elongation and strength. The study shows that the temperature of the deposited droplets has a smaller effect on the mechanical strength of the components than does the temperature of the platform.

Tailoring the microstructure to enhance the cryogenic mechanical properties of a Fe–34.5Mn–0.04C steel

Samples of a Fe–34.5Mn–0.04C (wt. %) austenitic steel with average grain size in the range from 0.45 to 21.0 μm were produced by cold rolling and subsequent annealing. The effects of microstructure on room–temperature (RT) and cryogenic tensile properties, and on the related fracture modes, were investigated. It is found that for fully recrystallized samples (those with average grain size in the range from 2.3 to 21 μm), both the yield strength and uniform elongation increase with decreasing grain size at both RT and −180 °C, while for smaller grain sizes (corresponding to partly recrystallized samples) the strength increases at the expense of uniform elongation. In general, samples with an average grain size of 2.3 μm show a better combination of strength and ductility than samples with larger grain sizes. A detailed characterization of the microstructure and fracture surfaces after failure reveals that grain refinement results in elimination of grain boundary martensite, thereby leading to a transition from transgranular and intergranular fracture modes to a ductile fracture mode, characterized by dimpled fracture surfaces. The results are discussed with respect to stacking fault energy and to the Mn and C content. The study provides important inputs for the design of advanced Fe–Mn–C steels for cryogenic applications.

Ultrafast Heating Heat Treatment Effect on the Microstructure and Properties of Steels

The adoption of the ultrafast heating (UFH) process has gained much attention in the last few years, as the green energy and minimization of CO2 emissions are the main aspects of contemporary metal science and thermal treatment. The effect of ultrafast heating (UFH) treatment on carbon steels, non-oriented grain (NGO) electrical steels, and ferriticor austenitic stainless steels is reported in this review. The study highlights the effect of ultrarapid annealing on microstructure and textural evolution in relation to microstructural constituents, recrystallization temperatures, and its effect on mechanical properties. A strong influence of the UFH process was reported on grain size, promoting a refinement in terms of both prior austenite and ferrite grain size. Such an effect is more evident in medium–low carbon and NGO steels than that in ferritic/austenitic stainless steels. A comparison between conventional and ultrafast annealing on stainless steels shows a slight effect on the microstructure. On the other hand, an evident increase in uniform elongation was reported due to UFH. Textural evolution analysis shows the effect of UFH on the occurrence of the Goss component (which promotes magnetic properties), and the opposite with the recrystallization g-fiber. The recovery step during annealing plays an important role in determining textural features; the areas of higher energy content are the most suitable for the nucleation of the Goss component. As expected, the slow annealing process promoted equiaxed grains, whereas rapid heating promoted microstructures with elongated grains as a result of the cold deformation.

Deformation mechanism of bimodal microstructure in Ti-6Al-4V alloy: The effects of intercritical annealing temperature and constituent hardness

The so-called bimodal microstructure of Ti-6Al-4V alloy, composed of primary α grains (αp) and transformed β areas (βtrans), can be regarded as a “dual-phase” structure to some extent, the mechanical properties of which are closely related to the sizes, volume fractions, distributions as well as nano-hardness of the two constituents. In this study, the volume fractions of primary α grains (vol.%(αp)) were systematically modified in three series of bimodal microstructures with fixed primary α grain sizes (0.8 μm, 2.4 μm and 5.0 μm), by changing the intercritical annealing temperature (Tint). By evaluating the tensile properties at room temperature, it was found that with increasing Tint (decreasing vol.%(αp)), the yield strength of bimodal microstructures monotonically increased, while the uniform elongation firstly increased with Tint until 910 °C and then drastically decreased afterwards, thereby dividing the Tint into two regions, namely region I (830−910 °C) and region II (910−970 °C). The detailed deformation behaviors within the two regions were studied and compared, from the perspectives of strain distribution analysis, slip system analysis as well as dislocation analysis. For bimodal microstructures in region I, due to the much lower nano-hardness of βtrans than αp, there was a clear strain partitioning between the two constituents as well as a strain gradient from the αp/βtrans interface to the grain interior of αp. This activated a large number of geometrically necessary dislocations (GNDs) near the interface, mostly with <c+a> components, which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region I. With increasing Tint, the αp/βtrans interface length density gradually increased and so was the density of GNDs with <c+a> components, which explained the continuous increase of uniform elongation with Tint in this region. For bimodal microstructures in region II, where the nano-hardness of βtrans and αp were comparable, neither a clear strain-partitioning tendency nor a strain gradient across the αp/βtrans interface was observed. Consequently, only statistically stored dislocations (SSDs) with <a> component were activated inside αp. The absence of <c+a> dislocations together with a decreased volume fraction of αp resulted into a dramatic loss of uniform elongation for bimodal microstructures in region II.

Texture modification and mechanical properties of AZ31 magnesium alloy sheet subjected to equal channel angular bending

Analysis of the recently proposed equal channel angular bending (ECAB) process is provided on thin hot-rolled AZ31 magnesium alloy sheets. In particular, effects of deformation temperature and strain path on the texture evolution and mechanical properties are systematically investigated under single pass ECAB at various temperatures and multi-pass ECAB process that involves changes in strain paths. It is found that simultaneous activation of multiple twinning types is successfully introduced during ECAB, which results in obvious tilted component of basal texture. Attributed to the domination of extension twins, weaker basal textures are detected after both single pass ECAB at 150 °C and three cross passes ECAB at 200 °C. After annealing, the basal texture is further weakened via twin-related recrystallization and the annealed microstructure is featured with mixture of basal and non-basal orientated grains. Additionally, the effect of grain orientation on the mode of plastic deformation and the roles of grain orientation and grain boundary on the local strain accommodation are coherently studied. This study reveals that over 60 % increase of uniform elongation with marginal reduction of tensile strength less than 5 % can be achieved for single pass ECAB at 150 °C and three cross passes ECAB at 200 °C, which is the result of larger fraction of grains favored with extension twinning and better local strain accommodation.

Enhancing the strain hardening and ductility of Mg-Y alloy by introducing stacking faults

Abstract Due to the insufficient slip systems, Mg and its alloys exhibit poor ductility during plastic deformation at room temperature. To solve this problem, alloying is considered as a most effective method to improve the ductility of Mg alloys, which attracts wide attentions of industries. However, it is still a challenge to understand the ductilization mechanism, because of the complicated alloying elements and their interactions with Mg matrix. In this work, pure Mg and Mg-Y alloys were comparatively studied to investigate the effect of Y addition on microstructure evolution and mechanical properties. A huge increase of uniform elongation, from 5.3% to 20.7%, was achieved via only 3 wt% addition of yttrium. TEM results revealed that the only activated slip system in pure Mg was basal slip, led to its poor ductility at room temperature. In contrast, a large number of stacking faults and non-basal dislocations with component were observed in the deformed Mg-Y alloy, which was proposed as the main reason for significant improvement of strain hardening and ductility. High resolution TEM indicated that most of the stacking faults were I1 and I2 intrinsic faults, which played a critical role in improving the ductility of Mg-Y alloy. Addition of Y into Mg alloy decreased the stacking fault energy, which induced high density stacking faults in the grain interior.

Strength-ductility modulation via surface severe plastic deformation and annealing

Surface severe plastic deformation (S2PD) techniques such as surface mechanical attrition treatment (SMAT) allow processing of metals to form a gradient of stored deformation energy and microstructure. Though strengthening of S2PD metals has been investigated in the literature, little information is available on the influence of subsequent annealing on microstructure and mechanical property evolution of the S2PD metals. Here, the SMAT process of Cu followed by heating at temperatures of 175 °C, 225 °C and 275 °C for 1 h is performed. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) are utilized to investigate microstructural evolution at different depths from the treated surface. To evaluate the spatial distribution of hardness, cross-section microhardness measurements are incorporated. The stored gradient of deformation has caused a reduction in the recrystallization temperature of the SMAT treated Cu, and following the annealing, microstructural evaluation reveals a recrystallized layer near the treated surface, the depth of which increased with a higher annealing temperature. Uniaxial tension testing is utilized to analyze the mechanical properties of the SMAT processed and annealed specimen, and a substantial increase in uniform elongation is observed with the 225 °C and 275 °C heat-treated specimens when compared to the as-SMAT processed state. This increase in uniform elongation is attributed to extra work-hardening rate due to the higher dislocation storage capacity of the recrystallized layer. This study shows that with annealing after SMAT, modulated grain structure gradients can be obtained. The strength-ductility combinations of the material can be flexibly altered in order to adapt to different applications with different mechanical property requirements by using different annealing parameters.

Investigation of Electroplastic Effect on Four Grades of Duplex Stainless Steels.

Since the late 1950s, an effect of electrical current in addition to joule heating on the deformation of metals called the Electroplastic Effect (EPE) has been known. It is used nowadays in the so-called Electrically Assisted Forming (EAF) processes, but the understanding of the phenomenon is not very clear yet. It has been found that EPE increases the formability of high stacking fault energy (SFE) materials, while low SFE materials reach fracture prematurely. Since Duplex Stainless Steels (DSSs) possess a microstructure consisting of two phases with very different SFE (low SFE austenite and high SFE ferrite) and they are widely used in industry, we investigated EPE on those alloys. Tensile tests at 5 A/mm2, 10 A/mm2 and 15 A/mm2 current densities along with thermal counterparts were conducted on UNS S32101, UNS S32205, UNS S32304 and UNS S32750. The DSS grades were characterized by means of optical microscopy, X-ray diffraction and their mechanical properties (ultimate tensile strength, total elongation, uniform elongation and yield stress). An increase in uniform elongation for the electrical tests compared to the thermal counterparts as well as an increase in total elongation was found. No differences were observed on the yield stress and on the ultimate tensile strength. Un uneven distribution of the current because of the different resistivity and work hardening of the two phases has been hypothesized as the explanation for the positive effect of EPE.

Ductile-brittle transition in a 9% Cr heat-resistant steel

The mechanical behavior of a P92-type steel was studied by tension and Charpy impact tests in the temperature range of 77–293K. The yield stress (YS) and uniform elongation increase concurrently with decreasing temperature because the steel exhibits work-hardening behavior. Transgranular dimpled fracturing remains the primary fracture mechanism under tension down to 193K. Brittle fracture takes place at lower temperatures. At 77K, the steel exhibits a well-defined yield plateau. Impact tests demonstrate that the ductile-brittle transition (DBT) occurs at 210K. At 193K, the energy absorption of 110J/cm2 at high impacts is attributed to extended areas of crack initiation and a stable crack propagation; the fast fracture load also has a high value. The steel becomes completely brittle at 77K, at which temperature the absorbed energy falls to 6J/cm2.

Effects of rolling temperature and subsequent annealing on mechanical properties of ultrafine-grained Cu–Zn–Si alloy

The effects of rolling temperature and subsequent annealing on mechanical properties of Cu–Zn–Si alloy were investigated by using X-ray diffraction, transmission electron microscope, electron back scattered diffraction and tensile tests. The Cu–Zn–Si alloy has been processed at cryogenic temperature (approximately 77K) and room temperature up to different rolling strains. It has been identified that the cryorolled Cu–Zn–Si alloy samples show a higher strength compared with those room temperature rolled samples. The improved strength of cryorolled samples is resulted from grain size effect and higher densities of dislocations and deformation twins. And subsequent annealing, as a post-heat treatment, enhanced the ductility. An obvious increase in uniform elongation appears when the volume fraction of static recrystallization grains exceeds 25%. The strength–ductility combination of the annealed cryorolled samples is superior to that of annealed room temperature rolled samples, owing to the finer grains, high fractions of high angle grain boundaries and twins.

Improvement of formability of ultrafine-grained materials by post-SPD annealing

Abstract Ultrafine-grained (UFG) IF-steel, as an example of an essentially single phase UFG microstructure, was annealed at different temperatures and time intervals in order to improve its formability by achieving a good strength–ductility–formability balance. In general, annealing increased the ductility and formability of UFG steel. Annealing at temperatures inside the recovery region caused a limited improvement in the formability of UFG steel due to the relief of internal energy without considerable grain coarsening. As the grain size increased to above 4 µm by annealing at temperatures inside the partially recrystallization region, the formability of UFG steel in the uniform region increased considerably, and localized deformation with early necking changed to a homogeneous mode as revealed by increased uniform thinning and enlargement of the membrane straining regime. Further grain coarsening resulted in a slight increase in uniform elongation both in uniaxial and biaxial tests. The UFG microstructure reduced the roughness of the free surface of biaxially stretched samples by decreasing the non-uniform grain flow, which leads to the so-called orange peel effect. Annealing of UFG microstructure did not degrade this positive effect due to the formation of sharp recrystallization textures although the annealed microstructures have relatively coarse grains. It can be concluded that a good balance between strength and uniform formability without an orange-peel effect can be achieved in UFG microstructures by well-design annealing processes.

Atomic-scale characterization of prior austenite grain boundaries in Fe–Mn-based maraging steel using site-specific atom probe tomography

The embrittlement and de-embrittlement behavior of an Fe–10Mn–1Pd (wt.%) maraging steel upon isothermal aging at 500°C is related to microstructural changes at prior austenite grain boundaries (PAGBs). Site-specific atom probe tomography measurements were performed to analyze the local chemistry of the PAGBs. Tensile tests and hardness measurements were conducted of the ternary alloy and of a binary non-hardenable Fe–10Mn alloy for comparison. Isothermal aging of the binary steel led to a decrease in strength along with a considerable increase in uniform elongation. The Pd-containing alloy, on the other hand, showed significant age-hardening, and an embrittlement and de-embrittlement transition was revealed. Ductile behavior was observed in the initial as-quenched and over-aged states, but there was zero tensile elongation in the intermediate under- and peak-aged regions, where intergranular fracture along the PAGBs occurred. In the brittle peak-aged state a large number of small nanometer-sized particles rich in Mn and Pd formed inside the grains and decorated the PAGBs. The precipitates grew in size on prolonged aging. Mn segregation to the PAGBs was revealed; the Mn concentration level at the boundaries varied with aging time and was highest in the peak-aged condition. Embrittlement and de-embrittlement mechanisms are discussed and compared to these observations.

Pulsed Nd:YAG laser shock processing effects on mechanical properties of 6061-T6 alloy

Abstract The aim of this paper is to investigate effects of single and double shot Nd:YAG laser shock processing (LSP) on residual stress, micro-hardness and tensile properties of 6061-T6 aluminum alloy. The X-ray diffraction technique was used to measure surface residual stress in LSP-treated 6061-T6 samples. The magnitude and directional dependence of the surface residual stress after single shot and double shot LSP were investigated with the sin2 Ψ method. The results show that laser shock processing can significantly increase surface compressive residual stress. In addition, micro-hardness of the LSP-treated sample was measured using a Vickers diamond indenter depending on the depth. The tensile tests of the single shot and double shot LSP-treated and untreated samples were carried out by the Schimadzu tensile testing machine having a video extensometer. Experimental results show that the values of micro-hardness, tensile strength and uniform elongation increase by LSP.

Influence of stacking fault energy and strain rate on the mechanical properties in Cu and Cu–Al–Zn alloys

The effects of stacking fault energy (SFE) and severe plastic deformation on the strength and ductility of Cu and Cu–Al–Zn alloys were systematically investigated. With lowering SFE, the crystallite size decreased while the microstrain, dislocation density and twin density all increased. Tensile testing results demonstrate that the tensile strength and uniform elongation increase with decreasing SFE, but the total elongation to failure first decreases with lowering SFE and then increases. The relationship between microstructure and mechanical behavior is briefly discussed in the paper.

Strain Rate and Temperature Dependent Plastic Deformation Behavior of Low-Carbon TRIP Steel

Low-carbon TRIP steel has high strength and excellent formability and is one of the key materials to meet the demand for weight reduction of automobile body parts. However, in order to disseminate the materials, the clarification of a complex plastic deformation mechanism depending on the strain rate and temperature and formulation of a proper constitutive equation is essential. The present investigation experimentally clarifies the quantitative effects of the strain rate and temperature on the material characteristics by means of tensile tests under strain rates from 1.7×10-4 to 1.7 x 10-1 and temperatures ranging from -50 to 250°C. The deformation and transformation behaviors substantially change in the range of 100 to 150°C. With regard to the relatively the low temperatures up to 100°C, retained austenite transforms into martensite. The increase in strain rate causes a rise in the flow stress of the ferrite phase and an increase in temperature, which suppresses the martensitic transformation. As a balancing effect, the stress-strain relationship expresses a high degree of strain rate independency. In the later stage of uniform deformation, the effective TRIP effect declines under the high strain rate and the uniform elongation decreases. However, the tensile strength is almost unchanged due to the very small amount of work hardening. Meanwhile, for the high temperature range above 150°C, the retained austenite transforms into bainite which requires a longer transformation time, which yields the strong strain rate dependency of the deformation behavior and the tensile strength and uniform elongation increase as the strain rate decreases.

Dynamic Strain Aging of Various Steels

The dynamic strain aging characteristics of two dual phase steels, a high strength low alloy (HSLA) steel, a 1008 steel and an interstitial free (IF) steel were determined from tensile properties at temperatures in the range 295 to 460 K (22 to 187 °C) and strain rates between 6 × 10-6 to 10-2s-1. All except the IF steel were found to be susceptible to dynamic strain aging, as evidenced by increases in tensile strength. The largest positive change was observed in the 1008 steel while the dual phase and HSLA steels showed much smaller increases. Also, large decreases (up to 75 pct) in uniform elongation were noted for the 1008 steel while the decreases were minimal for the dual phase and HSLA steels. The IF steel did not strain age and showed a slight increase in uniform elongation with increasing temperature. Based upon uniform elongation as an indicator of formability, formability might be improved in dual phase or HSLA steels by reducing the concentration of free interstitials in the ferrites through chemistry control.