Balance Of Ductility Research Articles

Enhanced mechanical behaviors of gradient nano-grained austenite stainless steel by means of ultrasonic impact treatment

The gradient nano-grained (GNG) structure was formed on the top layer of AISI 304 in the presence of coarse grains (CG) by means of ultrasonic impact treatment (UIT). The impact velocity was 5m/s and the coverage changed from 3000% to 9000% in order to obtain the microstructure GNG/CG materials. The tensile test and small punch test (SPT) were further carried out in order to investigate the mechanical behaviors of the GNG/CG 304 at the different stress states. The results indicated that the yield strength of GNG/CG 304 obtained by tensile test and SPT increased at the small expense of ductility. The stress triaxiality T played an important role in the deformation behavior of GNG/CG 304. An increased T-value in the region of biaxial stretching of small punch specimen (T=2/3) led to an improved ductility compared with that noted in the case of uniaxial tensile specimen. The strain rate sensitivity m of GNG/CG 304 was 0.0468 that was estimated to be 25-fold greater than the coarse material. The yield strength of GNG/CG 304 at 400°C was 1.6-fold greater than the coarse material at ambient temperature due to the thermal stability of the GNG layer. Thus, the GNG structure could improve the comprehensive mechanical performances of AISI 304 with a balance of strength and ductility.

Investigation on temperature uniformity of blanks by conduction heating

Abstract The austenitizing process of ultra-high strength steel by radiation heating occupied most of the cycle time, which would affect the application of hot stamping in automotive industry. The conduction heating had the advantages of rapid heating, high energy efficiency and good flexibility, which solved the deficiencies of radiation heating. However, the steady temperature distribution of the blanks by conduction heating in the regions closed to the copper electrodes was lower than that of the middle regions, which would affect the mechanical properties of the hot stamping parts. According to the principle of heat transfer, the unequal radiation heat transfer technology was proposed to solve the problem of the inhomogeneous temperature distribution of the blanks. The tests were carried out on the conduction heating test line to investigate the influence of the technology on the steady temperature distribution of the blanks. The results shown that the homogeneous temperature distribution of the blanks was achieved by the local and continuous unequal radiation heat transfer technologies, satisfying the requirement of hot stamping. And the material utilization ratio of the blanks was also improved. The tensile strength, elongation and the balance of strength and ductility of the high temperature regions exceeded 1490 MPa, 6% and 11 GPa•%, respectively. The technology solved the application deficiencies of conduction heating in hot stamping.

Hot stamped parts with desirable properties in medium Mn TRIP steels

Abstract With the increasing requirements of the automotive industry for reduced weight and safety improvement, high strength steel hot stamping parts have been increasingly applied in the automotive industry. But the process is along with higher equipment requirements and tool wear because of the high temperature.In this study, warm tensile deformation behavior of 6Mn, 6MnNb and 6MnNbMo steels was investigated, in conjunction with warm stamping to understand the role of Mn and Mo and evaluate the mechanical properties of Ushaped parts.The results show that the hotstamped6MnNbMo Ushaped parts at both 700 ° C and 760 ° C exhibited the desirable mechanical properties, e.g. the SDB (Strength Ductility Balance) values reached 23.6-36.8 GPa·%, which were more than double or triple times higher than those of hotstamped boron steels. The work hardening rate increased with addition of Nb and Mo, while it decreased with flow stress.As the stamping temperature decreased from 760 to 700 ° C, the fractions of soft phase ferrite and retained austenite increased, resulting in the enhanced elongation with reducing tensile strength from 1600-1814 MPa to 1361-1467 MPa.The combined addition of Nb and Mo raised UTS with a slight decrease in El, and improved the distribution uniformity of mechanical properties, which is of great importance when applying medium Mn steels to the automotive structural components for ensuring safety of passengers.

Comparative study on microstructure and mechanical properties of a C-Mn-Si steel treated by quenching and partitioning (Q&P) processes after a full and intercritical austenitization

A C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization (hereinafter referred as F-QP and I-QP process, respectively). The discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated. The results indicate that the austenite grains are mainly film-like in the F-QP samples, while blocky austenite grains account for the majority in the I-QP samples. And the later contains a larger amount of retained austenite than the former. In addition, the reason why the austenite fraction decreases with the increasing of partitioning time after the peak value can be related to the bainite transformation due to the dynamic change of phase region in the continue cooling transformation (CCT) diagram of untransformed austenite with carbon enrichment. The I-QP samples show higher product of strength and elongation (PSE) but relatively lower strength, while the F-QP samples are just the reverse. Notably, the one-step I-QP samples show an excellent strength–ductility balance, of which the tensile strength and total elongation are 1140–1280MPa and 17–22%, respectively. The exponential function and Olson-Cohen (O-C) model were utilized to fit the variation of retained austenite fraction with strain. By comparison, retained austenite in the I-QP sample is more sensitive to the increasing strain, i.e., lower mechanical stability.

Enhancement of Mechanical Biocompatibility of Titanium Alloys by Deformation-Induced Transformation

Metastable β-type titanium alloys are highly suitable for use as structural biomaterials applied to hard tissue, i.e., as cortical bone (hereafter, bone) replacing implants. However, their mechanical biocompatibitities, such as the Young’s modulus, strength and ductility balance, fatigue strength, resistance against fatigue crack propagation and fracture toughness, require improvenent for increased compatibility with bone. Through deformation, the metastable β-phase in a metastable β-type titanium alloy is transformed into various phases, such as α’ martensite, α” martensite, and ω-phases with exact phase depending by metastable β-phase stability. In addition, twinning is also induced by deformation. Deformation twinning effectively enhances the work hardening in the metastable β-type titanium alloy, leading to increased strength and ductility. This improvement is accompanied by with other deformation-induced transformations including the appearance of deformation-induced martensite and ω-phase transformation. The enhancement of the mechanical biocompatibility of various materials using the abovementioned deformation-induced transformation is described in this paper, for both newly developed metastable β-type Ti-Mo and Ti-Cr alloys for biomedical applications.

Polymer nanocomposites processed via self-assembly through introduction of nanoparticles, nanosheets, and nanofibers

Nanocomposites offer the theoretical potential to achieve mechanical properties surpassing those of conventional (micro-scale) composites. The underlying reasons for the high potential of nanocomposites include the uniquely high mechanical attributes of nano-scale reinforcement, effective control of defect size and growth by nano-spaced interfaces, and interactions between the polymer matrix and the large surface areas of nanomaterials. Attempts to produce nanocomposites via conventional processing techniques have encountered challenges associated with thorough dispersion and effective interfacial interactions of nano-scale reinforcement with the polymer matrix. In order to address these challenges, materials were processed into polymer nanocomposites via electrostatically driven layer-by-layer self-assembly. Electrostatically dispersed nanomaterials and oppositely charged polyelectrolytes were sequentially built upon a substrate (cellular scaffold). The self-assembled nanocomposites, after complementary cross-linking, provided a unique balance of strength and ductility, which surpassed those of conventional (micro-scale) composites. Self-assembly was found to be an effective approach to producing nanocomposites embodying uniformly dispersed nanomaterials with controlled interfacial interactions. This approach is highly versatile and enables introduction of diverse nanomaterials into polymer nanocomposites. The work reported herein evaluated introduction of diverse categories of nanomaterials incorporating nanoparticles, nanosheets, nanotubes, and nanofibers. This investigation also evaluated the potential for a biomimetic approach to processing of light-weight structural systems by self-assembly of polymer nanocomposites onto cellular scaffolds.

Enhancing strength and ductility of Mg-Zn-Gd alloy via slow-speed extrusion combined with pre-forging

Mg-1.58Zn-0.52Gd (wt.%) alloy was subjected to slow-speed extrusion at 350 °C and 400 °C combined with hot pre-forging. The extruded alloy without pre-forging exhibited a bimodal microstructure, consisting of fine dynamically recrystallized (DRXed) grains and coarse unDRXed grains elongated along the extrusion direction, and strong basal texture, while pre-forging changed the bimodal structure into fully DRXed microstructure with finer grains and weaker extrusion texture. The fine and homogeneous extruded microstructure in the pre-forged alloy was mainly ascribed to the combined effect of refined starting microstructure just before extrusion, continuous DRX mechanism during extrusion and pinning effect induced by fine Mg3Zn3Gd2 precipitates. The pre-forged alloy exhibited superior balance of strength and ductility, as well as yield asymmetry, to the non-forged alloy after extrusion. The pre-forged alloy extruded at 350 °C and 400 °C showed highly improved elongation of 27.9% and 34.8%, respectively, mainly due to the formation of fully DRXed microstructure with fine grains through suppressing the activity of {10 1¯ 1} contraction and {10 1¯ 1}-{10 1¯ 2} double twins during tensile deformation.

Enhanced mechanical properties for mill-annealed Ti-20Zr-6.5Al-4V alloy with a fine equiaxed microstructure

To achieve a better balance of strength and ductility for Ti-20Zr-6.5Al-4V alloy, the fully equiaxed microstructure was obtained by post-deformation annealing process following water quench. The experimental results on microstructure evolution during the applied heat treatment routes are discussed with respect to the mechanical properties.

High-throughput discovery and characterization of multicomponent bulk metallic glass alloys

This work describes a high-throughput experimental method to characterize compositional trends in the glass forming ability and mechanical behavior of a ternary metallic alloy system. Continuously-graded composition libraries of Cu-Zr-Ti ternary alloys were produced by laser deposition, and continuous regions of glass-forming compositions were rapidly identified through an optical microscopy technique. By varying the laser processing parameters and thereby the cooling rate of the melt within each library, the composition with the greatest glass-forming ability within the range studied was determined to be Cu51.7Zr36.7Ti11.6. An alternative deposition scheme was applied to fabricate libraries containing a large array of discrete compositions. Instrumented nanoindentation was performed on the discrete libraries to establish compositional trends in the measured properties. The indentation modulus was observed to be strongly correlated with the Ti-content over the entire region of study, while the hardness was more sensitive to Cu for high Zr-contents and to Ti at lower Zr-contents. These trends could inform the design of new metallic glass alloys possessing an optimized balance of both ductility and glass forming ability.

Microstructure and Mechanical Properties of a New Refractory HfNbSi<sub>0.5</sub>TiVZr High Entropy Alloy

A new refractory alloy HfNbSi0.5TiVZr was synthesized by induction levitation melting with the aim to achieve an excellent strength and toughness balance of the Hf-Nb-Ti-Zr based alloy. The as-cast alloy with density of ρ=7.75g/cm3 and microhardness of Hv=464 had the microstructure consisting of bcc solid solution with little vanadium rich phase and fine intermetallic phase presented dendritic structure. Mixing entropy and formation enthalpy can explain this behavior. After heat treatment at 1373K for 4 h, no new phase come into being but elements solute more fully. Compressive yield strength of the alloy gradually decreased from 1540MPa at room temperature to 371MPa at 1073K in as-cast state and decreased from 1483MPa at room temperature to 102MPa at 1073K after annealed. Comparing with the similar high entropy alloys, the structure combining silicide and continuous solid phase have a great benefit to the balance of strength and ductility.

Development of dilute Mg–Zn–Ca–Mn alloy with high performance via extrusion

Dilute Mg–0.21Zn–0.30Ca–0.14Mn (wt.%) alloy was newly developed as low-cost wrought Mg alloy. Excellent balance of strength and ductility was achieved via extrusion process by grain refinement and texture modification. The dilute alloy extruded at 300 °C exhibited a tensile yield strength of 307 MPa and an elongation of 20.6% due to a bimodal microstructure consisting of fine dynamically recrystallized (DRXed) grains of ∼2.3 μm and strongly textured coarse unDRXed grains. After extrusion at 350 °C, almost fully DRXed microstructure (DRX fraction of ∼0.96) and significantly weakened basal texture (maximum intensity of 3.2) with rare-earth texture component were achieved, consequently resulting in a high compression/tension yield ratio of ∼0.8 with tensile elongation of 30.0% and tensile yield strength of 164 MPa. Besides, fine spherical Mg2Ca and α-Mn phases dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth.

Development of Microstructural Damage in Ni-Based Alloys During Creep

Ni-based model alloys with a base composition of Ni-20 mass pct Cr-3 mass pct Mo that were precipitation strengthened by the γ′ phase were studied in regards to their failure mechanisms as part of the fundamental research for achieving a creep rupture strength of 100 MPa at 1023 K (750 °C) and 105 hours. The microstructure, which was interrupted by transient creep, as well as the minimum creep rate and accelerated creep at 1123 K (850 °C) and 80 MPa was observed. The microstructure around the grain boundaries was altered remarkably with strain-induced grain boundary migration, while the γ′ particle size increased linearly inside the grains with increasing temperature and time. Furthermore, the volume fraction of the γ′ phase and the amount of precipitation on the grain boundary were associated with the size of the precipitate-free zone (PFZ), which is a major factor in creep damage. The appropriate precipitations inside the grains and at the grain boundaries were very effective for suppressing PFZ. Consequently, the creep properties can be improved by controlling PFZ in the proximity of grain boundaries for a superior balance of creep strength and ductility.

Micro-processes of brittle fracture initiation in bainite steel manufactured by ausforming

In recent years, the high tensile steels have got much attention in steel structural fields. Especially, the steel plate which has bainitic structure manufactured by TMCP (thermo-mechanical control process) technology becomes widely used because it has good balance of strength, ductility and toughness. Microstructure of those material is basically bainite structures, which are consisted of bainitic ferrite, cementite and MA (Martensite-Austenite constituent). Furthermore bainite microstructure manufactured by modern TMCP has extremely fine structures due to its transformation process from heavily deformed band structure including sub-grain structures which were generated during large amount of rolling in unrecrystallized temperature range called “ausforming”.However, the theoretical mechanism of brittle fracture initiation of bainite is still not clear because of its complexity of its structure. According to previous research including the observation of the fracture surfaces or cross sections of fractured specimens, the prior phenomenon of brittle fracture may occur at either the boundary area between MA and matrix or inside of MA. However, the details of the prior event for the macroscopic fracture mechanism are still unknown. The purpose of this study is to clarify the precursor phenomenon and estimate the quantitative criterion of brittle fracture initiation of bainite steels, which have fine microstructures, by observing the process of it directly by the in-situ observation methods.

Ti–4.5Al–2Mo–1.6V–0.5Fe–0.3Si合金の強度–延性バランスに及ぼす組織因子と変形機構

The effects of difference in morphology, grain size and β fraction on tensile properties in Ti–4.5Al–2Mo–1.6 V–0.5Fe–0.3Si (Ti-9) alloy were quantitatively examined. Excellent balance of strength and ductility is exhibited in the bimodal morphology. Refinement of α-grain size results in simultaneous increase in strength and ductility. Herein, the Hall–Petch coefficients in the equiaxed morphology and bimodal morphology exhibit similar value of approximately 230 MPa/(µm−1/2), implying that the similar Hall–Petch effect is acted for both morphologies. During deformation, twinning in the α phase does not activate. On the other hand, according to the experimental results on strength and the Schmid law, we can speculate that 〈c+a〉 dislocation slip activates easier than the case in the Ti–6Al–4V alloy. In Ti-9 alloy, it is supposed that suppression of planar-dislocation-motion (under deformation) due to the decrease in Al content and the accommodation of strain incompatibility at α/β interface contribute to improve the ductility.

Effect of mold temperature and casting dimension on microstructure and tensile properties of counter-gravity casting Ti-6Al-4V alloys

The cast Ti-6Al-4V alloy bars with different section sizes were fabricated by investment casting at counter-gravity condition with the mold temperatures of 300 ° C and 650 ° C, respectively. The microstructure of the alloy was observed by means of OM and SEM, and the effect of mold temperature and casting dimension on tensile properties was studied. Results show that equiaxed grains are obtained regardless of the casting dimension. 3 grain size tends to increase with an increase in mold temperature. Hot isostatic pressing of the alloy was carried out for tensile properties’ comparison. Room temperature tensile test results show that Ti-6Al-4V alloy produced via counter-gravity casting has good balance of strength and ductility after hot isostatic pressing (HIP). The alloy shows higher ductility due to the elimination of porosity. In both cast and HIP status, the tensile strength is inclined to decrease with an increase in mold temperature, while the ductility is prone to slightly increase. Both the strength and ductility tend to decrease with an increase in the casting dimension.

Developing high strength and ductility in biomedical Co–Cr cast alloys by simultaneous doping with nitrogen and carbon

There is a strong demand for biomedical Co−Cr-based cast alloys with enhanced mechanical properties for use in dental applications. We present a design strategy for development of Co−Cr-based cast alloys with very high strength, comparable to that of wrought Co−Cr alloys, without loss of ductility. The strategy consists of simultaneous doping of nitrogen and carbon, accompanied by increasing of the Cr content to increase the nitrogen solubility. The strategy was verified by preparing Co−33Cr−9W−0.35N−(0.01–0.31)C (mass%) alloys. We determined the carbon concentration dependence of the microstructures and their mechanical properties. Metal ion release of the alloys in an aqueous solution of 0.6% sodium chloride (NaCl) and 1% lactic acid was also evaluated to ensure their corrosion resistance. As a result of the nitrogen doping, the formation of a brittle σ-phase, a chromium-rich intermetallic compound, was significantly suppressed. Adding carbon to the alloys resulted in finer-grained microstructures and carbide precipitation; accordingly, the strength increased with increasing carbon concentration. The tensile ductility, on the other hand, increased with increasing carbon concentration only up to a point, reaching a maximum at a carbon concentration of ∼0.1mass% and decreasing with further carbon doping. However, the alloy with 0.31mass% of carbon exhibited 14% elongation and also possessed very high strength (725MPa in 0.2% proof stress). The addition of carbon did not significantly degrade the corrosion resistance. The results show that our strategy realizes a novel high-strength Co−Cr-based cast alloy that can be produced for advanced dental applications using a conventional casting procedure. Statement of significanceThe present study suggested a novel alloy design concept for realizing high-strength Co−Cr-based cast alloys. The proposed strategy is beneficial from the practical point of view because it uses conventional casting approach—a simpler, more cost-effective, industrially friendly manufacturing process than other manufacturing processes such as thermomechanical processing or powder metallurgy. The developed alloys showed the excellent strength–ductility balance and significantly high strength comparable to that of wrought Co–Cr–Mo alloys, while maintaining acceptable ductility and good corrosion resistance. We described the relationship between microstructures and mechanical and corrosion prosperities of the developed alloys; this provides the fundamental aspect of the proposed strategy and will be helpful for further investigations or industrial realization of the proposed strategy.

Effect of Strain Rate on Deformation-Induced Martensitic Transformation of Quenching and Partitioning Steels

The Quenching and Partitioning (QP) steels have received increasing attention due to the balance of high strength and good ductility, which results from transformation-induced plasticity (TRIP) effect. In this study, the macroscopic flow behavior and deformation-induced martensitic transformation (DIMT) of QP steels at quasi-static strain rate and elevated temperature have been investigated by uniaxial tension and X-ray diffraction. In addition, temperature increment during quasi-static strain rate tensile tests are measured by infrared camera. A temperature dependent transformation kinetics model considering deformation induced heating is developed to study the explanation of strain rate dependent DIMT. The model indicates that the influence of deformation induced heating at quasi-static strain rate on DIMT is obvious. But, it is insufficient to explain strain rate dependent DIMT solely. Then, an assumption of temperature gradient is introduced to correct the model. The modified model incorporating temperature and strain rate well describes the DIMT behavior of QP steels at quasi-static strain rate and elevated temperature.

Microstructure, texture and mechanical properties in an as-cast AZ61 Mg alloy during multi-directional impact forging and subsequent heat treatment

Abstract Multi-directional impact forging (MDIF) and subsequent heat treatment were applied to an as-cast AZ61 Mg alloy and the microstructure, texture evolution and mechanical properties were systematically investigated. The microstructural evolution during MDIF was classified into twinning-active and dynamic recrystallization (DRX)-predominant stages. During the early forging passes, a complicated twinning behavior was observed, including {10–12} primary, {10–12}–{10–12} secondary and {10–12}–{10–12}–{10–12} ternary twins with various twin variants, and even detwinning behavior. With further continuous forging, {10–12} twins disappeared leading to the formation of coarse DRXed grains, and fine DRXed grains began to nucleate at serrated grain boundaries of the coarse DRXed grains due to discontinuous DRX mechanism and progressively consumed the coarse DRXed grains, eventually resulting in a bimodal microstructure. Meanwhile, as-cast random texture turned into a special non-basal texture. After subsequent heat treatment at 400 °C for 2 h, the coarse eutectic network completely disappeared with fine β-Mg17Al12 particles uniformly locating at grain boundaries and the non-basal texture stayed stable, thereby resulting in a good balance of strength (ultimate tensile strength of 259 MPa) and ductility (20.6%).

Beneficial influence of an intercritically rolled recovered ferritic matrix on the mechanical properties of TRIP-assisted multiphase steels

The present study deals with the microstructure and mechanical properties of intercritically rolled TRIP-assisted multiphase steels. It is shown that the occurrence of the TRIP effect in a recovered ferritic matrix brings about an improved strength–ductility balance with respect to a fully recrystallised ferrite matrix. On the other hand, the intercritical deformation does not influence the austenite transformation rate during straining at room temperature. The improvement of the mechanical properties results from the interactions between the transformation strain and the recovered ferrite.

Microstructure, mechanical properties and formability of friction stir processed interstitial-free steel

The microstructure, mechanical properties and stretch formability of fine-grained (FG) interstitial-free steel (IF-steel) formed by friction stir processing (FSP) was investigated systematically. One-pass FSP drastically refined the microstructure with aid of dynamic recrystallization (DRX) mechanism during processing and formed volumetric defect free basin-like processed region (PR) with a mean grain size of 5µm (initial grain size was 40µm). This microstructural evolution brought about a considerable increase in both hardness and strength values of IF-steel without considerable decrease in ductility values. Also, strain hardening dominated deformation behavior was obtained with the FSPed samples as an essential property for the engineering application. Coarse-grained (CG) IF-steel demonstrated high formability with an Erichsen index (EI) of 2.88mm. Grain refinement by FSP yielded very close EI value of 2.80mm with increasing punch load (FEI). Force–displacement curves obtained in each process conditions reflected a similar membrane straining regimes where samples uniformly thinned under biaxial tension loads with aid of strain hardening capability. The formation of FG microstructure by FSP reduced the roughness (orange peel effect) of the free surface of biaxial stretched sample by decreasing the non-uniform grain flow leading to the so-called orange peel effect. It is concluded that a good balance of strength, ductility and strain hardenability along with equivalence formability to CG condition can be achieved by FSP as a single step practical procedure.