Fine Substructure Research Articles

Temperature‐Dependent Dynamic Strain Aging in Selective Laser Melted 316L

Additively manufactured austenitic stainless steel AISI 316L (EN 1.4404, X2CrNiMo17‐12‐2) is used at higher temperatures, e.g., in space applications. However, the high‐temperature properties of such materials have not been analyzed in detail yet. Thus, selective laser melted (SLM) 316L is tested in the solution‐annealed condition by compression and tensile tests at temperatures between 25 and 877 °C. The compressive strength of SLM 316L is higher in comparison with the conventionally produced reference material due to hardening by a high dislocation density and a fine substructure. However, tensile tests reveal a loss in ductility of the SLM material at temperatures between 300 and 627 °C, where the elongation to fracture is reduced from 65% to 39%. Alloying elements cause serrated yielding in the affected temperature range. Together with an increased normalized work‐hardening rate and a negative strain rate sensitivity, dynamic strain aging is found to cause the reduction of ductility.

Melt Pool and Heat Treatment Optimization for the Fabrication of High-Strength and High-Toughness Additively Manufactured 4340 Steel

Additively manufactured (AM) components offer superior design flexibility compared to their conventionally manufactured counterparts and require process parameter optimization to achieve high-quality depositions with desirable and predictable mechanical properties. This study was focused on 4340 steel fabricated using laser powder bed fusion (LPBF), and 42 laser power and scan speed combinations have been systematically investigated to optimize the melt pool geometry and ensure fully dense parts. The AM material was compared with a wrought 4340 equivalent and studied in both as-fabricated and heat-treated conditions. Two heat treatments were designed and used to optimize the materials for strength and toughness, respectively. Microstructure, tensile, and fractographic studies were conducted to assess build integrity and establish processing–structure–performance relationships. Tensile properties of the AM materials in all studied conditions were equivalent or better than the comparable wrought materials. The high performance of the AM materials was attributed to the absence of both manganese sulfide inclusions and rolling-induced banding, typically found in the wrought materials. The fine cellular substructure is thought to additionally contribute to the high strength of the as-fabricated AM material. Complementary to the knowledge that has emerged from the experimental investigations, the dataset was further leveraged to make recommendations for future design of experiments to optimize AM build parameters in other material systems. A statistical Monte Carlo analysis was used to predict the interpolation error produced when using reduced datasets and to enable informed processing parameters selection. These findings are discussed to make recommendations for the use of AM materials for high-integrity structural applications.

Microstructure and Anisotropy of the Mechanical Properties of 316L Stainless Steel Fabricated by Selective Laser Melting

Significant anisotropy in mechanical properties was observed in 316L stainless steel (SS) that was subjected to selective laser melting (SLM) to produce a hierarchical structure, composed of molten pool, columnar grains, and a cellular substructure. Such anisotropy was induced by the geometric relationship between the boundary of the molten pool and the tensile force. The in situ tensile test showed initial deformation rapidly occurred at the boundary of the molten pool, followed by strain localization, and a lower ductility was obtained when loaded in the longitudinal direction (perpendicular to the molten pool). By contrast, the deformation was significantly constrained because of the geometry of the boundary of the molten pool, and substantial deformation occurred in the cellular substructure during loading in transverse direction (parallel to the molten pool). Finally, the quantitative analysis revealed that the high-level strength was attributed to the high-density dislocations and the fine cellular substructure.

Effect of cooling rate on the microstructure and mechanical properties of a low-carbon low-alloyed steel

Heavy plate steels with bainitic microstructures are widely used in industry due to their good combination of strength and toughness. However, obtaining optimal mechanical properties is often challenging due to the complex bainitic microstructures and multiple phase constitutions caused by different cooling rates through the plate thickness. Here, both conventional and advanced microstructural characterization techniques which bridge the meso- and atomic-scales were applied to investigate how microstructure/mechanical property-relationships of a low-carbon low-alloyed steel are affected by phase transformations during continuous cooling. Mechanical tests show that the yield strength increases monotonically when cooling rates increase up to 90 K/s. The present study shows that this is associated with a decrease in the volume fraction of polygonal ferrite (PF) and a refinement of the substructure of degenerated upper bainite (DUB). The fine DUB substructures feature C-rich retained austenite/martensite-austenite (RA/M-A) constitutes which decorate the elongated micrograin boundaries in ferrite. A further increase in strength is observed when needle-shaped cementite precipitates form during water quenching within elongated micrograins. Pure martensite islands on the elongated micrograin boundaries lead to a decreased ductility. The implications for thick section plate processing are discussed based on the findings of the present work.

Transferability of Ancestry‐Specific and Cross‐Ancestry CYP2A6 Activity Genetic Risk Scores in African and European Populations

The Nicotine Metabolite Ratio (NMR; 3-hydroxycotinine/cotinine), a highly heritable index of nicotine metabolic inactivation by the CYP2A6 enzyme, is associated with numerous smoking behaviors and diseases, as well as unique cessation outcomes. However, the NMR cannot be measured in nonsmokers, former smokers, or intermittent smokers, for example, in evaluating tobacco-related disease risk. Traditional pharmacogenetic groupings based on CYP2A6 * alleles capture a modest portion of NMR variation. We previously created a CYP2A6 weighted genetic risk score (wGRS) for European (EUR)-ancestry populations by incorporating independent signals from genome-wide association studies to capture a larger proportion of NMR variation. However, CYP2A6 genetic architecture is unique to ancestral populations. In this study, we developed and replicated an African-ancestry (AFR) wGRS, which captured 30-35% of the variation in NMR. We demonstrated model robustness against known environmental sources of NMR variation. Furthermore, despite the vast diversity within AFR populations, we showed that the AFR wGRS was consistent between different US geographical regions and unaltered by fine AFR population substructure. The AFR and EUR wGRSs can distinguish slow from normal metabolizers in their respective populations, and were able to reflect unique smoking cessation pharmacotherapy outcomes previously observed for the NMR. Additionally, we evaluated the utility of a cross-ancestry wGRS, and the capacity of EUR, AFR, and cross-ancestry wGRSs to predict the NMR within stratified or admixed AFR-EUR populations. Overall, our findings establish the clinical benefit of applying ancestry-specific wGRSs, demonstrating superiority of the AFR wGRS in AFRs.

Oxidation of amorphous HfNbTaTiZr high entropy alloy thin films prepared by DC magnetron sputtering

High entropy alloys represent a new type of materials with a unique combination of physical properties originating from the occurrence of single-phase solid solutions of numerous elements. The preparation of nanostructured or amorphous structure in a form of thin films promises increased effective surface and high intergranular diffusion of elements as well as a high affinity to oxidation. In this work, we studied HfNbTaTiZr thin films, deposited at room temperature by DC magnetron sputtering from a single bcc phase target. Films exhibit cellular structure (∼100 nm) with fine substructure (∼10 nm) made of round-shape amorphous clusters. The composition is close to equimolar with slight Ti enrichment and without any mutual segregation of elements. Oxidation at the ambient atmosphere leads to the formation of Ti, Zr, Nb, Hf, and Ta oxide clusters in the film up to the depth of 200–350 nm out of the total film thickness of 1650 nm. Oxygen absorption takes place preferentially in the large vacancy clusters located in between the amorphous cluster aggregates. The dominant type of defect are small open volumes with a size comparable with vacancy. The distribution of these defects is uniform with depth and is not influenced by the presence of oxygen in the film.

Equiaxed α microstructure evolution in wrought Ti-10Al-1Zr-1Mo-1Nb alloy during annealing

Evolution of α microstructure on the thermomechanical treated Ti-10Al-1Zr-1Mo-1Nb alloy during annealing was studied. The solution-treated materials were groove-rolled or uniaxially compressed in the α+β region and annealed at 1173 K. The flow softening behavior and crystal rotation in α platelets revealed an evolution of deformation texture. The volume fraction of equiaxed α grains was increased during annealing. Especially in the material compressed at a strain rate of 1 s−1, the equiaxed α grains developed within a shorter annealing duration than 1.8 ks. The deformation with higher strain rates promoted the division and fragmentation of α platelets during annealing. Transmission electron microscopy and X-ray diffraction analyses were employed to characterize dislocation components and structure, where the installed screw dislocations provided a fine substructure and high energy α/α boundaries in α platelets. The triple junction consisting of α/β boundaries and α/α boundaries may provide a site for thermal grooving, which induces the division and fragmentation of α platelets. Therefore, deformation at a higher strain rate is necessary in α+β processing to develop a fine equiaxed α microstructure for the Ti-10Al-1Zr-1Mo-1Nb alloy during annealing.

Microstructure and Mechanical Properties Evolution in HfNbTaTiZr Refractory High‐Entropy Alloy During Cold Rolling

The effect of cold rolling on the structure and mechanical properties of the HfNbTaTiZr refractory high‐entropy alloy with a single body‐centered cubic (BCC) phase structure is studied. The microhardness evolution during cold rolling to the thickness strain εth = 80% shows three distinct stages: an increase till εth ≈ 15%, a plato in the interval ≈15–40%, and again some increase at εth ≥ 40%. This behavior is found to be associated with an increase in dislocation density at the first stage, the formation of kink bands at the second stage, and the development of shear bands with fine lamellar internal substructure at the third stage. After cold rolling to 80%, the alloy demonstrates the yield strength of 1220 MPa, peak strength 1320 MPa, and elongation to fracture 3.4%. A short steady‐state flow stage is observed on the engineering stress–strain curve that can also be associated with kinking.

Effect of scan rotation on the microstructure development and mechanical properties of 316L parts produced by laser powder bed fusion

Additive manufacturing possesses appealing features for producing high-performance components, for a wide range of materials. One of these features is the ability to locally tailor the microstructure and in turn, the mechanical properties. This study investigates how the microstructure of stainless steel 316L parts produced by laser powder bed fusion are affected by alternating the laser scan orientation. The microstructure consists of large elongated grains with a fine cell substructure. This study established the correlation between the orientation of this substructure and the crystallographic orientation. The results show that by producing parts without any rotation a quite unique crystallographic orientation can be achieved. The grain structure primarily consisted of large 〈101〉 oriented grains, that were separated by thin bands of small 〈100〉 oriented grains with respect to the building direction. As rotation was added these bands were eliminated. Samples that were produced without any rotation generated the highest tensile strength (527 ± 5.4 MPa), yield strength (449 ± 2.4 MPa) and ductility (58 ± 1.3%). The lowest mechanical properties were obtained for samples that were produced using a scan rotation of 67° with the tensile strength of 485 ± 4.8 MPa, yield strength of 427 ± 5.4 MPa and ductility of 50 ± 1.3%. This indicates that cell orientation and crystallographic orientation plays an essential role in the tensile properties of 316L parts produced by laser powder bed fusion (L-PBF).

Effect of Scandium on Homogenization Treatment and Friction Stir Processing on Mechanical Properties of 7075 Aluminium Alloy Prepared by Foundry Route

The aluminium alloy (7075 series) with minor scandium (Sc) addition prepared by foundry route, was subjected to homogenization treatment at different temperatures from 300 to 500°C for 8h and also multi-pass friction stir processing (MP-FSP) was performed to improve surface properties of cast aluminium alloy. The effect of minor scandium (0.33wt.%) on the microstructure and mechanical properties of prepared alloy was investigated using optical microscopy, SEM, FESEM, XRD, TEM, Vickers’s hardness and tensile test. The homogenized microstructure was obtained showing fine grains because homogenization of cast structure and dispersion of second-phase particles also occur. The Al-Zn-Mg alloy with high strength, good corrosion resistance and good welding property is used widely in various industrial applications. In addition of scandium to increase the strength mainly comes from fine grain strengthening, substructure strengthening and precipitation strengthening caused by Al3 (Sc) and MgZn2 particles. These particles are formed during homogenization treatment of cast aluminium alloy and it has great influence on precipitation behaviour of Al3 (Sc) particles

Selective laser melting of 304L stainless steel: Role of volumetric energy density on the microstructure, texture and mechanical properties

The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm3. Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. A fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.

Effect of Scandium on Homogenization Treatment and Friction Stir Processing on Mechanical Properties of 7075 Aluminium Alloy Prepared by Foundry Route

The aluminium alloy (7075 series) with minor scandium addition prepared by foundry route, was subjected to homogenization treatment at different temperature from 300 to 500°C for 8h and also multi-pass friction stir processing (MP-FSP) was performed to improve surface properties of cast aluminium alloy. The effect of minor scandium (0.33wt.%) on the microstructure and mechanical properties of prepared alloy was investigated using optical microscopy, SEM, FESEM, XRD, TEM, Vicker’s hardness and tensile test. The homogenized microstructure was obtained showing fine grains because homogenization of cast structure and dispersion of second-phase particles also occur. The Al-Zn-Mg alloy with high strength, good corrosion resistance and good welding property is used widely in various industrial applications. In addition of Sc to increase the strength mainly comes from fine grain strengthening, substructure strengthening and precipitation strengthening caused by Al3(Sc) and MgZn2 particles. These particles are formed during homogenization treatment of cast aluminium alloy and it has great influence on precipitation behaviour of Al3(Sc) particles.

Ultra-high strength martensitic 420 stainless steel with high ductility

Martensitic 420 stainless steel was successfully fabricated by Selective laser melting (SLM) with >99% relative density and high mechanical strength of 1670 MPa, yield strength of 600 MPa and elongation of 3.5%. X-ray diffraction (XRD) and scanning electron microscopy disclosed that the microstructure of SLM 420 consisted of colonies of 0.5–1 μm sized cells and submicron martensitic needles with 11 wt. % austenite. Tempering of as-built SLM 420 stainless steel at 400 °C resulted in ultra-high strength material with high ductility. Ultimate tensile strength of 1800 MPa and yield strength of 1400 MPa were recorded with an elongation of 25%. Phase transformation analysis was carried out using Rietveld refinement of XRD data and electron backscattered diffraction (EBSD), which showed the transformation of martensite to austenite, and resulted in austenite content of 36 wt. % in tempered SLM 420 stainless steel. Transformation induced plasticity (TRIP), austenite formation and fine cellular substructure along with sub-micron martensite needles resulted in stainless steel with high tensile strength and ductility. The advanced mechanical properties were compared with conventionally made ultra-high-strength steels, and the microstructure-properties relationships were disclosed.

Intermediate shock sub-structures within a slow-mode shock occurring in partially ionised plasma

Context. Slow-mode shocks are important in understanding fast magnetic reconnection, jet formation and heating in the solar atmosphere, and other astrophysical systems. The atmospheric conditions in the solar chromosphere allow both ionised and neutral particles to exist and interact. Under such conditions, fine sub-structures exist within slow-mode shocks due to the decoupling and recoupling of the plasma and neutral species. Aims. We study numerically the fine sub-structure within slow-mode shocks in a partially ionised plasma, in particular, analysing the formation of an intermediate transition within the slow-mode shock. Methods. High-resolution 1D numerical simulations were performed using the (PIP) code using a two-fluid approach. Results. We discover that long-lived intermediate (Alfvén) shocks can form within the slow-mode shock, where there is a shock transition from above to below the Alfvén speed and a reversal of the magnetic field across the shock front. The collisional coupling provides frictional heating to the neutral fluid, resulting in a Sedov-Taylor-like expansion with overshoots in the neutral velocity and neutral density. The increase in density results in a decrease of the Alfvén speed and with this the plasma inflow is accelerated to above the Alfvén speed within the finite width of the shock leading to the intermediate transition. This process occurs for a wide range of physical parameters and an intermediate shock is present for all investigated values of plasma-β, neutral fraction, and magnetic angle. As time advances the magnitude of the magnetic field reversal decreases since the neutral pressure cannot balance the Lorentz force. The intermediate shock is long-lived enough to be considered a physical structure, independent of the initial conditions. Conclusions. Intermediate shocks are a physical feature that can exist as shock sub-structure for long periods of time in partially ionised plasma due to collisional coupling between species.

Effect of rolling temperature on the microstructure and mechanical properties of 6201 Al alloy

Abstract An 6201Al alloy was rolled at room temperature (RT), liquid nitrogen and alcohol mixed temperature (LAT) and liquid nitrogen temperature (LNT). The effect of the rolling temperature on microstructure and mechanical properties was investigated by means of hardness measurements, tensile tests, optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Microstructural observations reveal that the dislocation tangles and dislocation density increased with decreasing rolling temperature. The dislocation accumulation was enhanced, and the formation of dislocation tangles and cells occurred during deformation under low rolling temperature. The substructures of the rolled sample at LAT and LNT are similar, consisting of dislocation cells, high dislocation density with networks, dislocation array, dislocation tangles and subgrains. Compared with rolled samples at RT and LAT, the rolled samples at LNT presented strengthened hardness and strength. The improvement of mechanical properties contributes to the fine substructures and accumulation of higher dislocation density during low temperature rolling. In addition, with decreasing rolling temperature, the average dimple size decreased.

Evaluation of genetic structure across U.S. climate zones using prominent AI sires of Red Angus cattle

Climate variability can influence cattle performance. Therefore, the objective of this research was to determine fine-scale genetic diversity in Red Angus cattle in relation to climate zones. One hundred and seventy-five prominent Red Angus artificial insemination (AI) sires were sampled from five conterminous U.S. climate regions (Cool Arid, Cool Humid, Transition Zone, Warm Arid, and Warm Humid). Quantitative and molecular genetic approaches were used to evaluate genetic diversity for the cattle. The first method utilized neutral SNP to determine the genetic structure of the population. The second method used 66 SNP associated with traits potentially influenced by climate (body weight, heat stress, milk yield, heifer conception rate, and early embryonic survival) to detect loci under selection in each zone. Using 13,961 SNP, the genetic structure analysis revealed that there were eight sub-populations present within Red Angus. Furthermore, 23 of the 66 SNP were not in Hardy-Weinberg Equilibrium and loci under selection tests (P < 0.05). Chi-square tests revealed 7 of the 23 SNP to differ (P < 0.008) among climate zones. In conclusion, fine genetic substructure observed in Red Angus corresponded to U.S. climate zones. By identifying the genetic diversity within a prominent Bos taurus beef breed in relation to climate, management strategies can be implemented to utilize the genetic diversity of this breed to adapt to changing climates.

Hierarchical Parcellation of the Cerebellum.

Parcellation of the cerebellum in an MR image has been used to study regional associations with both motion and cognitive functions. Despite the fact that the division of the cerebellum is defined hierarchically-i.e., the cerebellum can be divided into lobes and the lobes can be further divided into lobules-previous automatic methods to parcellate the cerebellum do not utilize this information. In this work, we propose a method based on convolutional neural networks (CNNs) to explicitly incorporate the hierarchical organization of the cerebellum. The network is constructed in a tree structure with each node representing a cerebellar region and having child nodes that further subdivide the region into finer substructures. Thus, our CNN is aware of the hierarchical organization of the cerebellum. Furthermore, by selecting tree nodes to represent the hierarchical properties of a given training sample, our network can be trained with heterogeneous training data that are labeled to different hierarchical depths. The proposed method was compared with a state-of-the-art cerebellum parcellation network. Our approach shows promising results as a first parcellation method to take the cerebellar hierarchical organization into consideration.

Effects of Hot-Rolled Process and Heat Treatment on Microstructure and Mechanical Properties of Ti-6Al-4V ELI Alloy

The properties of titanium alloys significantly depend on the microstructure, which are correspond to the deformation conditions. However, because of its low thermal conductivity, sensitive to deformation temperature, narrow stable regions for hot working and structural heterogeneity, it does not achieve cosmically industrial production and application. In this paper, the effects of hot rolling deformation in single-phase (β) region, cross-phase region and heat treatment on the microstructure and mechanical properties of Ti-6Al-4V ELI alloy were systematically investigated. The relationship between microstructure and properties of alloy was also analyzed in order to a theoretical basis for the development of the rolling technology for the manufacture. The results indicated that hot rolling deformation in different region had significant effects on microstructure heterogeneity (the size and colony of α phase, lamellar microstructure of β transformed). It has been shown that fine and coarse lamellar α structure within grains and visible grain boundary α were characterized after the deformation above the β transformation temperature, which made high impact toughness. But in order to ensure in single phase region, the heat preservation method after passes of rolling may cause β grain coarsening (widmanstatten structure), leading to mechanical properties worsen. The fine crisscross substructures of α phase was obtained after deformation in cross-phase region, improving good mechanical properties. After solution treatment followed aging, the uniform type of microstructure was reached, which mainly displayed the change of contents and sizes of lamellar α phase.

Microstructure and properties of high strength and high conductivity Cu-Cr alloy components fabricated by high power selective laser melting

Although different kinds of metal materials have been built in the past years, it is difficult to fabricate the components of copper alloys with high strength and high conductivity due to their high reflectivity and thermal conductivity. In this paper, Cu-Cr alloy with high strength and high conductivity was successfully manufactured by high laser power selective laser melting. The microstructure, mechanical properties and conductivity were studied and compared before and after the heat treatment. The microstructure of the as-built sample was columnar grains with very fine cellular sub-structures and precipitates of Cr and Cr2O3. After heat treatment, the Cr particles precipitated from Cu matrix, resulting in simultaneous increase in strength and conductivity. The ultimate tensile strength of 468 MPa, yield strength of 377.33 MPa, and electrical conductivity of 98.31% IACS were achieved, which is even better than the samples fabricated by rolling with post heat treatment.

Processing Parameter DOE for 316L Using Directed Energy Deposition

The ability to produce consistent material properties across a single or series of platforms, particularly over time, is the major objective in metal additive manufacturing (MAM) research. If this can be achieved, it will result in widespread adoption of the technology for industry and place it into mainstream manufacturing. However, before this can happen, it is critical to develop an understanding of how processing parameters influence the thermal conditions which dictate the mechanical properties of MAM builds. Research work reported in the literature of MAM is generally based on a set of parameters and/or the review of a few parameter changes, and observing the effects that these changes (i.e., microstructure, mechanical properties) have. While these articles provide results with some insight, there lacks a standard approach that can be used to allow meaningful comparisons and conclusions to be made concerning the optimization of the processing variables. This study provides a template which can be used for making comparisons across DED platforms. The tests are performed with a design of experiments (DOE) philosophy directed to evaluate the effect of selected parameters on the measured properties of the DED builds. Specifically, a laser engineering net shaping system (LENS) is used to build multilayered 316L coupons and analyze how build parameters such as laser power, travel speed, and powder feed rate influence the thermal conditions that will define both microstructure and microhardness. A fundamental conclusion of this research is that it is possible to repeatedly obtain a consistent microstructure that contains a fine cellular substructure with a low level of porosity (less than 1.1%) and with microhardness that is equal to or better than wrought 316L. This is mainly achieved by maintaining an associated powder flow to travel speed ratio at the power level, ensuring an appropriate net heat input for the build process.