Dislocation Barriers Research Articles

Effect of interface type on deformation mechanisms of γ-TiAl alloy under different temperatures and strain rates by molecular dynamics simulation

Crystalline interface plays a significant role in strengthening lamellar γ-TiAl alloys through reconciling the strength and ductility. Herein, the effects of temperature and strain rate on the tensile deformation of three lamellar γ-TiAl interface models are investigated by molecular dynamics simulations. The three interfaces, pseudo twin (PT), rotational boundary (RB), and true twin (TT), exhibit different tensile responses due to the different interface effects: TT interface only acts as a barrier of dislocation traversing to facilitate crack extension; PT interface acts as both dislocation barrier and emission source and has a stronger release of strain energy than TT interface, retarding the crack extension; RB interface can retard and resist crack extension due to the blunting and deflection of the crack tip and the best interface geometry compatibility. The defect evolution indicates that the elevated temperature suppresses dislocation propagation at low strain rate, while the high strain rate causes small lamellar stacking faults and slit-shaped holes along tensile direction at low temperature. In addition, the dual conditions of high strain rate and low temperature induce the phase transition from FCC to BCC and then BCC to HCP. These findings provide a specific insight to understand the atomistic mechanism of interface-mediated deformation.

High-Strain-Rate Deformation Behavior of Co0.96Cr0.76Fe0.85Ni1.01Hf0.40 Eutectic High-Entropy Alloy at Room and Cryogenic Temperatures.

The deformation behaviors of Co0.96Cr0.76Fe0.85Ni1.01Hf0.40 eutectic high-entropy alloy (EHEA) under high strain rates have been investigated at both room temperature (RT, 298 K) and liquid nitrogen temperature (LNT, 77 K). The current Co0.96Cr0.76Fe0.85Ni1.01Hf0.40 EHEA exhibits a high yield strength of 740 MPa along with a high fracture strain of 35% under quasi-static loading. A remarkable positive strain rate effect can be observed, and its yield strength increased to 1060 MPa when the strain rate increased to 3000/s. Decreasing temperature will further enhance the yield strength significantly. The yield strength of this alloy at a strain rate of 3000/s increases to 1240 MPa under the LNT condition. Moreover, the current EHEA exhibits a notable increased strain-hardening ability with either an increasing strain rate or a decreasing temperature. Transmission electron microscopy (TEM) characterization uncovered that the dynamic plastic deformation of this EHEA at RT is dominated by dislocation slip. However, under severe conditions of high strain rate in conjunction with LNT, dislocation dissociation is promoted, resulting in a higher density of nanoscale deformation twins, stacking faults (SFs) as well as immobile Lomer-Cottrell (L-C) dislocation locks. These deformation twins, SFs and immobile dislocation locks function effectively as dislocation barriers, contributing notably to the elevated strain-hardening rate observed during dynamic deformation at LNT.

Alloy strengthening toward improving mechanical and tribological performances of CuxNi100−x/Ta nano multilayer materials

Alloy strengthening via introducing Ni atoms into Cu monolayer of Cu/Ta nano multilayer materials (NMMs) is used to enhance mechanical and tribological properties. Molecular dynamics simulations indicate that the introduction of solid-solution Ni atoms suppresses the dislocation multiplication and glide, leading to higher hardness, better tribological performances, and stronger deformation resistance for CuxNi100−x/Ta NMMs. The deformation resistance in nanoindentation is mainly determined by hardness and interlayer interface, where the interface acts as dislocation barrier and emission source to restrain main compression stress within about three monolayers. The resistance against friction-induced deformation relies on the comprehensive properties of strengthening monolayers and alloying interfaces, which induces further removal of Ta atoms. The atom-scale insights on the NMMs strengthening and the corresponding deformation mechanism are helpful for designing new wear-resistant materials.

Anisotropy in microstructure and mechanical properties of additively manufactured Ni-based GH4099 alloy

Additively manufactured Ni-based superalloys exhibit strong anisotropy due to microstructural differences resulting from their unique fabrication method. This study systematically investagiated the microstructure and mechanical properties of Ni-based GH4099 alloy fabricated by Selective Laser Melting (SLM) with subsequent heat treatment (HT). The causes of anisotropy in the deformation mechanism of alloys at room temperature and high temperatures are discussed. The results indicate that epitaxial grain growth occurs along the building direction, and after SLM, entangled dislocations gather at the cell boundaries. After HT, high-density dislocations are eliminated, while most grains remain in the as-deposited form. At room temperature, the deformation mechanism of the alloy remains consistent. Cracks generate and propagate inside grains, while grain boundaries provide a more substantial barrier for dislocations. Therefore, the difference in mechanical anisotropy depends on the difference in grain morphology in the build direction and perpendicular to the build direction. At high temperatures, fracture failure occurs at grain boundaries. The growth of carbides at high temperatures leads to the weakening of grain boundaries. The alloy exhibits different deformation mechanisms when loaded along the short and long axes of the grain. Only dislocations are activated when loaded along the short axis of the grain. Stacking faults and deformation twins provide higher plastic-deformation ability when loaded along the long axis of the grains. Moreover, dislocations also activate, however the dislocation density is lower compared to loaded along the short axis.

Designing ultra-strong and ductile hierarchical titanium alloys via interstitial solute-mediated multi-morphologic α-nanoprecipitates

In conventional duplex titanium (Ti) alloys, the soft primary α-precipitates (αp) are beneficial to ductility with the loss of high yield strength, while the continuous α-precipitates at grain boundaries (GBs, αGBs) often accompanied with α precipitate-free zones (α-PFZs) renders the strain incompatibility for limited ductility. Here, to overcome this longstanding issue, we utilize interstitial (O and N) atoms to notably strengthen the α-phase, and simultaneously architect the multi-morphologic secondary α-precipitates (αs) so as to improve the strain compatibility nearby GBs, thus develop a Ti-4.1Al-2.5Zr-2.5Cr-6.8Mo (wt.%) alloy with ultra-high yield strength of ∼1580 MPa and good ductility of ∼8.2%. The phase boundaries between the hard α-precipitates and the soft β-matrix not only act as dislocation barriers for strengthening, but also as sustainable dislocation sources for ductilizing to achieve a good combination of strength and ductility in our Ti alloys. This strategy not only sheds light on the understanding of the strength-ductility synergy of duplex Ti alloys, but also offers an available pathway to design ultra-strong and ductile alloys.

Irradiation hardening and deuterium retention behaviour of tungsten under synergistic irradiations of 3.5 MeV Fe13+ ions and deuterium plasma

This work investigates the irradiation hardening and deuterium (D) retention behaviour of tungsten (W) under synergistic irradiations of heavy ions and D plasma. 3.5 MeV iron (Fe13+) ion irradiation was performed on recrystallized W samples (RW) to produce displacement damage of 1 dpa. Then, low-energy (38 eV) D plasma exposure was conducted at 500 K. It is found that Fe ion irradiation creates substantial vacancy-type defects and dislocation loops/networks in RW. These irradiation-induced defects not only function as nucleation sites for dislocations that increase the activation probability of new dislocations and suppress the pop-in events, but also act as barriers for dislocations that result in irradiation hardening. Nano-indentation results show that the average hardness of RW-D, RW-Fe and RW-Fe-D increases from 5.26 GPa for RW to 5.28, 6.23 and 6.56 GPa, respectively. The strong interaction between dislocations and high density of damage-induced defects is suggested to be the chief source for the obvious irradiation hardening observed in the synergistic irradiation case. Besides, compared with RW-D, the total D retention in RW-Fe-D is increased by a factor of 2.65. NRA and TDS results suggest that Fe pre-irradiation not only increases D retention within the damage layer (within the first 1.3 μm), but also enhances that beyond the damage layer (> 1.3 μm) before surface blisters are formed. This work further improves the fundamental understanding on the microstructure evolution, D retention and nano-mechanical behaviour of W under synergistic irradiation effect of heavy ions and D plasma.

Grain boundary and triple junction characteristics analytics of additive manufactured Inconel 625 superalloy using selective laser melting

Selective laser melting (SLM) is a rapidly developing technology with many advantages over traditional manufacturing methods. SLM is gaining popularity in many industries due to its potential to revolutionise designs and efficiency. To facilitate more widespread adoption of the technology, it is vital to understand the microstructure characteristics of materials produced by it. In this study, IN625 superalloy were produced by SLM are analysed by microstructural characteristics including grain boundaries and triple junctions and mechanical properties by hardness. Electron backscatter diffraction (EBSD) and data analysis were employed to investigate the effect of SLM on grain structures, crystal orientation distribution, grain boundary characteristic distribution (GBCD) and triple junction characteristic distribution (TJCD) in IN625. Investigation of microstructure characteristics revealed that there is increase in the volume of grain boundaries and dislocation barriers perpendicular to the build direction. The grain boundaries in the SLM manufactured IN625 superalloy were found to be mostly low angle (<15°) and further investigation on triple junctions revealed that the majority of triple junction's present being the preferred type of triple junctions where two or more low angle or coincidence site lattice (CSL) grain boundaries meet. High volume of low angle boundaries, anisotropic microstructure and preferred type of triple junctions found in SLM manufactured IN625 leads to increase in the failure resistances when compared to conventional manufactured IN625. These microstructures were anisotropic in nature which leads to anisotropic mechanical properties.

Doubled strength and ductility via maraging effect and dynamic precipitate transformation in ultrastrong medium-entropy alloy

Demands for ultrahigh strength in structural materials have been steadily increasing in response to environmental issues. Maraging alloys offer a high tensile strength and fracture toughness through a reduction of lattice defects and formation of intermetallic precipitates. The semi-coherent precipitates are crucial for exhibiting ultrahigh strength; however, they still result in limited work hardening and uniform ductility. Here, we demonstrate a strategy involving deformable semi-coherent precipitates and their dynamic phase transformation based on a narrow stability gap between two kinds of ordered phases. In a model medium-entropy alloy, the matrix precipitate acts as a dislocation barrier and also dislocation glide media; the grain-boundary precipitate further contributes to a significant work-hardening via dynamic precipitate transformation into the type of matrix precipitate. This combination results in a twofold enhancement of strength and uniform ductility, thus suggesting a promising alloy design concept for enhanced mechanical properties in developing various ultrastrong metallic materials.

The Activation Energy of Strain Bursts during Nanoindentation Creep on Polyethylene.

In the present investigation, statistical characterization of strain bursts observed during the load-controlled deformation of high-density polyethylene (HDPE), which arise within the crystalline phase during plastic deformation, was carried out via high-resolution nanoindentation creep experiments. Discrete deformation processes occurred during the nanoindentation creep tests, which indicated that they arose from the break-off of dislocation avalanches, i.e., dislocation climb is a possible mechanism for indentation creep deformation. Characterization of the strain bursts, in terms of the associated height and number, demonstrated that these quantities followed a Gaussian distribution depending on the load and loading rate. This analysis enabled the accurate measurement of creep activation energy. Our method used nanoindentation tests to measure the creep activation energy of HDPE within both the crystalline and amorphous phases. The activation energy of the creep process within the crystalline phase was evaluated using two methods. The frequency of jumps within the crystalline phase, as a function of the strain rate, showed two peaks related to the 5 nm and 10 nm jump sizes that corresponded to the block size within the crystalline lamellae. The results indicated that the intervals coincided with the mean free path of dislocations and the block grain boundaries acted as dislocation barriers. From the dependence of burst frequency on the strain rate and temperature, the activation energy and thermally activated length of the dislocation segment for the plastic slip activation were determined to be 0.66 eV and 20 nm, respectively. Both numbers fit well to the Peterson's model for the nucleation and motion of thermally activated dislocation segments. A similar activation energy resulted from the differential mechanical analysis of the literature for the αI-transition, which occurred near room temperature in polyethylene. The transition was described as the generation of screw dislocation and its motion along a block grain boundary; therefore, this process is suggested to be the basic mechanism underlying the strain bursts observed in this study.

Alloy Design and Fabrication of Duplex Titanium-Based Alloys by Spark Plasma Sintering for Biomedical Implant Applications

Very often, pure Ti and (α + β) Ti-6Al-4V alloys have been used commercially for implant applications, but ensuring their chemical, mechanical, and biological biocompatibility is always a serious concern for sustaining the long-term efficacy of implants. Therefore, there has always been a great quest to explore new biomedical alloying systems that can offer substantial beneficial effects in tailoring a balance between the mechanical properties and biocompatibility of implantable medical devices. With a view to the mechanical performance, this study focused on designing a Ti-15Zr-2Ta-xSn (where x = 4, 6, 8) alloying system with high strength and low Young's modulus prepared by a powder metallurgy method. The experimental results showed that mechanical alloying, followed by spark plasma sintering, produced a fully consolidated (α + β) Ti-Zr-Ta-Sn-based alloy with a fine grain size and a relative density greater than 99%. Nevertheless, the shape, size, and distribution of α-phase precipitations were found to be sensitive to Sn contents. The addition of Sn also increased the α/β transus temperature of the alloy. For example, as the Sn content was increased from 4 wt.% to 8 wt.%, the β grains transformed into diverse morphological characteristics, namely, a thin-grain-boundary α phase (αGB), lamellar α colonies, and acicular αs precipitates and very low residual porosity during subsequent cooling after the spark plasma sintering procedure, which is consistent with the relative density results. Among the prepared alloys, Ti-15Zr-2Ta-8Sn exhibited the highest hardness (s340 HV), compressive yield strength (~1056 MPa), and maximum compressive strength (~1470). The formation of intriguing precipitate-matrix interfaces (α/β) acting as dislocation barriers is proposed to be the main reason for the high strength of the Ti-15Zr-2Ta-8Sn alloy. Finally, based on mechanical and structural properties, it is envisaged that our developed alloys will be promising for indwelling implant applications.

Enhanced mechanical and biocompatibility performance of Ti(1-)Ag(x) coatings through intermetallic phase modification

Advanced materials combining superior mechanical and biocompatibility performance are of significant interest to extend the lifetime of biomedical devices. In this work, Ag is alloyed with Ti to investigate the role of emerging Ti Ag intermetallic coatings with high mechanical hardness and exceptional biocompatibility. Thin films of Ti (1- x ) Ag (x) were deposited on 316 L steel and glass substrates using magnetron sputtering and subsequently heat-treated to aid Ti Ag intermetallic development. Mechanical properties were then measured and correlated to microstructural and morphological changes in the Ti Ag films. In the as-grown state, the Ti Ag matrix developed different intermetallic structures which increased the hardness of pure Ti films from 5 to >7 GPa. After heat treatment, a peak hardness of 7.39 GPa and elastic modulus of 105 GPa was achieved for a 43 at.% Ag film due to formation of the tetragonal Ti Ag phase and increase of upper surface oxides which act as dislocation barriers. However, at higher Ag concentrations, heat treatment leads to agglomeration of Ag around grain boundaries and decreases the crystallite size, leading to reduction in hardness to <3 GPa. The Ti rich films also depict better cytotoxicity performance following exposure to the L929 cell line, though excellent cell viability values >98% are observed for the entire Ti Ag range. While leached ion concentrations lower than 100 ppb demonstrate excellent biocompatibility of this Ti Ag alloy system. This work demonstrates the first successful attempt to develop biocompatible Ti Ag thin film coatings with high mechanical hardness with the potential to extend the lifetime of medical implants. • First study on biomechanical performance of Ti (1-x) -Ag (x) intermetallic films. • First report on external heat treatment to develop Ti Ag intermetallic thin films. • Ag addition causes transition to tetragonal Ti Ag structure with enhanced hardness. • Heat treatment leads to improved crystallinity and mechanical performance. • Ti Ag films exhibit excellent biocompatibility before and after heat treatment.

Micro-mechanisms of failure in nano-structured maraging steels characterised through in situ mechanical tests

High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time using in situ characterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements. It is seen that the phenomenon of planar slip leads to strain softening under uniaxial deformation and to crack branching under a triaxial stress state in hot rolled maraging steels. On the other hand, nano-structuring after HPT processing creates a large number of high angle grain boundaries as dislocation barriers, leading to strain hardening under uniaxial tension and nearly straight crack path with catastrophic fracture under triaxial stress state. Upon overaging, the hot-rolled sample shows signature of transformation induced plasticity under uniaxial tension, which is absent in the HPT processed overaged samples, owing to the finer reverted austenite grains containing higher Ni concentration in the latter. In the overaged fracture test samples of both the hot-rolled and HPT conditions, crack tips show a signature of strain induced transformation of the reverted austenite to martensite, due to the accompanying severe strain gradients. This leads to a higher fracture toughness even while achieving high strengths in the overaged conditions of the nanocrystalline HPT overaged samples. The results presented here will aid in design of suitable heat treatment or microstructure engineering of interface dominated nano-scale maraging steels with improved damage tolerance.

A new β-Ti alloy with good tensile properties by the mixed microstructure characteristics of α phase

In this study, the mechanical properties, microstructural characteristics and deformation behavior of a novel high-strength metastable β alloy Ti-6.5Cr-4.5Sn-4.5Mo (wt %) were investigated. By obtaining α phase mixed microstructure with different size, morphology and orientation combinations in β matrix, a good combination of tensile strength σb~1305 MPa and total elongation ɛf~13.4 % was achieved. The microstructural characteristics of α phase consisting of discontinuous thin grain boundary α phase (αGB), lamellar α colonies and acicular αs precipitates were obtained by the solution treatment above β transus for a short time plus subsequent aging treatment after the heavy cold rolling. Detailed TEM analysis shows that it is difficult to deform for acicular αs precipitates due to their fine-scale, leading to a large number of α/β interfaces acting as dislocation barriers, which may ultimately result in an improvement on strength of the alloy. Compared with αs, the αGB and lamellar α colonies are soft, which could produce the large enough deformation and allow dislocations to generate and accumulate within them, so that it is beneficial to improve the mechanical properties of the alloy. Stress-relaxation experiments show that the sample with a uniform and finer distribution of α precipitate has a larger value of mobile dislocation ρm/ρm0 after relaxation, implying that most of the mobile dislocations remains in the sample, which results in the ultrahigh strength meanwhile accepted ductility in the specimen.

Dislocation dynamics in heterogeneous nanostructured materials

Crystalline materials can be strengthened by introducing dissimilar phases that impede dislocation glide. At the same time, the changes in microstructure and chemistry usually make the materials less ductile. One way to circumvent the strength–ductility dilemma is to take advantage of heterogeneous nanophases which simultaneously serve as dislocation barriers and sources. Owing to their superior mechanical properties, heterogeneous nanostructured materials (HNMs) have attracted a lot of attention worldwide. Nevertheless, it has been difficult to characterize dislocation dynamics in HNMs using classical continuum models, mainly due to the challenges in describing the elastic and plastic heterogeneity among the phases. In this work, we advance a phase-field dislocation dynamics (PFDD) model to treat multi-phase materials, consisting of phases differing in composition, structural order, and size in the same system. We then apply the advanced PFDD model to exploring two important but divergent materials design problems in HNMs: dislocation/obstacle interactions and dislocation/interface interactions. Results show that the interactions between a dislocation and distribution of obstacles varying in structure and composition cannot be understood by simply interpolating from their individual interactions with a dislocation. It is also found that materials containing interfaces with nanoscale thicknesses and compositional gradients have a much higher dislocation bypass stress than those with sharp interfaces, providing an explanation for the simultaneous high strength and toughness of thick interface-containing nanolaminates as observed in recent experiments.

Segregation enabled outstanding combination of mechanical and corrosion properties in a FeCrNi medium entropy alloy manufactured by selective laser melting

• A face-centered cubic single-phase FeCrNi medium entropy alloy with a high Cr (~35 at.%) content was successfully manufactured by selective laser melting (SLM). • This as-SLMed MEA presents a hierarchical microstructure of coarse columnar grains and submicron cell structures. Cr, and also interstitial C , are found segregated to the cell boundaries. • The as-SLMed FeCrNi MEA shows a good combination of strength (σ0.2 = 745 MPa, σUTS=1007 MPa) and ductility (εf=31%), as well as a superb corrosion resistance ( i corr =0.06 μA/cm 2 ). Selective laser melting (SLM) has the advantage in preparing supersaturated solid solutions due to its unique thermal field and high solidification rate. In this study, a face-centered cubic single-phase FeCrNi medium entropy alloy (MEA) with an ultrahigh Cr content (~35 at.%) was additively manufactured by SLM. The as-built MEA shows a hierarchical microstructure of coarse columnar grains and submicron dislocation cell structures, where the cell boundaries are probed segregated with Cr and C and decorated with nano carbides. The appearance of these dislocation barriers results in an excellent combination of strength ( σ 0.2 =745 MPa, σ UTS =1007 MPa) and ductility ( ε f =31%). The current MEA also shows a superb corrosion resistance with a corrosion current density of 0.06 μA cm −2 in 3.5 wt.% NaCl solution, which is far lower than that of 316 L. The high content of solutioned Cr in the MEA ensures sufficient Cr supply to form an integrated Cr 2 O 3 passive film, and the large number of cell boundaries acting as the diffusion channels lead to the fast formation of a stable passive film over the alloy surface.

Mechanical properties of Al6005A-nTiC nano-composite obtained by milling, hot extrusion and FSW

An aluminium matrix nano-composite (AlMNC) was obtained by a powder metallurgy (P/M) process, consisting of an ex situ reinforcement of pre-alloyed AA6005A powders with 3% vol. of TiC nanoparticles (nTiC), high-energy mechanical milling (HEMM) and hot extrusion powder (HEP). AA6005A powder was chosen as matrix because it is a heat-treatable medium strength Al alloy with excellent corrosion resistance, extrusion performance and good weldability; and the nTiC particles due to their low density, good chemical stability and excellent combination of hardness and elastic modulus. HEMM allowed obtaining an ultra-fine grained (UFG) matrix, a uniform dispersion of the nano-reinforcement and its proper incorporation in the ductile matrix. Simultaneously, the repeated plastic deformation of matrix powder led to a high density of crystal defects, and the large number of dislocation barriers produced by nTiC particles are responsible of the obtained nanostructured composites. HEP allowed obtaining high quality extrudates with an a homogeneous distribution of nanoparticles. The hardness of the nanocomposite is almost 43 % higher than that of the unreinforced AA6005A, showing important increases in yield strength and ultimate tensile strength with a 5% elongation. The extruded profiles were Friction Stir Welded (FSW) obtaining sound welds. The grain size refinement on the stir zone (SZ), led to a slight increase in hardness in the welds.

Selective laser melting of Nano-TiN reinforced 17-4 PH stainless steel: Densification, microstructure and mechanical properties

In this study, TiN-reinforced 17-4 PH stainless steel was produced by selective laser melting (SLM). It was aimed to utilize nano-sized TiN particles both as inoculants to obtain an equiaxed microstructure in as-built condition and as dislocation barriers to improve mechanical properties. SLM process parameters development was conducted for TiN-reinforced 17-4 PH stainless steel powders. Consistent with the literature, it was observed that SLM processing window shifts to higher energy densities with the addition of ceramic particles. Moreover, it was found that smaller point distance values favor continuous melt tracks and thus, higher densities. TiN-reinforced composites were seen to exhibit a very fine and equiaxed microstructure, effectively eliminating directional solidification and consequent anisotropy. Both strength and ductility in as-built condition increased significantly for the TiN-reinforced composites. Strength further increased after heat treatment with a compromise in ductility due to excessive hardening from both TiN particles and Cu-rich precipitates. Yet, TiN incorporation was found promising for high-temperature applications in the future, where standard 17-4 PH stainless steel fails due to precipitate coarsening.

Angular-dependent interatomic potential for large-scale atomistic simulation of iron: Development and comprehensive comparison with existing interatomic models

The development of classical interatomic potential for iron is a quite demanding task with a long history background. A new interatomic potential for simulation of iron was created with a focus on description of crystal defects properties. In contrast with previous studies, here the potential development was based on force-matching method that requires only ab initio data as reference values. To verify our model, we studied various features of body-centered-cubic iron including the properties of point defects (vacancy and self-interstitial atom), the Peierls energy barrier for dislocations (screw and mix types), and the formation energies of planar defects (surfaces, grain boundaries, and stacking fault). The verification also implies thorough comparison of a potential with 11 other interatomic potentials reported in literature. This potential correctly reproduces the largest number of iron characteristics which ensures its advantage and wider applicability range compared to the other considered classical potentials. Here application of the model is illustrated by estimation of self-diffusion coefficients and the calculation of fcc lattice properties at high temperature.

In situ high-energy X-ray study of deformation mechanisms in additively manufactured 316L stainless steel

A key structural material planned to be used in the core of the Transformational Challenge Reactor is the additively manufactured 316L stainless steel (AM 316L SS). While the deformation of conventionally-manufactured 316 SS have been extensively studied, the deformation mechanism of AM 316L SS is less known. Here the room-temperature tensile behavior of AM 316L SS was studied in situ using high-energy X-ray diffraction and X-ray tomography. Diffraction yielded the lattice strains and the dislocation densities of the AM 316L SS, which were compared to those of a conventionally processed 316H SS. It was found that while the conventional 316H SS had three distinct deformation stages, namely stage II athermal hardening, stage III dynamic recovery, and stage IV, the AM 316L experienced only the stages III and IV after an initial transitional period. The plastic instability stresses (PIS), i.e. the true ultimate tensile strength, were nearly the same in both materials. The AM 316L had very low dislocation barrier strength throughout the plastic deformation, which, in combination with the invariant PIS, explained its high dislocation storage capability and the attractive combination of strength and ductility. Tomography revealed morphological changes of the built-in pores in the AM 316L during deformation and fracture, and it was concluded that large pores close to the surface might have significant roles in the necking stage.

Microstructure affected residual stress prediction based on mechanical threshold stress in direct metal deposition of Ti-6Al-4 V

Although the material microstructure attributes play an important role in the mechanical properties of the additively manufactured (AM) part, their impact is largely ignored in the mechanics modeling of AM processes. A physics-based mechanical threshold stress (MTS) model is proposed to consider the material internal attributes, such as grain size and dislocation to dislocation barrier during simulation of the material flow stress and deformation. This model is based on the dislocation dynamics and thermal activation theory with a combination of three main components of athermal stress, thermal stress, and threshold stress. The MTS flow stress model is embedded into a fully coupled thermo-mechanical incremental plasticity model to predict the residual stress during AM processes from the prediction of temperature field and associated thermal stresses. The experimental measurements of residual stress are conducted on additively manufactured Ti-6Al-4 V samples for the model validation. The proposed model provides insight into the impact of microstructure on residual stress prediction in the additive manufacturing process modeling.