Dislocation Strengthening Research Articles

Microstructure and enhanced strength-ductility of TiNbCu alloys produced by laser powder bed fusion

The β-type Ti35NbxCu alloys (x = 0, 1, 3, 5, and 7 wt%) were produced by laser powder bed fusion (LPBF) and the influence of Cu content on the microstructure and mechanical properties was studied. The results indicate that when the content of Cu is less than 3 wt%, Cu can diffuse completely into the Ti matrix to form solid solution without element segregation. Moreover, the elastic modulus of Ti35Nb1Cu alloy (49.1 ± 1.6 GPa) is reduced by 33 % compared to that of Ti35Nb alloy (73.2 ± 1.4 GPa). When the content of Cu exceeds 3 wt%, the eutectoid reaction is activated during the cyclic heating and rapid cooling of LPBF, and the partially supersaturated β phase is transformed into nano-scale α and Ti2Cu phases (β→α+Ti2Cu). Nano-scale Ti2Cu particles precipitated along β grain boundary results in a pinning effect, which inhibits the growth of β grain, hinders dislocation movement, and improves the strength of the alloy. However, when the content of Cu is 7 wt%, the dislocation density (4.179 × 1015 m−2) and the ratio of brittle phase Ti2Cu (8.5 %) are too high, and the stress concentration caused by the initiation of inherent micro-cracks are the main reason for the tensile brittle fracture of Ti35Nb7Cu alloy. As such, the Ti35Nb5Cu alloy has the highest tensile strength of 867.3 ± 24 MPa and the elongation reaches 11.7 ± 0.5 %. Therefore, solution strengthening of Cu, fine grain strengthening of β, dislocation strengthening, and precipitation strengthening of Ti2Cu are the main reasons for the excellent comprehensive mechanical properties of Ti35Nb5Cu alloy.

Effect of hatch spacing on the characteristics of LPBF 2195 Al-Li alloy

Focusing on the manufacturing challenges associated with the formation of high-strength Aluminum-lithium (Al-Li) alloys, this study explores the laser powder bed fusion (LPBF) additive manufacturing method. The influence of hatch spacing on the characteristics of 2195 Al-Li alloy fabricated by LPBF is investigated. The findings suggest that variations in hatch spacing affect the overlap between "layer-to-layer" and "channel-to-channel", resulting in a maximum relative density of 99.06% and microhardness of 130.0 HV, respectively. The grain morphology exhibits epitaxial growth of columnar grains along the maximum temperature gradient. The crystalline phases, such as T2 (Al6CuLi3) and θ′/Ω (Al2Cu), are widely dispersed within the α-Al matrix, where the formation of Mg-Ag atomic clusters promotes the nucleation of the Ω phase. The 2195 specimen exhibits an ultimate tensile strength (UTS) of 295.03 ± 5.77 MPa and an elongation of 5.01 ± 0.13%. The hatch spacing significantly affects the forming quality, while metallurgical defects, fine grain strengthening, dislocation strengthening, and secondary phase strengthening collectively determine the mechanical properties of the 2195 Al-Li alloy.

Optimizing the friction behavior of medium entropy alloy via controllable coherent nanoprecipitation

In recent years, FeCrNi medium entropy alloy, a new material with high hardness, strength, ductility, and wear resistance, has been widely studied. In this work, the effect of precipitation volume fraction on the friction behavior of FeCrNi is studied by molecular dynamics simulation. With the increase of precipitation volume fraction, the average friction coefficient shows an upward trend. When the volume fraction of precipitation is between 2.33% and 3.10%, the wear resistance of FeCrNi would be enhanced after the nanoscratching, and the normal force is large, which means that a certain precipitation volume fraction strengthens FeCrNi. Low precipitation volume fraction can effectively reduce the wear volume and wear rate during scratching, thus effectively lowering frictional force and friction coefficient. The interaction between dislocation and precipitation is an important factor that hinders dislocation propagation, leading to the strong dislocation strengthening and the increase of wear volume. This trend is manifested as the increase of normal force and frictional force. The frictional properties of FeCrNi can be optimized with a certain precipitation volume fraction. The findings give a guiding significance for the effect of multiple precipitation on frictional properties of FeCrNi.

Strengthening mechanisms and microstructural evolution of medium manganese steel under dynamic and static loading

The study analyzes the deformation strengthening mechanisms of medium manganese steel under dynamic and static loading conditions. Different austenite volume fractions (7.4 %, 19.1 %, 30.9 %, 37.7 %) were obtained by varying the heat treatment temperatures (640 °C, 660°C, 680 °C, 700 °C). Results show a linear increase in austenite volume fraction with higher annealing temperature, while carbon content in austenite decreases. Yield strength initially decreases and then increases with annealing temperature. The mechanical properties of all four temperature-tested steels show a clear strain rate dependence in the high strain rate range, with strength increasing with strain rate. Quasi-static conditions no significant strain rate sensitivity. 680 °C test steel's quasi-static deformation is mainly governed by the transformation-induced plasticity (TRIP) effect and dislocation slip in body-centered cubic (bcc) crystals. Dynamic deformation after a strain of 0.14 shows a weakened TRIP effect, and dislocation strengthening in bcc crystals becomes dominant despite a large amount of untransformed austenite. During quasi-static deformation, 700 ℃ test steel exhibits strain hardening due to the TRIP effect at low strains and significant dislocation strengthening in bcc crystals at high strains. Although the TIRP effect is initially suppressed during dynamic deformation, it lasts for a long time. The combined effects of the TRIP effect and martensite dislocation strengthening determine the deformation strengthening mechanisms. Additionally, an optimized Voce model with strain rate strengthening term is proposed. It can well describe the mechanical behavior under dynamic strain rate.

Fabrication of TiB-reinforced titanium matrix composite via spark plasma sintering and vertical bidirectional reactive hot rolling

TiB/Ti6Al4V composite was prepared using spark plasma sintering followed by bidirectional reaction hot rolling. The TiB whiskers were synthesized in situ during the rolling process, thereby preserving a relatively high structural integrity. TiB whiskers were distributed within the composite in three distinct locations: intragranular, intergranular, and transgranular distribution. After eight passes of vertical bidirectional reaction hot rolling, the average grain size of the matrix was refined to 1.36 μm, and the grains exhibited random crystallographic orientations. Tensile properties in different directions demonstrated high consistency, with a strength of 1220 MPa ± 30 MPa and an elongation of 8.0% ± 0.5%. The primary strengthening mechanisms in the composite included grain refinement, dislocation strengthening, and load transfer strengthening from the TiB.

Influence of bimodal microstructure on strength and ductility of as-extruded Mg-Gd-Y-Zr alloy

This paper studied the role of layered bimodal structure in the strength-ductility balance of Mg-8Gd-4Y-0.4Zr (wt%) alloys under as-extruded conditions. The various bimodal microstructure, consisting of fine dynamically recrystallized grains and elongated un-dynamically recrystallized grains along extrusion direction, were controlled by different extrusion ratios. The mechanical properties were then discussed in detail. The local strain evolution was investigated in the alloys based on in situ testing and analyzed with electron backscattered diffraction and digital image correlation. The fracture behavior of the alloys was characterized by 3-Dimensional X-Ray Microscopy. The alloys with an extrusion ratio of 7.5 exhibited high tensile yield strength of 338 MPa and ductility of 13.2%, resulting from grain boundary strengthening of fine DRXed grains, and dislocation strengthening of elongated un-DRXed grains and Mg5RE particles. The strain evolution analysis showed that the slips and rotations of DRXed grains contributed to relieving strain localization at the interface. Additionally, the layered structure assisted local strain dispersion and uniformed strain distribution, improving the strength and ductility of alloys. The microcracks primarily nucleated and propagated at the un-DRXed regions. The DRXed regions can refine microcracks and enhance deformation compatibility.

NbC reinforced FeMnCoCr high entropy alloy with excellent mechanical properties and wear resistance

In this paper, a NbC reinforced FeMnCoCr dual-phase high entropy alloy was used. After hot rolling, cold rolling and annealing, partial recrystallization sample and fully recrystallized fine-grained sample were obtained. The annealed samples are a single FCC phase, with micron-sized NbC particles of different shapes including columnar, granular and polygonal are distributed in the matrix. The yield strength of the partial recrystallization sample is 708 MPa, which was 1.4 times of the yield strength of the fine-grained sample, and the tensile elongation is 39.4%. The main reasons for the high yield strength of partially recrystallized samples is dislocation strengthening. The recrystallization volume fraction of partially recrystallized samples is 66.2%, and the un-recrystallization grains contain high dislocation density, and the increment of dislocation strengthening is 144 MPa. The work hardening rate of partially recrystallized samples is lower than that of fine-grained samples. Due to the high dislocation density in the partially recrystallized samples, there are fewer dislocations that can be activated and propagated during the tensile process. The high yield strength and the existence of a large number of micron-sized NbC make the NbC reinforced FeMnCoCr high entropy alloy have good wear resistance.

Effect of nitrogen on microstructural evolution and mechanical properties of low carbon reduced activation ferritic/martensitic steel with boron microalloying

In this work, the microstructural evolution and mechanical properties of 0.01 wt% boron microalloying low carbon reduced activation ferritic/martensitic (RAFM) steel with various nitrogen content were studied. The results show that nitrogen could greatly reduce the dimension and volume fraction (fV) of M23C6 carbides and increase the fV of MX carbonitrides. Fine BN inclusions with the dimension of ∼200 nm could form with the addition of 0.01 wt% nitrogen, while clustered BN inclusions with the dimension of ∼6 μm could be found by 0.02 wt% nitrogen added. The higher nitrogen could result in the higher fV and number density of MX carbonitrides, which contribute to the higher dislocation density and dislocation strengthening. However, the strength and toughness of RAFM steel deteriorated by clustered BN inclusions.

Preparing large-scale, uniform, and high-performance Cu–Cr–Zr strips by a novel continuous expanding extrusion process

A novel continuous expanding extrusion process for manufacturing Cu–Cr–Zr strips was proposed in the present study. The U-shaped strips with a cross-section of 420 mm × 19 mm were successfully prepared by taking the upward continuous casted Φ 28 mm rods as raw material. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were employed to characterize the microstructure and precipitates, respectively, which revealed that the microstructure of the continuous extruded strip is very fine and uniform across the entire cross-section and the Cr precipitates are in a dispersed distribution in the matrix helping to improve the strength of the alloy. The average grain size is approximately 1 μm and much smaller than that of the as-cast state (in millimeters). The average micro-hardness, tensile strength, and electrical conductivity of the extruded strip are 120 HV, 328.8 MPa, and 82.6 % IACS, respectively, increasing approximately by 31.6 %, 29.3 %, and 125.5 % in comparison to these of the as-cast state. In addition, the fracture elongation can maintain a very high level (average value ∼28.9 %). The strengthening mechanisms were then quantitatively calculated, which showed that grain boundary strengthening, precipitation strengthening, and dislocation strengthening take comparable contributions. Thus, our investigations provide new insights into manufacturing high-performance Cu–Cr–Zr strips.

Simultaneously enhancing the strength and electrical conductivity of Cu-Ni-Sn alloy through plastic deformation of an intermetallic compound

The precipitation strengthening can significantly increase the strength but inevitably sacrifices the electrical conductivity of Cu-Ni-Sn alloys. In this study, a Cu-9Ni-6Sn alloy was aged at various temperatures for different durations and then subjected to severe plastic deformation. When the alloy was aged at 400 °C for 36 h, the uniformly distributed fine precipitates totally transitioned to discontinuous precipitates. And formed nanofibers of intermetallic compounds after plastic deformation. The tensile strength and electrical conductivity of the treated alloy were 1178 ± 3 MPa and 25.0 ± 0.1 % IACS, respectively, which were close to those of Cu-Be alloys. Compared to traditional alloys, the strength and conductivity were both improved. The main strengthening mechanisms were dislocation and fiber strengthening. Considering these mechanisms, the theoretical yield strength was determined to be 1091 MPa, which was close to the experimental value. This work provides a new approach for the preparation of copper alloys with excellent comprehensive performance.

Strengthening Hf0.95Ta0.05B2 ceramic with ultrafine grains and high-density dislocations

Full densification and fine microstructures are the two key optimization targets of ceramic materials. Although fine Hf0.95Ta0.05B2 powder (~ 0.36 μm) has been synthesized, it was still difficult to obtain densified Hf0.95Ta0.05B2 ceramics with ultrafine grains (< 1 μm) using conventional high temperature sintering. Increasing sintering pressure could provided higher densification driving force, but it usually negatively promoted grain growth for nanoceramics. Our strategy was to gain the fully dense Hf0.95Ta0.05B2 ceramic under a high pressure at a selected temperature with retarded grain growth. In this work, fully dense Hf0.95Ta0.05B2 ceramic was prepared at 1700 °C under a high pressure of 200 MPa. The limited grain growth was achieved with the average grain size of 0.6 μm. Therefore, the mechanical properties were significantly improved, including Vickers hardness (24.8 GPa) and fracture toughness (4.2 MPa.m1/2), which were ascribed to Hall-Petch and dislocation strengthening mechanism.

Effects of heat treatment processes on the mechanical properties, microstructure evolution, and strengthening mechanisms of Al–Mg–Zn–Cu alloy

In this study, the mechanical properties and strengthening mechanisms resulting from the evolution of microstructure in an Al-4.39Mg-2.78Zn-0.42Cu (wt%) alloy were investigated through various heat treatment processes. Compared with the conventional T6 process, both the final thermomechanical treatment (FTMT) and the T6+deformation (T6D) process were found to significantly increase the strength of the alloy. In FTMT and T6D treated alloys, the distribution of nano-sized T-Mg32(Al, Zn, Cu)49 phase and dislocations were observed throughout the microstructure. Furthermore, the FTMT process led to a decrease in the solid solution amount of Mg atoms in the alloy, resulting in an improvement of the Portevin-Le Chatelier (PLC) effect. The increase in yield strength of the FTMT-treated and T6D-treated alloys was attributed to the combined effects of enhanced precipitation strengthening and introduced dislocation strengthening. However, the contributions of grain boundary strengthening and solid solution strengthening remained relatively unchanged. This study presents a strategy for producing Al–Mg–Zn–Cu alloys with excellent mechanical performance by manipulating their microstructure and strengthening mechanisms.

Microstructure and mechanical properties of 7075-T6 aluminum alloy plates by welding with weld reinforcement rolling

The welding with weld reinforcement rolling (WR) process was proposed to improve the deteriorated mechanical properties of traditional arc welded (TW) joints of the 7075-T6 aluminum alloy. In this novel process, the welding process and joint strengthening process were performed simultaneously. A comprehensive investigation of the microstructure evolution during the WR process was conducted. Ultimate tensile strength and elongation determined from tensile testing of the joints were 427.4 MPa, 5.5 % for WR joint and 301.3 MPa, 3.4 % for TW joint. The microhardness of the fusion zone (FZ) and heat-affected zone 1 (HAZ1, the dissolution zone) in the WR joint exceeded that of the TW joint. These results suggest that the application of rolling force not only affected the FZ but also simultaneously strengthened the HAZ1 through deformation while having a comparatively less effect on the heat-affected zone 2 (HAZ2, the over-ageing zone). Quantitative assessments of the effects of different strengthening mechanisms on the strength were conducted, and dislocation strengthening was found to be the predominant factor in increasing the strength of the WR joint. Deformation within both the FZ and HAZ1 during the WR process was mainly due to dislocation slip, while sub-grain rotation was also observed.

Strong and ductile medium-entropy alloy via coupling partial recrystallization and hierarchical precipitation

Developing alloys with high strength and large ductility has always been a relentless pursuit. Employing hierarchical microstructures ushers in a crescendo of achieving this goal. Partial recrystallization and hierarchical precipitation are two commonly used approaches to producing hierarchical microstructures. However, employing one of them often embrittles the material and reduces work-hardening capability. Herein, we report that via coupling partial recrystallization and hierarchical precipitation, a strong and ductile medium-entropy alloy (MEA) can be successfully developed. In the model (Ni3CrV)100-xAlx (x = 0, 3, 6 and 9 at.%) MEAs, increasing the Al content not only retards recrystallization due to the resultant solute drag and Zener pinning, but also promotes hierarchical precipitation through elemental partitioning, which produces hierarchical precipitates embedded in the partially recrystallized matrix with large chemical complexity. As a result, the dual-hierarchical structure endows (Ni3CrV)91Al9 alloy with a tensile yield strength of 1151 ± 33 MPa and an ultimate tensile strength of 1448 ± 17 MPa, as well as a uniform elongation of 17% ± 1%. The complex hierarchical structure brings multiple strengthening mechanisms, i.e., hetero-deformation-induced (HDI) strengthening, precipitation strengthening and dislocation strengthening, which lead to the observed high strength. Also, such complex structure heterogeneities facilitate multiple deformation behaviors, i.e., planar slip, wavy slip, stacking faults, dislocation networks and twinning deformation at different loading stages, which give rise to the large ductility and pronounced work-hardening capability.

Microstructural and mechanical behaviour of friction welded SS316L components fabricated by selective laser melting

The research study on solid state welding, especially rotary friction welding (FW) on the selective laser melted (SLM) components is limited. The present study is performed to deduce the scope of FW on SLM stainless steel (SS) 316 L parts and also to understand the plastic deformation effect on SLM components. The obtained results can be considered as a reference for the dissimilar welding joints of SS316L with other metallic systems in future studies. The microstructural analysis revealed significant changes in microstructural features like grain morphology and grain orientation. Also, it showed the conventional three distinct zones during the FW process, viz., base metal (BM), heat affected zone (HAZ) and weld zone (WZ). The columnar grains in the BM change into elongated deformed grains while moving towards the WZ. The findings also confirmed the existence of homogeneous equiaxed grains due to complete recrystallization in the WZ. The refined grains, resulted in improved hardness by 23 % at the WZ, when compared with SLM sample. Tensile test results demonstrated an enhancement of yield strength of about 190 MPa, however by sacrificing the ductility. WZ has achieved higher hardness and strength than the SLM component, even though the dislocation density is very high in later one. To understand this coupled effect, work hardening calculations and strengthening contributions were evaluated. The results from strengthening mechanisms confirmed that Hall-Petch strengthening is the dominant strengthening contribution in the welded samples and in case of as built parts it was due to dislocation strengthening.

Effects of Structural Radiation Disorder in the Near-Surface Layer of Alloys Based on NbTiVZr Compounds Depending on the Variation of Alloy Components

This research investigated how changes in the composition of Nb–Ti–V–Zr-based alloys affect their resistance to radiation damage and the preservation of strength characteristics when exposed to the heavy ions Kr15+ and Xe23+. These heavy ions simulate the impact of nuclear fuel fission fragments on the material. The primary objective of this study was to explore how variations in alloy components influence radiation resistance and the retention of alloy strength properties. Accumulation of radiation defects can potentially lead to embrittlement and a decrease in resistance to external factors during operation. An analysis of the X-ray diffraction data obtained from the initial alloy samples, in relation to the variations in the number of components, revealed that an increase in the number of components leads to the formation of a denser crystal structure. Additionally, this resulted in the emergence of a dislocation strengthening factor associated with changes in crystallite size. Concurrently, when assessing changes in the strength characteristics of the irradiated alloys, it was observed that the NbTiV and NbTiVZr alloys demonstrated the highest resistance to strength property degradation, specifically a 2.5- to 5-fold increase in resistance against a significant decrease in hardness. It was confirmed that the significant factor contributing towards the enhancement and preservation of the structural and strength properties is the dislocation strengthening mechanism. An increase in dislocation strengthening effectively enhances resistance against destructive embrittlement, particularly when exposed to high-dose irradiation.

Enhancing mechanical performance of Ti–Mo–Si titanium matrix composites via Al-doped MoSi2 addition

Ti–Mo–Si titanium matrix composites (TMCs) with high Si content are potential for high-temperature applications but show limited mechanical properties at room temperature due to the severe Ti–Si interfacial reaction. In this work, we propose a novel reinforcement alloying strategy to improve the interface bonding between matrix and reinforcement and enhance the room temperature mechanical properties of the Ti–Mo–Si TMCs. Specifically, the MoSi2 compound was mechanical alloyed with minor Al, then the effects of Al-doped MoSi2 addition on the microstructure, mechanical properties, fracture behavior and strengthening mechanisms of the Ti–Mo–Si composites were studied systematically. With the addition of Al-doped MoSi2, the matrix microstructure of the Ti–Mo–Si composites was refined and the mechanical properties were improved significantly. Notably, the TMC reinforced by 4 wt% Al-doped MoSi2 shows higher tensile strength and bending strength (766 MPa and 2616 MPa) than those of TMC (733 MPa and 2236 MPa) without Al-doping, respectively. The increase in strength is mainly attributed to the decrease in the volume fraction of Ti5Si3 and the tight interface bonding between the matrix and the reinforcement, leading to a high load transfer efficiency and a uniform strain distribution. In addition, dislocation strengthening and grain boundary strengthening play the key role in governing overall strength the Ti–Mo–Si composites.

Importance of cold rolling and tempering on the microstructure evolution, precipitation behavior and mechanical responses of 9Cr3Co3W1Cu ferritic/martensitic steel

The effects of thermomechanical treatment (cold rolling + tempering) on the microstructure evolution, precipitation behavior, and mechanical properties of the 9Cr3Co3W1Cu martensitic steel (G115) were investigated, by adjusting the cold rolling reduction (0, 20% and 40%), tempering temperature (750 °C and 800 °C) and time (30 min and 120 min). The results indicate that the G115 steel with 20% cold-rolling reduction rate followed by tempering at 800 °C for 30 min exhibits the optimum mechanical response (yield strength (σs) 899 MPa, elongation (δ) 26%), compared with that of 40% cold-rolled (σs 812 MPa, δ 17%) and undeformed steels (σs 670 MPa, δ 15%). The optimum yield strength in 20%-800 °C-30 min steel is attributed to the dislocation strengthening and precipitation strengthening, for the relatively low recrystallization rate and high density Cr23C6 and NbC precipitates, and the good ductility is attributed to the low recrystallization ratio and the uniform microstructure. As a comparison, in 40%-800 °C-30 min steel, a ferrite martensite dual phase structure formed. Due to the hardness difference between the two phases, there may be significant inharmony during the deformation process, leading to premature fracture.

Effect of solid solution temperature on microstructure and mechanical properties of Sc-containing 7085 alloy

The microstructure, mechanical properties, and electrical conductivity of 7085 alloy sheet without and with Sc (0.25 wt%) at different solid solution temperatures were investigated by using OM, SEM, TEM, EBSD, XRD, and the strengthening mechanism of the alloys were analyzed. The results show that the micrometer-scale second phases in the 7085 alloy are mainly the T(AlZnMgCu) phase and the Al7Cu2Fe phase, while Al3(Sc, Zr) particle and the AlZnMgCuSc phase are also formed in the 7085-Sc alloy. The Mg, Zn, and Cu elements in the soluble second phase of both alloys are mainly dissolved as the solid solution temperature increases from 460 °C to 510 °C. The 7085-Sc alloy shows a smaller grain size, higher ultimate tensile strength (UTS, 624 MPa) and yield strength (YS, 580 MPa) after treatment of 480 °C/1 h + 120 °C/24 h. The high strength of 7085-Sc alloy could be attributed to fine-grained strengthening & dislocation strengthening of Al3(Sc, Zr) particles and the precipitation strengthening of MgZn2 phases. The optimum solid solution temperature of 7085 alloy should be controlled in the range of 470–480 °C and that of 7085-Sc alloy is higher, should be 480–490 °C.

High strain rate plastic deformation driven microstructure evolution in AlCoCrFeNi dual-phase high entropy alloy

High strain rate plastic deformation under laser shock processing (LSP) has been applied to the AlCoCrFeNi dual-phase high entropy alloy (HEA) for optimizing the microstructure and properties. Phase constituents, microstructure evolution of the HEA before and after LSP were investigated. Microstructures of the HEA samples were examined using scanning electron microscopy and transmission electron microscopy. Results showed that the microstructure in the LSPed region was obviously refined. High density dislocations were produced in the alloy and dislocation slip was the main plastic deformation mode during LSP treatment. When the laser energy increased to 5 J and 7 J, nano-precipitates, twins and stacking faults appeared, and the plastic deformation mode was a mixture of deformation twinning and dislocation slip. The refined grains, dislocation structures, deformation-induced twins and stacking faults were analyzed and associated with the microhardness. The microhardness of the HEA was significantly improved after LSP, which can be attributed to the synergistic strengthening of grain refining strengthening, precipitation strengthening and dislocation strengthening.