Intragranular Dislocation Research Articles

Achieving high strength and low yield ratio by constructing the network martensite-ferrite heterogeneous in low carbon steels

In this research, focusing on low-carbon steel, a martensite-ferrite heterogeneous structure dual-phase (MFDP) steel with a network morphology where ferrite is surrounded by martensite was obtained via cyclic annealing and subcritical quenching heat treatment processes. With the initial microstructure of ferrite and lamellar pearlite, a spherical pearlite and martensitic structure surrounding the ferrite was first obtained by applying the cyclic annealing process near the Ac1 temperature. Subsequently, the annealed structure was subjected to subcritical quenching heat treatment, thereby establishing a network-like martensite-ferrite dual-phase heterogeneous structure and named N-760 °C and N-780 °C. In comparison with the ferrite-martensite dual-phase steel where ferrite envelopes martensite, N-780 °C witnessed a marked increase in tensile strength and uniform elongation, while the yield ratio dropped by 20 %. Through cyclic loading and unloading tensile tests, it was found that the N-760 °C showed a more obvious heterogeneous deformation-induced (HDI) strengthening effect. The results from electron backscattering and transmission electron microscopy indicate that, in the N-760 °C, a small quantity of dislocations is produced in the ferrite due to the martensitic phase transformation prior to the tensile test. During the tensile process, as the strain increases, the ferrite undergoes significant deformation, and the intragranular dislocations re-arrange to form dislocation cells and deformation-induced grain boundaries (SIBs). Meanwhile, geometrically necessary dislocations (GNDs) accumulate at the ferrite/martensite interface. Therefore, the non-coordinated deformation between the mesh-like dual-phase microstructure offers additional HDI strengthening for MFDP steel.

Deformation anisotropy and lamellar α re-orientation of TC17 alloy under the different slip modes of HCP structure

Deformation anisotropy and re-orientation for α phase during isothermal compression of TC17 alloy with basketweave microstructure were investigated systematically by quantitative analysis the influence of tilt angle between compression axis and c-axis on LABs density, rotate factor and ratio of activated slip system. The test results revealed that the tilt angle between compression axis and c-axis affects the deformability of α grain by affecting the rotation tendency and the number of activated slip systems. It results in the different prone of lamellar α with different orientation to globularization. Therein, the variation of the rotation factor with tilt angle conforms to the quadratic function, reaching a maximum value around 45°. When the rotation factor is small, the ratio of (01–11) [1–210] slip system with small Burgers vector at the phase boundary is large, the rotation tendency is poor, and the intragranular dislocation slip replaces the grain rotation as the strain accommodation mechanism, the lamella is more prone to globularization. And when the tilt angles are between 0°–13°, 13°–35°, 35°–78° and 78°–90°, the most easily activated slip systems are pyramidal <a+c> slip, basal <a> slip, pyramidal <a> slip and prismatic <a> slip, respectively. The more slip systems may be activated, the lamella is more prone to globularization. In addition, isothermal compression destroys the initial preferred orientations of the α grains and causes the lamellar α to rotate towards the direction that forms (0 0 0 1) plane texture, which is formed due to the activation of the prismatic <a> slip. Then, the (0 0 0 1) plane texture will be weakened due to a large number of randomly oriented grains generated by recrystallization under larger height reduction.

Microstructural evolution and deformation mechanisms of superplastic aluminium alloys: A review

Aluminium alloy is one of the earliest and most widely used superplastic materials. The objective of this work is to review the scientific advances in superplastic Al alloys. Particularly, the emphasis is placed on the microstructural evolution and deformation mechanisms of Al alloys during superplastic deformation. The evolution of grain structure, texture, secondary phase, and cavities during superplastic flow in typical superplastic Al alloys is discussed in detail. The quantitative evaluation of different deformation mechanisms based on the focus ion beam (FIB)-assisted surface study provides new insights into the superplasticity of Al alloys. The main features, such as grain boundary sliding, intragranular dislocation slip, and diffusion creep can be observed intuitively and analyzed quantitatively. This study provides some reference for the research of superplastic deformation mechanism and the development of superplastic Al alloys.

Substantial Improvement in Mechanical Properties and Anti‐corrosion Properties of Rolled Zn–Mg–Sr Alloys as an Orthopedic Implant

This study investigates a Zn–Mg alloy, enhanced for biodegradability and biocompatibility, by introducing trace amounts of Sr and subjecting it to hot‐rolling. The resulting Zn–Mg–Sr alloy shows a reduced average grain size to 3.35 μm and a transformed Mg2Zn11 phase from lamellar to finely dispersed particles, while maintaining a uniform SrZn13 phase distribution. The as‐rolled alloy exhibits significantly improved mechanical properties, with a tensile strength of 340 MPa and 40% of elongation, attributed to grain‐boundary strengthening, reinforcement by second‐phase particles, and the presence of intragranular dislocations and nanoscale precipitates. This microstructural refinement enhances the elongation by hindering crack propagation. Corrosion resistance tests reveal the superior performance of the as‐rolled alloy, owing to the finely distributed eutectic particles, which mitigated localized corrosion and altered the corrosion morphology from localized to finer corrosion pits. In vitro biocompatibility assessments show over 80% cell viability in C3H10 cultures with 50% Zn–Mg–Sr alloy extract, indicating low cytotoxicity. Furthermore, the alloy exhibits promising bone‐promoting properties, highlighting its potential for biomedical applications.

Effect of hydrogen on the microstructural evolution and local mechanical properties of 430 ferritic stainless steel at high-temperature conditions

The present study investigated the impact of hydrogen on the microstructural evolution and mechanical properties of 430 ferritic stainless steel at high-temperature conditions. Hydrogen trap sites were analyzed at high temperatures using thermal desorption spectrometry through high-temperature hydrogen atmosphere charging and electrolytic hydrogen charging. The microstructural evolution was characterized using scanning electron microscopy and electron backscatter diffraction. The nanoindentation technique was employed to analyze the local mechanical properties of ferrite matrix, grain boundaries, and M23C6/matrix interfaces. Hydrogen trap sites in 430 ferritic stainless steel included ferrite crystal lattice, grain boundaries, intragranular dislocations, and M23C6/matrix interfaces where hydrogen could not intrude at room temperature. At high temperatures, hydrogen caused a decrease in grain size. Furthermore, at high temperatures, hydrogen altered the crystal orientation and texture of the ferrite matrix. There was an increase in the number of ferrite grains with (001) and (101) orientation increase and the formation of a new α-fiber texture. Hydrogen inhibited the triggering of the slip system in body centered cubic crystal lattice and induced an increase in dislocation density and internal stress. While hydrogen at high temperatures had a minimal impact on the matrix strength limit, it reduced the strength limit of the near grain boundaries and (Fe, Cr)23C6 precipitates. The influence of hydrogen on the microstructural evolution and mechanical properties of 430 ferritic stainless steel could be elucidated by the mechanisms of hydrogen enhanced decohesion, hydrogen-enhanced localized plasticity, and hydrogen enhanced specific crystal plane rotation suggested in the present study.

Fracture behaviors and microstructure evolution of an Al–Mg–Mn-Sc-Zr alloy at elevated and cryogenic temperature

The mechanical properties and fracture behavior of Al-5.8Mg-0.4Mn-0.25Sc-0.1Zr alloy were investigated across cryogenic temperatures (−196.15 °C ∼ −56.15 °C) and elevated temperature (room temperature ∼ 250 °C), with results indicating sensitivity of the alloy's mechanical properties to temperature. Firstly, the tensile test at elevated temperatures shows a decrease in strength and strain hardening exponent (n) due to thermal softening effects, while elongation increases and then decreases, peaking at 100 °C. Within the temperature range of −196.15 °C ∼ −56.15 °C, the yield stress (YS), ultimate tensile strength (UTS), and ductility tend to increase with the decreasing temperature gradually, which can be attributed to the decrease in lattice thermal vibration energy at low temperatures, resulting in an elevated external force requirement for dislocation motion. Meanwhile, throughout the entire high-temperature range, the alloy exhibits ductile fracture, with the number of dimples peaking at 100 °C. Differently, when the alloy is deformed at low temperatures, dislocations tend to accumulate within the grains, and with the grain boundaries acting as weak bonding areas between grains and experiencing significant stress concentration, the fracture mechanism gradually transitions from transgranular to intergranular fracture. Microstructural analysis at low temperatures shows minimal texture content changes, while at elevated temperatures, regular changes in texture components occur due to grain orientation rearrangement. Furthermore, high-resolution transmission electron microscopy was employed to investigate the evolution of precipitates at various temperatures, and the results shows that uniform and fine Al3(Sc1−x, Zrx) precipitates are distributed within the crystal, acting as effective dislocation pinning sites and leading to a uniform intragranular dislocation arrangement after tensile test at cryogenic temperature. Notably, after tensile test at higher temperatures, a few precipitates lose coherency with the matrix and started growing, which diminishing grain boundary pinning effect and promoting recrystallization.

Effect of final rolling temperature on the microstructure and properties of high-manganese austenitic low-temperature steel

The effect of final rolling temperature ranging from 962 to 696 °C on as-rolled high-Mn austenitic low-temperature steel was investigated using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The microstructure of the experimental steel remained austenite without suffering ferrite phase transformation, even when produced with a final rolling temperature as low as 696 °C. A decrease in the final rolling temperature of the steel was accompanied by a decrease in grain size from 20.2 to 6.0 μm, accompanied by an increase in dislocation density from 3.01 × 1014 to 10.9 × 1014 m−2. In addition, M23C6 carbides precipitated at the grain boundaries when the final rolling temperature was below 782 °C. Grain refinement and increased dislocation density significantly enhanced the strength of the material. However, an increase in the intragranular dislocation density compromised grain deformation ability. Moreover, the precipitation of M23C6 reduced the plasticity and toughness of the material. Considering the balance between yield strength and low-temperature toughness, the optimal final rolling temperature was 898 °C; this produced a material with a yield strength of approximately 507 MPa and a low-temperature impact energy absorption of approximately 143 J.

A novel strategy to enhance the mechanical properties and corrosion resistance of ultra-high strength Al-Zn-Mg-Cu alloy: Pre-recovery Multi-Stage Solution Treatment (P-MST)

This study proposed a novel solution treatment method: Pre-recovery Multi-Stage Solution Treatment (P-MST), which involved a pre-recovery treatment before the conventional MST. Hardness testing results indicated that the recrystallization initiation temperature of the alloy is around 240 °C. Through in-depth analysis of the microstructure, mechanical properties, and corrosion resistance of samples subjected to conventional MST (450 °C/2h + 460 °C/2h + 470 °C/2h, water quenched), P-MST-350 °C (350 °C/12h + MST), and P-MST-250 °C (250 °C/12h + 350 °C/12h + MST), it was found that under the treatment of 350 °C/12 h and 250 °C/12 h + 350 °C/12 h, some MgZn2 phases were dissolved, resulting in a decrease in the area fraction of residual secondary phases and refinement in their size, leading to a higher degree of solid solution for Zn and Mg in the alloy. Simultaneously, pre-recovery treatment not only suppressed recrystallization phenomena and decreased intragranular dislocation density, but also caused the intragranular aging precipitates to transition from η' + η to GP zones + η′, with smaller precipitate sizes. Consequently, the alloy's tensile strength, elongation, and corrosion resistance showed significant improvement. Compared to the MST sample, the P-MST-250 °C sample exhibited enhancements in yield strength, ultimate tensile strength, and elongation from 725 MPa, 741 MPa, and 4.1 % to 778 MPa, 802 MPa, and 6.5 %, respectively. Quantitative calculations demonstrated that solid solution strengthening, low-angle grain boundary strengthening, and aging precipitation strengthening contributed approximately 9.6 MPa, 26.3 MPa, and 43.3 MPa, respectively, while the dislocation strengthening decreased 27.3 MPa to the yield strength of the alloy.

Uniaxial tensile behaviors and Hall-Petch relationship of polycrystalline 316LN stainless steel via molecular dynamics simulation

A molecular dynamics (MD) simulation method was used to investigate the effects of element position, tensile strain rate (ε̇) and average grain diameter (d) on the uniaxial tensile properties of 316LN stainless steel at room temperature. It was found that the mechanical properties (elastic modulus, yield strength and tensile strength) of 316LN stainless steel were independent of the random position of the elements used in the model. Since the time of atoms relaxing to the equilibrium position was determined by the tensile strain rate, the mechanical properties deeply depended on the tensile strain rate. The fluctuations of simulated mechanical properties decreased with the increase number of randomly oriented grains in the molecular dynamics model, and the minimum number of grains should not be less than 500. Intragranular dislocation movement and grain boundary sliding were the main factors leading to plastic deformation, and their competition led to the normal to inverse Hall-Petch (HP) transformation of yield strength at the critical average grain diameter of 17 nm. When d was bigger than 17 nm, the relationship between yield strength and d was a normal HP relationship, σy(MPa) = 162 + 465.5d −0.5; while d was smaller than 17 nm, the forgoing relationship transformed into an inverse HP relationship, σy(MPa) = 3930-25d−0.5.

The transition of strain rate sensitivity induced by Fe solutes in electrodeposited nanocrystalline Ni–Fe alloy

A face-centered cubic nanocrystalline (NC) Ni–Fe alloy was fabricated by adjusting electrodeposition parameter. By evaluating the hardness of NC Ni–Fe via nanoindentation at the selected strain rates from 0.005 s−1 to 0.2 s−1, a transitional point of strain rate sensitivity index (m) was exhibited, contrary to the constant value that reported in other works. Based on X-ray diffraction and transmission electron microscopy analysis, which evidenced the apparent lattice expansion as much more Fe solutes dissolved in intragrain rather than grain boundary (GB), we attributed the unexpected enhancement of m value at lower strain rates to pronounced GB-mediated plasticity facilitated by residual dislocations near GBs, whilst the lower m value at higher strain rates mainly to the intragranular dislocation motions. This work contributes directly to understanding the profound effect of alloying on the mechanical behaviors of NC materials.

Effect of grain size of nanocrystalline metals on the loss modulus

The unique distribution of grain boundaries (GBs) and grains in nanocrystalline metals makes it interesting to study the mutually exclusive properties of modulus and damping. This study carried out large-scale calculations about the grain size effects through molecular dynamics and comprehensively analyzed the static modulus and damping properties of nanocrystalline metals using the loss modulus as the merit value. The research reveals that as grain size increases, the dissipation in polycrystalline metals transitions from being dominated by GB sliding in small grains to being jointly dominated by GB sliding and intragranular dislocation in larger grain structures. This transition is similar to the one observed in plastic flow stresses in nanocrystalline materials under finite deformation, i.e., the transition from the inverse Hall-Petch mechanism to the Hall-Petch mechanism. The mechanistic conversion of these two dissipations is demonstrated by quantitative characterization. This study is instructive for the mechanical design and development of nanocrystalline metals.

Observation of Grain Boundary Sliding in a Lamellar Ultrafine‐Grained Steel

The deformation behavior of a ferrite steel with ultrafine‐grained (UFG) lamellar microstructure generated by linear flow splitting is investigated and compared to a coarser cold‐worked reference state, using a set of complementary local deformation and microstructural characterizations methods. The pile‐up around indentations shows a pronounced anisotropy for the UFG lamellar microstructure indicating the relative motion of grains along their elongated boundaries. This observation is confirmed by stepwise compression testing of micropillars along the normal direction of lamellar‐shaped grains using a new faceted pillar geometry to image the initial microstructure and its evolution throughout the test. The surface roughening in pillar compression testing can be categorized into the formation of discrete steps at the surface along particular grain boundaries and a more gradual roughening that is attributed to intragranular dislocation slip. Potential mechanisms for the observed grain boundary sliding are discussed taking several factors such as the strain rate sensitivity and potential Coble creep rates into account. In conclusion, a grain boundary sliding process carried by grain boundary dislocations appears to be the most likely explanation for the observed behavior.

Low-temperature superplastic deformation mechanism of ultra-fine grain Ti–6Al–4V alloy by friction stir processing

The ultra-fine grain Ti–6Al–4V alloy sheet was successfully achieved by friction stir processing (FSP). The low-temperature superplastic deformation mechanism of Ti–6Al–4V alloy under the strain rate of 3 × 10−4 s−1 at 600 °C was studied by scanning electron microscope, electron backscatter diffraction and superplastic tensile test. It is found that the grain size of Ti–6Al–4V alloy by FSP is refined from 7.48 μm to 0.82 μm, and there is no preferential crystallographic orientation of the uniform grain. After FSP, the β phase of the base material (BM) in the grain boundary is refined and homogenized. During the superplastic tensile processing, with the increase of strain, the grain is refined continuously, and the α→β phase transition is induced by the dislocations pile-up at the β grain boundary, and the content of β phase increases. The grain near the fracture was refined to 0.35 μm, and the stress concentration led to the formation of cavities near the β phase, which eventually caused the fracture. The dynamic recrystallization in superplastic tensile processing is mainly geometric dynamic recrystallization and discontinuous dynamic recrystallization. The elongation of superplastic tensile (1033%) is 19.7 times higher than that of BM (50%). The order of superplastic deformation mechanism in the low-temperature superplastic deformation of the ultra-fine grain Ti–6Al–4V alloy sheet is as follows: intragranular dislocation slip and diffusion, grain boundary sliding, and phase boundary sliding. Phase boundary sliding is the main deformation mechanism for low-temperature superplastic deformation.

Phase-field modeling of stored-energy-driven grain growth with intra-granular variation in dislocation density

We present a phase-field (PF) model to simulate the microstructure evolution occurring in polycrystalline materials with a variation in the intra-granular dislocation density. The model accounts for two mechanisms that lead to the grain boundary migration: the driving force due to capillarity and that due to the stored energy arising from a spatially varying dislocation density. In addition to the order parameters that distinguish regions occupied by different grains, we introduce dislocation density fields that describe spatial variation of the dislocation density. We assume that the dislocation density decays as a function of the distance the grain boundary has migrated. To demonstrate and parameterize the model, we simulate microstructure evolution in two dimensions, for which the initial microstructure is based on real-time experimental data. Additionally, we applied the model to study the effect of a cyclic heat treatment (CHT) on the microstructure evolution. Specifically, we simulated stored-energy-driven grain growth during three thermal cycles, as well as grain growth without stored energy that serves as a baseline for comparison. We showed that the microstructure evolution proceeded much faster when the stored energy was considered. A non-self-similar evolution was observed in this case, while a nearly self-similar evolution was found when the microstructure evolution is driven solely by capillarity. These results suggest a possible mechanism for the initiation of abnormal grain growth during CHT. Finally, we demonstrate an integrated experimental-computational workflow that utilizes the experimental measurements to inform the PF model and its parameterization, which provides a foundation for the development of future simulation tools capable of quantitative prediction of microstructure evolution during non-isothermal heat treatment.

Creep Performance Study of Inconel 740H Weldment Based on Microstructural Deformation Mechanisms

Abstract The creep behavior of Inconel 740H weldment at the temperature of 760 °C is investigated experimentally and analytically using the deformation-mechanism-based true stress (DMTS) model. The Inconel 740H weldment specimens are prepared with the gas tungsten arc welding technique. Creep testing is performed on the Inconel 740H weldment specimens under a range of stress levels from 190 MPa to 447 MPa at the temperature of 760 °C. The DMTS model is employed to analyze the creep curves and creep rates. The model parameters for Inconel 740H weldment are determined from the analyses of the creep testing data in combination with that from the previous studies of similar materials based on the same creep mechanisms that involve grain boundary sliding and intragranular dislocation climb-plus-glide with dislocation multiplication. The creep life predictions of the DMTS model for Inconel 740H weldment agree very well with creep rupture test data within a temperature range of 700–800 °C. The fractured surfaces and longitudinal sections of creep-tested Inconel 740H weldment specimens are examined using scanning electron microscopy, which corroborates the DMTS model inference that the creep failure of Inconel 740H weldment is in a mode of predominantly intergranular fracture. The present study suggests that grain boundary sliding is the most significant controlling factor for the creep failure of Inconel 740H weldment.

Effect of cooling temperature on microstructure and mechanical properties of Ti microalloyed steel

Ti is a relatively inexpensive alloying element. Ti micro-alloying enables steel to have higher strength, which has already been applied to steel products in many domains, including engineering machinery. In this work, the effects of cooling temperature upon the mechanical properties and microstructure of Ti microalloyed steel were researched by OM, TEM, tensile and impact tests. The consequences show that as the final cooling temperature (FCT) decreased, the steel plate microstructure changed from PF + P + B (less) to GB + PF (less)+ P (less), and the grain size decreased obviously. As the temperature further decreased to 607°C, the intragranular dislocation tangles and subgrains increased significantly. The size and number of large TiC precipitates in the steel plate decreased with the decrease of the final cooling temperature. Additionally, the mean size of precipitates decreases gradually. As the FCT varied between 672°C and 645°C, the steel plate strength and low temperature toughness were significantly improved. As the temperature is further reduced to 607°C, the strength increase was reduced. Due to the improvement of grain uniformity, the low temperature toughness was further enhanced to 156 J.

Powder bed fusion repair of titanium with surface damage: Molecular dynamics study on microstructure and mechanical properties

Additive manufacturing has been emerging as a highly efficient method that minimizes the consumption of virgin materials and energy to rapidly repair the damaged parts and potentially recover the original performance. Here, the microstructures and mechanical properties of polycrystalline titanium (Ti) with surface damage after powder bed fusion (PBF) repair and post-polishing treatment are investigated by molecular dynamics simulations. It is found that during PBF repair of the surface damage by melting Ti nanoparticles, grain boundaries (GBs) in polycrystalline Ti are more susceptible to damage from temperature elevation than the grain interiors. During the solidification process, grains grow in the form of columnar crystals. After PBF repair and post-polishing treatment, the yield strength of damaged samples is recovered to 96.8% of that of perfect ones. This restoration can be attributed to the increased capacity of carrying dislocations due to that the GB directional growth generates dislocation walls after PBF repair. More importantly, the flow stress exceeds that of the perfect sample due to the increase in stacking faults and the appearance of twins after PBF repair, which enhance the resistance of dislocation motion. The fatigue performance of the damaged samples processed by combinatorial PBF repair and post-polishing treatment is found to surpass that of perfect samples. This is attributed to that the elimination of rough surface and the reduction of intragranular dislocations avoid dislocation entanglement that otherwise results in strain concentration, and also prevent the premature occurrence of twins and the rapid increase in stacking faults during the cyclic loading, thus retarding the nucleation of intergranular cracks. Our molecular dynamics simulation results could help shed light on the atomic mechanism of microstructure evolution during PBF repair and mechanical performance after PBF repair.

Hot isostatic pressing induced precipitation strengthening at room and high temperature of Ni-Fe-Cr-Al-V high-entropy alloy manufactured by laser powder bed fusion

The highly adjustable properties of high-entropy alloys (HEAs) offer great potential for developing superior materials for critical structural applications in high-temperature conditions. In this study, Ni-Fe-Cr-Al-V HEA was manufactured by laser powder bed fusion, which has a unique microstructure composed of dislocation substructure and Face-centered cubic + L12 coherent structure and achieves the desired strength-ductility combination. Hot isostatic pressing post-treatment was applied on the laser powder bed fusion-processed HEA to further improve the relative density and optimize the mechanical properties. During the hot isostatic pressing process, the precipitation of L12 and B2 phases can be ascribed to the precipitation modes dominated by continuous precipitation and discontinuous precipitation, respectively. With the tensile deformation temperature increasing from 773-1,173 K, the softening degree of HEA increases continuously, and the dominant deformation mechanism evolves from intragranular dislocation slip to grain boundary sliding. At temperatures below 773 K, precipitation strengthening significantly improves tensile strength and ductility. At 1,173 K, the grain boundary strength decreases and grain boundary area increases, which promotes grain boundary sliding and contributes to plastic deformation, resulting in significant softening.

Effects of P segregation on deformation mechanism in Ni-P nanocrystalline by atomic simulations

By using hybrid Monte Carlo-molecular dynamics (MC-MD) simulations, the segregation behavior of solute P atoms in the grain boundaries (GBs) of nanocrystalline (NC) Ni-P alloy was studied under different temperatures. The effects of P segregation on the mechanical properties and quantitative deformation mechanism of the Ni-P alloy were also investigated at high temperatures (800 K). The results show that the temperature strongly influences the segregation behavior of P atoms. Uniform P segregation occurs along GBs at 300 K, while at elevated temperatures (e.g., 1000 K), P atoms are concentrated at triple junctions (TJs), forming Ni-P precipitates. The uniform grain boundary segregation of P leads to Ni-P alloy with larger grain sizes and higher strength, while Ni-P precipitates at TJs result in alloy with smaller grain size, higher yield and flow stresses, which could be attributed to the Zener pinning effect. Moreover, quantitative analysis of the deformation mechanisms reveals that for NC Ni-P alloy with small grain size, Ni-P precipitates show significant effect on the competition between intracrystalline deformation and grain boundary deformation. A low percentage of elastic strain on the GBs was observed in the elastic deformation stage. The plastic deformation process of the non-segregated sample with grain size of 5 nm is dominated by grain boundary deformation, whereas for non-segregated samples of 10 and 15 nm, the strain is accommodated by both intragranular dislocation slip and grain boundary deformation. Importantly, Ni-P precipitates can reduce the degree of grain boundary deformation in alloys with small grains and thereby stabilize the GBs. The less grain boundary deformation caused by uniform grain boundary segregation of P and the more dislocation slip play a major role in improving the mechanical properties of the alloy with large grains. The results and findings of this study provide further insights into the segregation-induced strengthening mechanisms of NC Ni-P alloys.

Grain size dependence of grain rotation under high pressure and high temperature

Grain rotation caused by the movement of dislocations is a determinant factor for the mechanical behavior of metals. In general, the grain rotation may be mediated by grain boundary dislocations (GB-dis) and intragranular dislocations (In-dis), which are closely associated with grain size. Few works have investigated how grain size depends on grain rotation, and the competitive mechanism between GB-dis and In-dis remains unclear. The present work investigates the structural evolution and deformation of coarse-grained tungsten under high pressure. The results show that under high pressure, the nano-sized grains preferentially rotate with dislocation climbing in GBs. Under high pressure, In-dis migrate faster across coarse grains and are absorbed by GBs on the other side, resulting in grain rotation. Elevated temperature also facilitates the migration of In-dis to arrive GBs where they can be absorbed by GBs, thus promoting grain rotation. The theoretical results show that grain rotation occurs easily under high pressure and high temperature. With increasing grain size, the stress-induced rotation mechanism goes from being dominated by GB-dis to being dominated by In-dis migration. The competitive relationship between GB-dis and In-dis during grain rotation is elaborated, providing a new strategy for designing materials under high pressure.