Back Stress Research Articles

Microstructure and mechanical properties of Al0.5CoCrFeNi HEA prepared via gas atomization and followed by hot-pressing sintering

Pre-alloyed Al0.5CoCrFeNi high-entropy alloy (HEA) powders were prepared via gas atomization and followed by hot-pressing sintering (HPS) at 1200 °C for 1 h to fabricate bulk materials. The phase stability, microstructure evolution and mechanical properties of Al0.5CoCrFeNi HEA were mainly investigated. Results indicated that the atomized powders exhibited a fine-grained FCC + dendritic-like ordered BCC (B2) dual-phase structure. Rapid solidification was favorable to the metastable B2 phase formation during gas-atomization. Partial of the dendritic-like B2 phase dissolved after annealing (>600 °C), resulted in a homogeneous dual-phase structure with irregular B2 phase dispersed in the FCC matrix. The homogeneous structure was “inherited” in the sintered bulks and coarsened after HPS. Tensile test results suggested that the sintered bulks possessed excellent comprehensive mechanical properties with yield strength, ultimate tensile strength and elongation of 587.3 ± 18.7 MPa, 960.2 ± 23.6 MPa and 32.6 ± 5.1%, respectively. The excellent mechanical response during deformation was mainly attributed to the synergistic deformation of the FCC/B2 phase and considerable back stress hardening mechanism.

Effect of microstructure on sag resistance of ultra-high strength spring steel evaluated by hysteresis loop

Sag refers to the reduction in the free height of coil springs during actual usage, hence sag resistance is one of the crucial properties of spring steel and its improvement is essential for prolonging the service life of springs. However, limited research has been conducted on the effect of microstructure on sag resistance in recent years. This study aims to investigate the relationship between microstructure and sag resistance in 65SiCrV6 spring steel. The steel was quenched from various temperatures to obtain microstructures with varying sub-block size, and tempered at different temperatures to obtain microstructures with distinct tempered carbides morphology. The microstructures were characterized and the sag resistance was evaluated by hysteresis loop effect formed during cyclic tensile test. The results revealed that smaller sub-block sizes of martensite and higher proportions of high-angle grain boundary (HAGB) are beneficial for improving sag resistance. Incoherent granular cementite precipitated in the tempered microstructure improves sag resistance by enhancing interactions with dislocations, and coarsened cementite does not reduce the sag resistance due to the generation of back stress in heterogeneous microstructure. The hysteresis loop effect resulting from heterostructure as well as interactions between dislocations and tempered carbides increases with rising tempering temperature until it reaches a saturation state. This study highlights the importance of optimizing the microstructure with both small sub-block size and incoherent cementite precipitation through appropriate heat treatment process to enhance sag resistance, meanwhile provides valuable insights into understanding the micromechanism of sag resistance in spring steel.

Tuning heterogeneous microstructure and improving mechanical properties of twin-roll strip cast low carbon steels by on-line heat treatments

Coarse austenite grains are unavoidable in the low carbon steels processed by twin-roll strip casting. Unfortunately, the polygonal ferrite (PF) grains cannot be refined by conventional on-line cooling process, resulting in low strength. In this work, an on-line heat treatment including reaustenitizing and subsequent controlled cooling was performed in laboratory to form heterogeneous microstructures composed of PF and acicular ferrite (AF). It has been founded that increased prior austenite grain size and the cooling rate led to more AF laths, which inhibited the growth of PF grains. When the prior austenite grain size is less than 60 μm, the formation of AF is suppressed. The tensile tests indicate that the heterostructured steels exhibited higher yield strength than the samples without AF. The improvement of yield strength can be attributed to the grain boundary strengthening, dislocation strengthening and back stress strengthening. The higher AF volume fraction leads to higher dislocation density in PF grains, and more AF/PF interfaces may increase the amount of accumulated geometrically necessary dislocations. Therefore, the contributions of dislocation and back stress strengthening increase with the increase of AF volume fractions. High dislocation density and the generation of back stress contribute a high strain hardening rate to delay the necking and sustain good plasticity. Moreover, the heterostructured steels exhibit the two-stage work hardening behavior during tensile test, which indicates that plastic deformation starts in PF grains while the AF laths remain in elastic state, and then plastic deformation occurs in both strain hardened PF grains and AF laths.

In-situ EBSD tensile revealing the evolution mechanism of high angle grain boundaries in Al–Zn–Mg alloy profile with heterogeneous structures

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs.

Synergic strength-ductility enhancement in bimodal titanium alloy via a heterogeneous structure induced by the 〈c + a〉 slip

In this work, a heterogeneous structure (HS) with an alternating distribution of coarse and fine α lamella is fabricated in bimodal Ti6242 alloy via insufficient diffusion of alloying elements induced by fast heating treatment. Instead of a distinct interface between the primary α phase (αp) and β transformation microstructure (βt) in the equiaxed microstructure (EM), all αp/βt interfaces are eliminated in the HS, and the large αp phases are replaced by coarse α lamella. Compared to the EM alloy, the heterostructured alloy exhibits a superior strength-ductility combination. The enhanced strength is predominantly attributed to the increased interfaces of α/β plates and hetero-deformation induced (HDI) strengthening caused by back stress. Meanwhile, good ductility is ascribed to its uniform distribution of coarse and fine α lamella, which effectively inhibits strain localization and generates an extra HDI hardening. This can be evidenced by the accumulated geometrically necessary dislocations (GNDs) induced by strain partitioning of the heterostructure. Significantly, the HDI causes extra 〈c+a〉 dislocations piling up in the coarse α lamella, which generates an extra strain hardening to further improve the ductility. Such hetero-interface coordinated deformation mechanism sheds light on a new perspective for tailoring bimodal titanium alloys with excellent mechanical properties.

Excellent strength-ductility synergy of Cu-Al alloy with a gradient nanograined-nanotwinned surface layer

Gradient nanostructured (GNS) metallic materials have shown significant potential in achieving excellent mechanical properties. However, the underlying strengthening mechanisms resulting from plastically inhomogeneous deformation in GNS materials, particularly those involving nanotwinned structures, remain inadequately understood. In this study, a gradient nanograined-nanotwinned (GNNT) surface layer was generated on a Cu-Al alloy using the severe plastic deformation technique known as single-point diamond turning. Uniaxial tensile and localized micropillar compression tests were conducted to compare the mechanical properties of GNNT samples with conventional gradient nanograined (GNG) and uniform coarse-grained (CG) Cu/Cu-Al counterparts. The results revealed that the GNNT samples exhibit excellent strength-ductility synergy, with a yield strength of approximately 329 MPa, tensile strength of around 477 MPa, and uniform ductility of about 48 %, thereby demonstrating remarkable strain hardening capacity and mechanical stability. These characteristics are further supported by the observed strong back stress effect, well-preserved mobile dislocation density, and effective suppression of grain coarsening in the GNNT surface layer. In contrast to the evident mechanically induced grain coarsening through grain boundary (GB) migration in the GNG surface layer, grain coarsening through twin boundary (TB) migration in GNNT surface layers is effectively inhibited by Lomer-Cottrell (L-C) locks formed by TB-stacking fault and TB-TB intersections, resulting in significantly enhanced structural stability. Furthermore, the L-C locks also extensively contribute to the high density of mobile dislocations observed in the GNNT samples. These findings provide valuable insights into the optimization of GNS structures for achieving excellent strength-ductility synergy in metallic materials.

Characteristics of nurses with musculoskeletal disorders from Dr Soedarso Regional Hospital, Pontianak, Indonesia

Musculoskeletal disorders (MSDs) can have a significant negative impact on quality of life, resulting in reduced ability to work, absenteeism, and possibly switching occupations. The purpose of this study was to investigate the relationship between the prevalence and severity of MSDs and the nurse characteristics (NCs) of nurses working in the Dr Soedarso Regional Hospital (DSRH) inpatient department. An analytical and descriptive cross-sectional methodology was used to examine 134 nurses from Inpatient Wards A and B. Total sampling was used to select the respondents. The level of exposure to the conditions investigated by the quick exposure check (QEC), namely, static and dynamic movements in the back, shoulders/arms, neck, and work-related stress, was significantly correlated with gender and neck (0.050), a history of education-related physical strain, such as back mobility (0.021), years of work-related strain on the neck (0.040), and work-related stress (0.033). There were no significant correlations found between age and static and dynamic movements of the back, shoulders/arms, and neck. Gender, education level, and employment duration all have a strong correlation with MSDs among DSRH inpatient nurses.

Modulating hierarchical microstructural components to enhance strength-ductility synergy of a high-entropy alloy containing brittle sigma phase

Although brittle intermetallic phases are usually unwanted in the design of strong and ductile alloys, high strength and good ductility can be achievable in high-entropy alloys (HEAs) containing brittle phases. Here, we present an investigation on modulating the hierarchical microstructural components in a prototype metastable HEA (Fe30Cr25Co25Mn10Ni10, at. %) containing sigma (σ) phase to optimize the mechanical properties. An excellent combination of tensile strength (999.3 MPa) and ductility (57.7 %) is achieved in alloy sample containing recrystallized grains of various sizes, which is mainly attributed to the continuous transformation-induced plasticity (TRIP) effect and the back stress effect caused by the pile-up of geometrically necessary dislocations at various interfaces in the hierarchical microstructures. The decrease in fraction of hard domains (σ particles, non-recrystallized and small-sized grains) can lead to the decrease of tensile strength due to the weakened back stress effect. However, an excessively high fraction of hard domains (particularly with small-sized grains and σ particles) suppresses the TRIP effect, facilitating crack propagation and reducing ductility. The back stress effect can be rationally controlled through tuning the fraction and size of small-sized grains (accompanied with fine σ particles) by thermo-mechanical treatments, which can also enable the activation of TRIP effect in grains of various sizes to render high strain-hardening rate and good strength-ductility synergy. This provides a strategy for achieving an optimum combination of strength and ductility for metastable HEAs containing fine brittle phases via alleviating the embrittling effect of brittle phases and retaining the precipitation strengthening effect.

A finite-deformation isotropic non-associative viscoplasticity/damage coupled thermodynamic model for ductile fracture of thick adhesive composite joint

This paper develops a finite-deformation isotropic non-associative viscoplasticity/damage coupled model to predict ductilefracture behaviors of thick adhesive composite joint within the framework of irreversible thermodynamics. First, a damage variable is introduced into the elastic constitutive model, along with the Drucker-Prager’s type yielding function and plastic potential function which take into account the variable hydrostatic pressure and non-associative plasticity. An exponential damage potential function is developed to derive the damage evolution law relating to the thermodynamic force. Second, both kinematic hardening and isotropic hardening are considered that are represented by the back stress, hardening stress and their conjugate relationships with the corresponding internal variables, derived by the Helmholtz free energy. Third, an extended version of the Perzyna’s type model is developed by incorporating an over-stress function to derive the consistency plasticity factor for the viscoplasticity/damage coupled model. Fourth, the strain, stress, back stress, thermodynamic force, damage variable and tangent modulus are updated within the corotated configuration in the integration procedure. To simplify numerical computation, the stress and strain at time n + 1 are initially updated using the frozen damage variable and the back stress at time n, and the latter two values are independently updated at time n + 1. Finally, the developed model and numerical algorithm by implicit FEA are employed to predict the strain, stress, back stress, damage and fracture behaviors of the dog-bone MMA specimens under tensile loads and the thick MMA ductile adhesive specimens under shear loads.

A Full-Field Crystal Plasticity Study on the Bauschinger Effect Caused by Non-Shearable Particles and Voids in Aluminium Single Crystals

In the present work, the goal is to use two-scale simulations to be incorporated into the full-field open software DAMASK version 2.0.3 crystal plasticity framework, in relation to the Bauschinger effect caused by the composite effect of the presence of second-phase particles with surrounding deformation zones. The idea is to achieve this by including a back stress of the critical resolved shear stress in a single-phase simulation, as an alternative to explicitly resolving the second-phase particles in the system. The back stress model is calibrated to the volume-averaged behaviour of detailed crystal plasticity simulations with the presence of hard, non-shearable spherical particles or voids. A simplified particle-scale model with a periodic box containing only one of the spherical particles in the crystal is considered. Applying periodic boundary conditions corresponds to a uniform regular distribution of particles or voids in the crystal. This serves as an idealised approximation of a particle distribution with the given mean size and particle volume fraction. The Bauschinger effect is investigated by simulating tensile–compression tests with 5% and 10% volume fractions of particles and with 1%, 2%, and 5% pre-strain. It is observed that an increasing volume fraction increases the Bauschinger effect, both for the cases with particles and with voids. However, increasing the pre-strain only increases the Bauschinger effect for the case with particles and not for the case with voids. The model with back stress of the critical resolved shear stress, but without the detailed particle simulation, can be fitted to provide reasonably close results for the volume-averaged response of the detailed simulations.

Cryogenic tensile behavior of carbon-doped CoCrFeMnNi high-entropy alloys additively manufactured by laser powder bed fusion

Cryogenic tensile behaviors of carbon-doped CoCrFeMnNi high-entropy alloy (C-HEA) printed by laser powder bed fusion (LPBF) were systematically explored. The LPBFed C-HEA exhibits excellent cryogenic tensile properties with not only high yield strength but also largely extended elongation as compared to those under room temperature deformation. In particular, the elongation of the C-HEA is twice as high at 77 K compared to 298 K. The strain hardening rate of LPBFed C-HEA under cryogenic deformation is much higher than that under plastic deformation at the room temperature, which contributes to the dramatic enhancement of uniform elongation by delaying plastic instability. Because the flow stress of C-HEA is significantly increased by temperature decrease, it can exceed the critical twinning stress at the early-stage deformation at 77 K. Deformation twins and nano-carbides synergistically contribute to the high back stress evolution of the C-HEA under cryogenic tensile deformation. This study can provide a new perspective on developing high-performance alloys for use in additive manufacturing in cryogenic applications.

Plastic anisotropy in yield stress of drawn pearlitic steels

Plastic anisotropy in the yield stress of pearlitic steels evolved by drawing was studied. The tension and compression tests were performed in parallel and perpendicular directions to the drawing direction. The yield stress in the parallel tension and vertical compression tests increased with the increase in drawing strain. On the other hand, in the parallel compression, the yield stress decreased significantly at the small drawing strain and then increased at the larger strain. The difference between the yield stress at the tension and at the compression did not change largely after the start of the drawing. The observation of microstructure indicated no significant heterogeneity of the deformation microstructure and texture at the different locations in the radial direction of the drawn samples, meaning that the plastic anisotropy was brought by reasons other than the non-uniformity of the deformation condition. The measurement of the distribution of the local misorientation in ferrite suggested the large plastic deformation even at smaller strain by drawing. This result indicates that the residual stress in plastically deformed ferrite brings back stress, which can be the source of the plastic anisotropy.

Optimizing microstructure and mechanical properties of bimodal-structured magnesium matrix composites by regulating the remelting time of powder thixoforming

A kind of bimodal graphene oxide (GO) reinforced ZK60 Mg matrix composite, characterized by the constitutes of fine grain (FG) zones reinforced by GO and coarse grain (CG) zones, was prepared via powder thixoforming, and its microstructure was optimized through adjusting the partial remelting time to achieve a desirable strength-ductility coordination. Both the volume fractions and grain sizes of CGs could be regulated at different levels by changing the heating duration. The composite thixoformed at 60 min, having 30.1 vol% CGs with average grain size of 15.2 μm, achieves the highest yield strength of 192 MPa and ultimate tensile strength of 316 MPa, as well as a high elongation of 21.8% and toughness of 61.9 MJ m−3, possessing the most remarkable coordination of strength and ductility among the as-reported homogeneous-structured Mg-6Zn composites reinforced by carbonaceous nanomaterials. The macroscopic strain of the composites becomes more uniform as the remelting time increases due to the enhanced co-deformation ability between FG and CG constitutes, which contributes to higher ductility. The back stress strengthening makes a large contribution to the strengthening of the composites. Bimodal structure induced toughening, enhanced dislocation storage capability of the FG zones, and crack blunting contribute to the toughness of the composites. This work provides a promising approach for designing and fabricating bimodal-structured metal matrix composites with good comprehensive mechanical performances.

High strength and high work hardening rate in oxygen gradient Ti-15Mo alloy

The low work hardening is a prominent deficiency for high-strength titanium (Ti) alloys. The gradient design of oxygen content was adopted to realize the coupling deformation of {332}<113> twinning and dislocation slip in the Ti-15Mo alloy. This oxygen gradient alloy exhibited an optimal balance of yield/tensile strength (700 and 848 MPa) and elongation (25 %), with remarkable work hardening behavior. The dominated dislocation slip deformation and the solution strengthening of oxygen atoms in the oxygen-rich region resulted in a remarkable increase in yield strength. The successive formation of {332}<113> twins and piled-up geometrically necessary dislocations around the twin boundaries in the oxygen-free region induced remarkable back stress strengthening, maintaining the high work hardening rate, which resulted in a stable increase in strength. The twins and dislocations formed at the crack tips effectively hindered the cracking behavior, avoiding premature necking. The present study provides a novel idea for designing oxygen layer-distributed Ti alloys, which further improves the strength–ductility tradeoff.

Microstructural origin of high strength and high strain hardening capability of a laser powder bed fused AlSi10Mg alloy

Compared to a cast AlSi10Mg alloy, a laser powder bed fused (LPBF) AlSi10Mg alloy shows superior yield strength and strain hardening capability. However, the underlying microstructure origin has not been comprehensively understood. In this work, the microstructural evolution of an LPBF AlSi10Mg alloy during tensile deformation was investigated. Synchrotron X-ray diffraction characterization shows that both stress and strain exhibit significant partition between an Al phase and a Si phase upon tensile deformation. This leads to a significant strain gradient between those two phases, which is evident by the high density of dislocations in the cell boundaries of the deformed alloy. The strain gradient results in long-range internal stress, also known as back stress, in the cell boundaries, and in turn leads to enhanced strength and strain hardening in the LPBF AlSi10Mg alloy. Quantitatively analyses via loading-unloading-reloading tests show that during the tensile deformation, the back stress contributes 135 MPa to the yield strength of the alloy, which continuously increases with increasing the strain beyond the yielding point. This work illuminates the microstructural origin of the back stress in the LPBF AlSi10Mg alloy, i.e. the back stress arises from the stress/strain partition between the Al and Si phases in the cellular structures, and the back stress leads to significant strengthening of the alloy upon tensile deformation. This work may also provide guidance for manipulating the mechanical properties of additively manufactured Al-Si alloys for specific application needs.

Numerical investigation of internal damage in heterogeneous-structured laminates using a 3D multiple physical mechanisms based constitutive model

Heterogeneous-structured laminates (HSLs), which consist of multiple layers of metallic materials arranged in successive pairs of coarse-grained (CG) and nano-grained (NG) layers, have been recently reported to display excellent balance of strength and ductility. However, it is argued that accompanying state-of-art numerical models developed to simulate the deformation of this specific class of composite materials have limitations for investigating their underlying damage evolution. Addressing this issue is essential to support the rational design, optimisation and application of HSLs, especially when subjected to contact and dynamic processes. For this reason, a novel 3D numerical framework for HSLs is proposed and tested in this research considering published experimental findings and dislocation theories. This framework comprehensively considers the evolution of various types of dislocations and back stress while being coupled with the Johnson Cook damage criterion. The HSL specimens simulated here were made of alternating layers of CG and NG copper separated by interface affected zones. Following initial microhardness and uniaxial tensile simulations on homogenous copper with different grain sizes, simulations of HSLs were conducted to study the effect of different layer thickness and the volume fraction of the NG layer. Overall, a good correlation between numerical and experimental results was achieved. An important and distinguishing characteristic of this research is that the proposed model enables the evolution of internal damage and the synergetic effect between the CG and NG layers to be investigated. Through the evaluation of the damage accumulation factor in the NG layer, the simulations results yielded quantitative information which aligned with the following known experimental observations: 1) the smaller the layer thickness, then the smaller the internal damage and 2) the internal damage increases with the increase in volume content of the NG layer. In addition, for a set simulated strain of 10%, the developed model could be used to show that the damage accumulation factor in the NG layer was 10 times lower than that in its counterpart, i.e., a stand-alone NG layer not sandwiched between two CG layers.

ERGONOMIC ANALYSIS OF HOSPITAL WORKPLACE ENVIRONMENT: THE CASE OF A PUBLIC HOSPITAL IN DUZCE

With this study, it was aimed to investigate the ergonomics of the furniture used for the employees in a public hospital in Düzce. With the data obtained and interpreted for this purpose, it was possible to use the furniture used by the hospital staff more ergonomically. Within the scope of the study, 242 employees, consisting of nurses, midwives, health officers and employees working in polyclinics, were reached by face-to-face interview method and data were obtained. As a result of the statistical evaluation of the data obtained, it was determined that 26.4% of the participants did not have any knowledge about ergonomic working conditions, and 45% had partial knowledge. The situation that the participants are most satisfied with regarding the ergonomic designs of the work areas is the presence of sufficient amount of antiseptic and disinfectant substances where deemed necessary. In addition, the participants are pleased with the existence of the necessary compartments to ensure privacy, the regular cleaning of the work areas, and the adequate lighting of the work areas. It has been determined that the participants are least disturbed by whether the electrical cables are collected safely, whether the seats are suitable or not, and whether the office temperature is appropriate. Within the scope of the study, it was observed that the most basic health problem experienced by the employees depending on their working environment was the feeling of general fatigue and exhaustion, followed by low back and back pain and stress.

Strength-ductility synergy in a novel carbon nanotube-high entropy alloy co-reinforced aluminum matrix composite

Here we present a novel design strategy for the manufacturing carbon nanotube (CNT)-high entropy alloy (HEA) particles co-reinforced aluminum matrix composites (AMCs) with heterogeneous structures. This strategy involves in situ synthesis of CNTs@HEA hybrid reinforcement. The results demonstrate that the fabricated CNTs@HEA/Al composites have bimodal grain structure and nano-reinforced particle spatial distribution. The dispersion of CNTs within the Al matrix was achieved via the transport of micron-sized HEA particles. Simultaneously, the formation of Al4C3 and Cr7C3 occurred at the HEA/CNTs/Al interface, thereby augmenting the load-bearing capacity of CNTs. The enhanced strength of CNTs@HEA/Al composites is mainly attributed to the synergistic effect of load transfer, back stress strengthening, Orowan strengthening, grain refinements and stacking faults (SFs) strengthening. Moreover, the bimodal grains and SFs of the composite contribute sustained strain hardening, resulting in satisfactory tensile ductility.

Identifying Risk Factors for Preexisting or Developing Low Back Pain in Youth, High School, and Collegiate Lacrosse Players Using 3-Dimensional Motion Analysis.

Low back pain (LBP) is a common condition that can affect athletes of all ages. The risk factors for LBP onset and worsening associated with the lacrosse shooting motion are not yet known. To identify training and biomechanical factors associated with preexisting LBP and development of LBP over 6 months in youth, high school, and collegiate lacrosse players. Case-control study; Level of evidence, 3. A total of 128 lacrosse players were enrolled in this study between January 2016 and January 2019. Player characteristics, lacrosse experience, and participation in other sports were self-reported. At baseline and 2-, 4-, and 6-month follow-ups, the players self-rated the presence and severity of LBP using a numeric pain rating scale (0-10 points). Participants were grouped according to LBP symptoms: no LBP at any time point (n = 102), preexisting LBP (n = 17), or developed LBP within the 6-month period (n = 9). The lacrosse shooting motion was captured via 3-dimensional motion analysis, and kinematic and kinetic variables were recorded. A Low Back Stress Index was used to estimate lumbar stress as a function of pelvic acceleration at the time of maximum lateral trunk lean during the shot. Univariate analyses of covariance and logistic regression models were used to address study aims. Compared with the no-LBP group, the preexisting LBP group demonstrated 13.9% to 22.9% lower maximum angular velocities at the pelvis, trunk, and shoulders in the transverse plane (P < .05), 19.3% less collective pelvis-shoulder rotation in the transverse plane (P = .015), and 4.5% more knee flexion excursion (P = .063). The developed-LBP group produced 2.3% to 11.1% higher angular velocities in the pelvis, trunk, and shoulder and generated maximum pelvic acceleration values 36% to 42% higher than the remaining groups (P < .05 for both). Mean Low Back Stress Index values were not statistically significant among the groups (no LBP: 12,504 ± 13,076 deg2/s2; preexisting LBP: 8808 ± 10,174 deg2/s2; developed LBP: 19,389 ± 13,590 deg2/s2; P = .157). Preexisting LBP was associated with significantly restricted motion of the pelvis, trunk, and shoulders during a lacrosse shot. Excessive pelvic acceleration may be related to the development of LBP in lacrosse players.

Tensile properties and strengthening behavior of CoCrFeNiW high entropy alloys with heterogeneous structures

Co30Cr30(FeNi)40−xWx (x=0−8 at.%, simplified as HW0−HW8, respectively) high entropy alloys (HEAs) were fabricated by vacuum arc-melting. The microstructure and tensile properties of as-cast and as-annealed alloys were investigated. The results show that HW2 and HW4 have a single FCC phase. The fine and granular μ phase is dispersed in FCC matrix and the area fraction increases with the increase of W content and annealing temperature. The soft FCC matrix and hard μ phase constitute the heterostructure with strain incompatibility. With the increase of W content from 0 to 8 at.%, the yield strength and tensile strength increase from 278 and 629 MPa to 530 and 839 MPa, respectively. The strain maintains 33%. The as-annealed HW8 exhibits superior yield strength (810 MPa) and tensile strength (1087 MPa). The yield strength improvement is attributed to solid-solution, precipitate, and back stress strengthening. Back stress strengthening plays a dominant role in improving tensile strength and plasticity by inducing high hardening behavior in heterogeneous structures.