Plastic Slip Research Articles

In-situ EBSD study of the multiple deformation modes of a TiNb/Nb/NiTi multilayer composite

In the current work, a Ti72.8Nb27.2/Nb/Ni50.9Ti49.1 multilayer composite was fabricated to elucidate the contribution of multiple deformation modes to the mechanical properties with respect to the material. The composite, which consists of a TiNb component, intermediate Nb layers and NiTi component, shows a well-bonded metallurgical interface. The in-situ EBSD results indicated that B2 and β-Nb experienced elastic deformation at macroscopic strains below 1.3% during loading. As the strain range increased from 1.3% to 9.5%, B2 underwent stress-induced martensitic transformations (SIMTs) from B2 to B19′, while β-Nb experienced mainly plastic deformation. In the case of β-TiNb, the process of elastic deformation, plastic slip, and deformation twinning occur within the macroscopic strain ranges of 0%–2.5%, 2.5%–5.3%, and 5.3%–9.5%, respectively. The deformed composite, comprising of B19′ martensite, β-TiNb/Nb, TiNb twins, and B2 parent phases, exhibited both elastic recovery (ranging from 9.5% to 8.8%) and partially reversible B19′ to B2 SIMTs (ranging from 9.5% to 7.8%). Once unloaded, a significant macroscopic residual strain is retained at 7.8%, since the partially reversible B19′ → B2 SIMTs, potential plastic deformation and deformation twinning.

The role of hydrides and precipitates on the strain localisation behaviour in a zirconium alloy

In service, zirconium alloys undergo aqueous corrosion and hydrogen is absorbed, which can lead to the formation of hydrides. Hydrides, as well as the precipitates in zirconium alloys, affect the mechanical performance, though their contribution to plasticity is not yet well understood. This study uses a combination of high-resolution digital image correlation and electron backscatter diffraction to quantify the strain partitioning between the different phases. Following uniaxial tensile deformation, it was found that the average strain within the δ-ZrH and Zr(Fe,Cr)2 was lower than in the α-Zr. Shear bands in the α-Zr matrix were observed to interact with small and medium sized hydride in multiple ways, such as terminating at the interface, cracking and shear around the interface or causing plastic slip in the hydride. Large hydrides showed a particularly detrimental effect on deformation behavior, with large strain localizations and cracking observed at interfaces, which are typically precursors to failure. In contrast, the Zr(Fe,Cr)2 precipitates remained undeformed and forced the expected metal-matrix lattice rotation. The importance of these observations in context of fuel cladding integrity is discussed.

Investigation on the crystal plasticity of α phase Ti-6Al-4V based on nano-indentation simulation and experiment

Ti-6Al-4V parts are usually hypothesized as isotropic mechanical properties during micro-machining operation. However, the crystal anisotropic mechanical property is an influential factor in the machining of crystals with different sizes and orientations. The hypothesis causes distinct variation in predicting machining force, and consequently the surface integrity cannot be exactly predicted in the micro-machining. Actually, Ti-6Al-4V parts are pseudo isotropy composed of grains with different orientations, elastic modulus, and hardness. Simultaneously, the initiation mode of plastic slip system varies with the grain orientation when the crystal is subjected to an applied load. It is challengeable to evaluate the effect of crystal orientation on its macroscopic properties, either by traditional experimental methods or macroscopic finite element analysis. Nano-indentation provides one novel method to investigate the plastic deformation of selected grains with specific orientations to characterize their mechanical response properties. This research aims to investigate the plastic behavior of α phase Ti-6Al-4V with different orientation grains by means of nano-indentation experiment and crystal plastic finite element method. In the study, single crystals of α phase are classified into “hard grains” (θ < 30°), “soft grains” (θ > 60°) and “normal grains” (30° < θ < 60°) by defining the crystal inclination angle (θ). The elastic modulus and hardness of hard grains are 9% and 28% larger than those of soft grains, respectively. This difference in mechanical properties depends on the slip system that dominates the plastic deformation behavior. The plastic deformation of soft grains is dominated by prismatic slip (48%), while hard grains are accommodated to plastic deformation by basal slip (51%). The average total cumulative shear strain of the soft grains is double that of the hard grains at a maximum load of 10 mN. Meanwhile, the sensitivity analysis is performed to evaluate the effects of the three sets of < a > slip systems and the two sets of < c + a > slip systems on plastic deformation with different grain orientations. The research contributes to the understanding of the plastic deformation mechanism in micro-machining and the exactly predicting the surface integrity generated from micro-machining.

Room‐Temperature Wide‐Gap Inorganic Materials with Excellent Plasticity

AbstractIn general, inorganic non‐metallic materials exhibit brittleness, and achieving plasticity in wide‐gap semiconductors or dielectrics poses an even greater challenge. Historically, silver halides have been suggested to be ductile; however, their deformability under different load modes has not been well demonstrated, and the underlying mechanisms are not fully understood. In this study, the authors demonstrate the excellent plasticity of AgCl and AgBr polycrystals at room temperature under tension, bending, compression, and roller pressing. In particular, the rolling reduction rate of AgCl/AgBr exceeds 97%, corresponding to the plastic extensibility from 3600% to 4200%. The metal‐like plasticity and multiple slip systems are attributed to the ionic features, specifically the Coulombic nature of the Ag‐Cl/Br interactions, and the appreciable polarization of the anions. Such less localized diffuse bonding can be readily switched upon atomic gliding, thus ensuring slip without cleavage. This study contributes to the advancement of the understanding and development of wide‐gap plastically deformable inorganic materials for applications in flexible, shape‐conformable high‐power electronics and dielectrics.

Influence of Thermo-Mechanical Process and Nb-V Microalloying on Microstructure and Mechanical Properties of Fe–Mn–Al–C Austenitic Steel

In present study, a novel Nb-V microalloyed Fe–Mn–Al–C steel and a non-microalloyed Fe–Mn–Al–C austenitic steel were treated with different thermo-mechanical processes. The microstructure of the test steels was observed by scanning electron microscope, transmission electron microscope, electron probe micro-analyzer, and X-ray diffractometer. A tensile test was conducted to estimate the mechanical properties of the test steels. Results show that the equiaxed austenitic grains are obtained in both test steels, while hot rolling reduces the grain size significantly. The grain size of the Fe–Mn–Al–C steel decreases by 40%–55% after the addition of Nb-V due to the precipitation of nanoscale (Nb,V)C particles within the austenite matrix. Compared with the solid solution treated specimens, the strength of the hot-rolled or aged specimens is improved. Meanwhile, the strength of the Fe–Mn–Al–C steel with Nb-V microalloying is also increased by 55 MPa due to the precipitation strengthening and fine grain strengthening, while the elongation is decreased. The Nb-V microalloyed Fe–Mn–Al–C steel, after hot rolling + aging treatment, obtains the maximum strength, with the yield and tensile strength of 669 MPa and 1001 MPa, respectively. The strengthening mechanisms that contribute significantly to the yield strength are solid solution strengthening and dislocation strengthening. They are 185 MPa and 211 MPa, respectively, for the Nb-V microalloyed Fe–Mn–Al–C steel at hot rolling + aging conditions. Meanwhile, the segregation of carbon atoms after aging treatment also improves the yield strength significantly.

Crack mechanism correlated with Sn grain orientations on Ni metal surface subjected to 1000 thermal shocks

With the rapid development of 3D packaging, it is of great practical significance to understand the failure mechanism of solder joints in heterogeneous integration devices. In this study, the grain orientation and microstructure evolution of SAC305 solder joints were investigated in thermal shock tests for fan-out wafer level packaging. The heterogenous evolution of microstructure in Sn–3Ag-0.5Cu (SAC305) solder joints was investigated using electron backscattered diffraction (EBSD) and nanoindentation analysis. When the thermal shock reaches 1000 cycles, the local deformation caused the heterolattices in the joint to rotate toward the corners due to the thermal expansion. During deformation, sub-grain formation is accompanied by an increase in dislocations, the accumulation of plastic slip, and the activation of slip systems. The energy stored by this deformation is released in the form of cracks. The work in this paper provides new insights into the early stages of grain orientation, microstructural deformations and the accumulation of stored energy associated with these deformations, providing a scientific basis for the application and failure analysis of large size fan-out welded joints in complex working conditions.

Efficient numerical strategies for an implicit volume fraction transfer scheme for single crystal plasticity including twinning and secondary plasticity on the example of magnesium

AbstractIn this work an efficient algorithm for a fully implicit single crystal plasticity routine including twinning and secondary plasticity is presented and implemented for the example of magnesium. The material model uses the volume fraction transfer scheme for the evolution of twinning, that is, plastic slip in newly formed twins (“secondary plasticity”) is resolved. This is considered particularly important, if the volume fractions of some twins reach the order of magnitude of the parent phase. However the resolution of secondary plasticity also implies a large number of unknowns, which is significantly reduced by a newly proposed algorithm. For magnesium a hardening model based on basal, prismatic, pyramidal a and pyramidal c+a slip modes as well as a tension and a compression twinning mode and yields a total of 18 slip systems and 12 twinning systems. As a special feature of the proposed algorithm, the total amount of 246 unknowns, due to simultaneous slip in the parent phase and the twinned phases, is reduced to only 31 unknowns. Additionally, thermodynamic consistency is ensured by including the second law with the Clausius–Duhem equation, which considers the change of free energy upon twinning as an additional driving force for twinning. Further, the setup of the time discrete nonlinear equation system using midpoint rule, as well as the analytical solution of the algorithmic tangent are given in detail. Finally, the implemented model is tested in finite element simulations and compared to single‐ and polycrystal compression and tension experiments.

Investigation of strain partition behavior in the lamellar microstructure of dual-phase titanium alloy based on crystal plasticity simulations

A well-designed microstructure plays a crucial part in achieving favorable properties and optimizing the service performances of titanium alloys, requiring thorough investigations of the microstructure effect on the heterogeneous deformation and strain partition behaviors. The mechanical responses of titanium alloy with lamellar microstructure are analyzed from lamellae features' crystallographic and geometric aspects through crystal plasticity simulations. It is found that the coarser β and finer α lamellae exhibited improved strain accommodation capacity which mainly benefitting from the large amount of plastic slip occurring in the coarse β lamellae of lamellar microstructure. Furthermore, the interlamellar strain partition coefficient evolves dynamically and gradually stabilizes after 5% deformation. Considering the geometric and crystallographic orientations of α phase, angles γ1 and γ2 are used to evaluate the strengthening effects of α lamellae precipitating from 6 types of β variants. Simulated results reveal that a more uniform strain partition occurs when cosγ1 is in the range of 0.8–1.0 and cosγ2 is lower than 0.3, while a higher deformation resistance occurs when cosγ1 and cosγ2 are in the range of 0.2–0.4 and 0.9 to 1.0, respectively. Therefore, finding an appropriate balance by manipulating the microstructural characteristics can simultaneously enhance the plasticity/strength of a titanium alloy. The obtained results will facilitate understanding the interlamellar strain partition behavior of titanium alloys and theoretically guide the microstructure designing for mechanical properties optimization.

Physical model test on the mechanical behavior and progressive failure of tunnel-type anchorages

This paper presents the results of tensile load tests performed with instrumented model of a tunnel-type anchorage (TTA) installed in a rock mass simulated model material in a rectangular chamber with digital image correlation (DIC) capability. Digital images of the model were captured during the test; the processing of these images using the DIC technique yielded the surrounding rock displacement and strain fields. The data from local sensors and DIC indicated the following: The horizontal displacement field of the surrounding rock undergoes an evolution from trumpet-shaped to spindle-shaped, whereas the vertical displacement field experiences an evolution from an inclined layered to the vertical stripe in the upper part of the plug body and an approximate arc distribution in the lower part of the plug body. The progressive failure of the TTA can be divided into four main stages according to the following: stable bearing stage I, failure stage II of shear crack initiation in the surrounding rock at the back end of the plug body, double failure stage III of compression−shear (and tension−shear) failure of the surrounding rock and slip failure of the interface, and failure stage IV of the crack in the surrounding rock extending to the ground surface and interface debonding. The progressive failure of the TTA changes from early rock mass failure to double failure, which includes rock mass and interface slip failure. During the progressive failure of the TTA, the interface undergoes elastic deformation, plastic slip, and debonding state in turn. The shape of the interface stress distribution gradually shifts from a double-peak shape with two large ends and a small middle to a double-peak shape with the peak at the rear end shifting forward, and finally into a trapezoid-like shape with an overall decrease in stress.

Achieving superelasticity in additively manufactured Ni-lean NiTi by crystallographic design

Superelastic metallic materials possessing large recoverable strains are widely used in automotive, aerospace and energy conversion industries. Superelastic materials working at high temperatures and with a wide temperature range are increasingly required for demanding applications. Until recently, high-temperature superelasticity has only been achievable with multicomponent alloys fabricated by complex processes. In this study, a novel framework of multi-scale models enabling texture and microstructure design is proposed for high-performance NiTi fabrication via laser powder bed fusion. Based on the developed framework, a Ni-lean Ni(49.4 at.%)-Ti alloy is, for the first time, endowed with a 4% high-temperature compressive superelasticity. A 001 texture, unfavorable for plastic slip, is created to realize enhanced functionality. The unprecedented superelasticity can be maintained up to 453 K, which is comparable with but has a wider superelastic temperature range (∼110 K) than rare earth alloyed NiTi alloys, previously only realizable with grain refinement, and other complicated post-processing operations. At the same time, its shape memory stability is also improved due to existing textured 100 martensite and intergranular precipitation of Ti2NiOx. This discovery reframes the way that we design superior performance NiTi based alloys through directly tailoring crystallographic orientations during additive manufacturing.

Multiscale analysis of microstructural effects on the fracture properties of laser repaired cracks with micro/nano composite material addition

The effects of microstructure characteristics on the cumulative plastic slip (CPS) in the dominant slip system (DSS) at meso-scale and the J-integral at macro-scale are studied by the combined crystal plasticity finite element method and variable-node finite element method. It is found experimentally that the grain size of repaired layer is much smaller than that of substrate, and the J-integral of the repaired specimen is also greatly reduced when appropriate amount of nano tungsten carbide (WC) is added at the crack tip. The added nano-WC during laser repairing process plays a role of Orowan strengthening on the one hand, and refine the grain of repaired layer and improve the fracture properties on the other hand. In addition, the numerical results reveal that, when the added nano-WC locates near the crack tip, the CPS in the DSS the crack tip will be reduced and prevented from reaching the critical value.

Nano-twinning and uncommon α´-martensite formation as a result of very high cycle fatigue of metastable austenitic stainless steel at 573 K

Microstructural changes were investigated regarding the existence of the true endurance limit of a cyclic loaded metastable austenitic stainless steel AISI 347 in the very high cycle regime at 573 K. The fatigue tests were performed in stress control with a stress load ration R = -1 using a servo-hydraulic testing system with a load frequency of 980 Hz. Local cyclic plastic deformation and slip bands formation were observed even in the specimens that achieved the endurance limit at 5 × 108 cycles. Moreover, in these specimens further microstructural effects like nano-twins and α´-martensite formation were detected by transmission electron microscopy. While nano-twin formation takes place in the bulk material, the α´-martensite nucleates at the specimen's surface. Therefore, the α´-martensitic transformation is not the main mechanism responsible for the true endurance limit observed in the investigated AISI 347 at 573 K.

Sensitivity of local cyclic deformation in lath martensite to flow rule and slip system in crystal plasticity

The prediction of the cyclic deformation behaviour in lath martensite-based high-strength steels requires constitutive models that reflect the local stress and strain fields as accurately as possible. At the same time, the constitutive models should act as efficiently as possible in order to achieve the required high number of cycles in a finite time. Only few research works have studied the sensitivity of the local cyclic deformation in lath martensite to the power law-based flow rule (Hutchinson or Chaboche–Cailletaud) and the active body-centred cubic (bcc) slip systems ({110}〈111〉 and {112}〈111〉) in the crystal plasticity finite element method (CPFEM). This paper, therefore, aims to provide some guidance in the selection of suitable flow rule and slip systems. Based on full-field micromechanical modelling of a medium-carbon steel under symmetric strain-controlled cyclic loading, it can be shown that the two most commonly used flow rules according to Hutchinson and Chaboche–Cailletaud are equally capable of predicting the local stress and strain distributions within the hierarchical martensitic microstructure. However, using the Hutchinson flow rule increases the computational performance for the quasi-rate-independent problem considered here. The local distributions found differ strongly from those in the parent austenitic microstructure. If plastic deformation is assumed not only on the slip systems {110}〈111〉, as often done, but also on the {112}〈111〉 type, a redistribution of the bimodal distributed local stresses occurs at a significantly lower stress level. The unimodal distributed local strains are less affected by this. In addition, it is found that slightly different critical resolved shear stress (CRSS) values for both slip system types influence the local stress and strain distributions less severely than the additional plastic slip activation in the material.

Design of Fe36.29Cr28.9Ni26.15Cu4.17Ti1.67V2.48C0.46 HEA using a new criterion based on VEC: Microstructural study and multiscale mechanical response

The present work updated empirical parameter ranges with a high adjustment to the experimental results as obtained by the means of the exploratory data analysis (EDA) method. A new high entropy alloy with FCC structure reinforced by precipitation of intermetallic phases fabricated by vacuum-argon induction melting was designed and modeled. The main results showed that the valence electron concentration (VEC) is the predominant empirical phase prediction parameter in high entropy alloys and that the lattice packing factor is necessary to determine the presence of intermetallic phases effectively. Phase stability ranges based on VEC associated with the base crystal structures and the presence of intermetallics corroborated computationally by CALPHAD and experimentally were reported. The interdendritic zone proved to be a preferential precipitation zone with precipitates of σ, TiC, and γ' phases. The γ' phase showed a size difference between the dendritic and interdendritic zone associated with diffusive Ti mechanisms. The nanoscale mechanical response determined that dislocation creep and reinforcement in the interdendritic zone are predominant creep mechanisms that obtained an effective entanglement of the dislocations increasing the strain hardening coefficient. The mechanical response of the alloy obtained is superior to the average of the alloys with FCC structure. It maintains a high ductility that allows reaching an energy absorption and damage tolerance of 43.56 GPa% showing severe plastic slip lines being in the range of use for aerospace applications and hydrogen tanks.

Correlation between dislocation hardening and the geometrically-necessary-dislocation densities in a hexagonal-close-packed Zr-2wt%Ti alloy

Geometrically necessary bands (GNBs) that are reflected by variations in the geometrically necessary dislocation densities (GNDDs) often appear at early stages of deformation. Studying the hardening variation inside and outside of GNBs is helpful for understanding how GNBs are formed to accommodate deformation heterogeneity. The former requires an accurate estimation of the slip activities at the mesoscale. Making use of the facts that the lattice rotation induced by prismatic <a> slips does not affect the <0001> direction, and one resulting from basal <a> slips does not influence an axis ⊥ <0001>, the slip activities identified by both the crystal plasticity model and slip traces for the electron backscatter diffraction microstructures after compression were verified. An abundance of GNBs was observed. The hardening/softening effect in the GNBs is always compensated by a softening/hardening behavior near the GNBs. Such microstructural features are also reflected in the profiles of frequency as a function of strength for both high and low GNDD regions: they share the same shape and peak positions. This confirms that the high GNDD region is always accompanied by a low GNDD region, and the GNDDs are independent of grain strength. Strong dependence of slip activities on grain orientations was identified, while the GNDDs are independent of orientations. The present paper provides experimental evidences of the identified slip activities and a way to explore the correlation between dislocation hardening and the GNDDs.

Coarse-Graining of a Discrete Model for Edge Dislocations in the Regular Triangular Lattice

We consider a discrete model of planar elasticity where the particles, in the reference configuration, sit on a regular triangular lattice and interact through nearest neighbor pairwise potentials, with bonds modeled as linearized elastic springs. Within this framework we introduce plastic slip fields, whose discrete circulation around each triangle detects the possible presence of an edge dislocation. We provide a $\Gamma$-convergence analysis, as the lattice spacing tends to zero, of the elastic energy induced by edge dislocations in the energy regime corresponding to a finite number of geometrically necessary dislocations.

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

Modelling of hydrogen-assisted damage at the deforming single crystal crack-tip

A coupled framework of dislocation density-based crystal plasticity model and slip-rate based hydrogen transport model is developed to simulate hydrogen-assisted damage at the deforming crack-tip. Chemical potential-based boundary conditions and mobile dislocation-assisted hydrogen transport account for the evolving hydrogen concentration. A novel fracture indicator parameter is proposed to quantify the damage that considers the combined effect of local hydrogen concentration, accumulated plastic slip and stress triaxiality. Experimentally-informed critical value for hydrogen concentration is considered to model the crack initiation. Depending on the crystal orientation, the damage is shown to be associated either with an individual hydrogen embrittlement mechanism (hydrogen-enhanced localized plasticity, and hydrogen-enhanced decohesion) or their synergistic effect at the crack tip.

Fracture behavior of B2 phase matrix of Ti2AlNb-based alloy with microcracks of different orientations

The influence mechanism of crack orientation on B2 phase crack propagation behavior under cyclic loading is studied by using the meta-atom molecular dynamics (MAMD) method, and the simulation results are compared and verified based on the theory of linear elastic fracture mechanics. According to the different propagation speed of crack in B2 phase matrix, the crack propagation process can be divided into three stages: elastic stage, rapid propagation stage and slow propagation stage. The toughness or brittleness of the crack mainly depends on the competition between the cleavage fracture of the atomic bond and the plastic slip at the crack tip. The crack propagation behavior is obviously orientation dependent. Orientation I and orientation II expand in the form of ductile fracture. Orientation III shows obvious brittleness and expands in the form of cleavage fracture. Orientation IV (Orientation I-IV represent the four angles between the crack plane and the crystal orientation) shows both ductile fracture and brittle cleavage fracture. The defects and stress concentration at the crack tip provide a driving force for martensitic nucleation, and the hcp phase produced by martensitic transformation effectively increases the toughness of the crack. The simulation results of molecular dynamics are basically consistent with the theoretical prediction of linear elastic fracture mechanics. This suggests that the variation of the orientation could tune the crack tip plasticity and toughening in B2 phase matrix.

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

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