Mechanical Properties Of Amorphous Materials Research Articles

The change of glass transition temperature under general stress state in amorphous materials

The phenomenon of the change in glass transition temperature (Tg) upon externally applied stresses in amorphous materials has been known for a long time. However, most of the existing works focus on the effects of hydrostatic stress state, while the effect of a general stress state on glass transition behaviors remains largely unknown. In this work, molecular dynamics (MD) simulations are first performed on typical amorphous polymers and alloys to probe the change of Tg under different stress states. It is found that Tg changes linearly with pure hydrostatic stress, while under shear stress, Tg changes nonlinearly with the stress magnitude. This further leads to an asymmetric change in Tg under uniaxial tension and compression loading conditions. Based on the above observations, a theoretical model based on the free volume theory and Eyring model is proposed to quantify the change of Tg under general stress states. Comparison with simulation results and existing experimental data reported in the literature demonstrate that the theoretical model can capture the stress dependence of Tg in amorphous materials. Potential applications in tuning Tg through external stress and the sensitivity of Tg to hydrostatic and shear stresses for different amorphous materials are subsequently discussed. The current work reveals the dependence of Tg on general stress states, pointing out potential routes to tune the mechanical properties of amorphous materials.

Decoupling between enthalpy and mechanical properties in rejuvenated metallic glass

The mechanical properties of amorphous materials are closely related to their energy states (enthalpy). Usually, a higher energy state corresponds to larger plasticity and lower hardness. Here, we explore the correlation between the enthalpy and hardness of rejuvenated Au-based metallic glasses (MGs). We found that the rejuvenated MG with a high-energy state presents a hardness variation trend different from before, i.e. the hardness increases instead of keeping on decreasing with increasing enthalpy when the enthalpy is higher than an intermedia value. It is revealed that both the reactivations of β- and α-relaxation are beneficial to increasing the enthalpy but show opposite effect on the hardness. This study shows that this decoupling may be originated from the beginning of the reactivation of α-relaxation. Our work provides new insights into designing MGs with excellent mechanical properties by rejuvenating relaxed MGs via the furthest reactivation of β-relaxation and avoiding reactivation of α-relaxation.

Development of Mg/Al/Si/O ReaxFF Parameters for Magnesium Aluminosilicate Glass Using an Artificial Neural Network-Assisted Genetic Algorithm

Commercial high-strength S-glass fiber used in structural composites mainly consists of SiO2, Al2O3, and MgO. There is no established reactive force field to characterize S-glass fiber. In this study, a newly developed artificial neural network (ANN)-assisted genetic algorithm (GA) is applied to optimize a new ReaxFF parameter set to describe Mg/Al/Si/O interactions in S-glass and other magnesium aluminosilicate (MAS) glass compositions. The training set includes the density functional theory data of the energy response of various Mg/Al/Si/O crystals during volumetric expansion and compression and Mg migration inside Mg/Al/Si/O crystals. Test molecular dynamics simulations showed the characteristics of tectosilicate MAS glasses. Different structural properties, including oxide coordination, density, structural factors, and mechanical properties, showed fair agreement with references from experiments and other simulations. A newly developed GA-ANN parametrization algorithm assisted the training process. This force field can be used for virtual composition mapping to develop new glass fiber materials. We also believe our force field would support computational studies of mechanical properties of amorphous materials used in geochemistry, construction, and protective material applications.

Accessing a broader range of energy states in metallic glasses by variable-amplitude oscillatory shear

The influence of variable-amplitude loading on the potential energy and mechanical properties of amorphous materials is investigated using molecular dynamics simulations. We study a binary mixture that is either rapidly or slowly cooled across the glass transition temperature and then subjected to a sequence of shear cycles with strain amplitudes above and below the yielding strain. It was found that well annealed glasses can be rejuvenated by small-amplitude loading if the strain amplitude is occasionally increased above the critical value. By contrast, poorly annealed glasses are relocated to progressively lower energy states when subyield cycles are alternated with large-amplitude cycles that facilitate exploration of the potential energy landscape. The analysis of nonaffine displacements revealed that in both cases, the typical size of plastically rearranged domains varies depending on the strain amplitude and number of cycles, but remains smaller than the system size, thus preserving structural integrity of amorphous samples.

Time-resolved shear transformations in the transient plastic regime of sheared amorphous silicon.

The accumulation of shear transformations (STs) in space and time is responsible for plastic deformation in amorphous solids. Here we study the effect of finite strain rates on STs during simulations of athermal shear deformation in an atomistic model of amorphous silicon. We present a time-resolved analysis of STs by mapping the plastic events identified in the atomistic simulations on a collection of Eshelby inclusions, which are characterized in terms of number, effective volume, lifetime, and orientation. Our analysis led us to distinguish between small and large events. We find that the main effect of a lower strain rate is to allow for a larger number of small events, roughly identified by an effective volume γ_{0}V_{0}<20 Å^{3}, while the number and characteristics of larger events are surprisingly independent of the strain rate. We show that at low strains, the decrease of the stress observed at lower strain rates is mainly due to the excess of small events, while at larger strains, when the glass approaches the yield point where a shear band forms, larger events start to play a role and organize due to their elastic interactions. This phenomenology is compared with the predictions of mesoscale elastoplastic models. The technique developed here can be used as a systematic tool to analyze plasticity during molecular dynamics simulations. It can also give valuable information to develop physically grounded mesoscale models of plasticity, providing quantitative predictions of the mechanical properties of amorphous materials.

The influence of complex thermal treatment on mechanical properties of amorphous materials

We study the effect of periodic, spatially uniform temperature variation on mechanical properties and structural relaxation of amorphous alloys using molecular dynamics simulations. The disordered material is modeled via a non-additive binary mixture, which is annealed from the liquid to the glassy state with various cooling rates and then either aged at constant temperature or subjected to thermal treatment. We found that in comparison to aged samples, thermal cycling with respect to a reference temperature of approximately half the glass transition temperature leads to more relaxed states with lower levels of potential energy. The largest energy decrease was observed for rapidly quenched glasses cycled with the thermal amplitude slightly smaller than the reference temperature. Following the thermal treatment, the mechanical properties were probed via uniaxial tensile strain at the reference temperature and constant pressure. The numerical results indicate an inverse correlation between the levels of potential energy and values of the elastic modulus and yield stress as a function of the thermal amplitude.

Thermomechanical behavior of amorphous alloys based on titanium at the temperature range of the glass transition and crystallization

Knowledge and systematic research on the mechanical properties of amorphous materials in the transition temperature range from amorphous to crystalline is a fundamental scientific interest. Planning the use of these materials must be based on thermomechanical properties information. TiZrCuPd amorphous alloys belong to the potentially lead-free solders of relatively large glass forming ability. The paper presents results of the differential scanning calorimetry, thermomechanical, microstructural and phase analysis of the TiZrCuPd amorphous alloys containing 14 at% Pd and 2 and 3 at% Sn additions at the expenses of Cu content. The samples were investigated in the tensile-load modulated and dilatometric modes in the temperature range of the glass transition and crystallization. The relation between two steps of crystallization process, plastic flow and resulting ability for the deformation was analyzed and related to the microstructure and phase analysis. The determined temperature range of glass transition and primary crystallization show enhanced ductility in the dependence on the Sn additions and homogeneous deformation in this range. The engineering strain of alloys reveals its dependence on the temperature, especially visible in the sample for the 2 at% Sn additive.

A multiscale model for amorphous materials

In this work, we proposed a Cauchy-Born rule (CBR) based multiscale model to study mechanical properties of amorphous materials. In this work, we combine a coarse-grained Parrinello-Rahman (CG-PR) method and the Multiscale Cohesive Zone Model (MCZM) method to model the Lennard-Jones (L-J) binary glass and amorphous silicon (a-Si) solid. The proposed CG-PR method applies the CBR to a representative volume element of an amorphous material with representative microstructure pattern, whose side dimension is about twice of the cutoff distance of interatomic interaction. Numerical simulations were carried out, and it is found that CG-RP method can reproduce the stress-strain relations extrapolated from large scale MD simulations for both L-J binary glass as well as amorphous silicon (a-Si).The CG-PR method is then combined with MCZM method to simulate failure process of amorphous materials. We found that (1) the CG-PR method can capture the history-dependent inelastic stress-strain relation in amorphous materials, and (2) the CG-PR enhanced MCZM method can simulate both brittle and ductile fracture in both a-Si solid and L-J binary glass. Moreover, the multiscale methodology developed here may be extended to study mechanical properties of a variety of other non-crystalline materials.

Mechanical Relaxation-to-Rejuvenation Transition in a Zr-based Bulk Metallic Glass

The relaxation of amorphous materials, i.e., aging, would largely endanger their performances in service. Here we report a mechanical relaxation-to-rejuvenation transition of a Zr35Ti30Be27.5Cu7.5 bulk metallic glass (BMG) in elastostatic compression at ambient temperature, thus provide an accessible way to tailor the mechanical properties of amorphous materials. To unravel the structural evolution underlying the observed transition, atomistic simulations parallel with the experimental tests on a typical model glass system Zr60Cu40 were performed, which successfully reproduced and thus upheld the experimentally observed mechanical relaxation-to-rejuvenation transition. The variations of coordination number and atomic volume during the transition are evaluated to indicate a de-mixing tendency of the constituent atoms in the rejuvenation stage. This de-mixing tendency largely explains the difference between mechanical rejuvenation and thermal rejuvenation and reveals a competitive relationship between activation enthalpy and activation entropy in the stress-driven temperature-assisted atomic dynamics of BMG, such as diffusion and plastic deformation etc.

Mechanical Properties of Nanocrystalline Coatings Prepared by Thermal Spraying Process

The uniaxial tensile tests are generally used to measure the yield stress and/or tensile stresses. The majority of the results, resulting from this method and deferred in the literature on the mechanical properties of amorphous materials, were obtained in the ribbons form generally produced by melt spinning. This limitation in the shape of the specimens for tensile tests was inherent in the manufacturing techniques of the amorphous materials which required cooling rates of about 106°C/s. The fabrication of amorphous coatings by plasma spraying, in tubular form, should improve rigidity of the specimens, increase their size and especially get a more reliable means to the amorphous coatings under the tensile test. Amorphous coatings of NiCrBSi alloys are produced by thermal plasma spraying. The thickness of the resulting amorphous deposit may be up to 500 μm. The uniaxial tensile tests were carried on Instron machine at room temperature. Microstructural studies are investigated by means of X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopy. The test methodology used in this study shows that it is possible to obtain a comprehensive assessment of the mechanical properties of the amorphous coatings deposited by thermal plasma in tubular form. Plasma sprayed coatings in tubular form, under the uniaxial tensile test in the amorphous state, crystallized and nanocrystallized, fracture without bursting compared to samples in the form of amorphous ribbon.

Determination of the mechanical properties of amorphous materials through instrumented nanoindentation

A novel methodology based on instrumented indentation is developed to determine the mechanical properties of amorphous materials which present cohesive-frictional behaviour. The approach is based on the concept of a universal hardness equation, which results from the assumption of a characteristic indentation pressure proportional to the hardness. The actual universal hardness equation is obtained from a detailed finite element analysis of the process of sharp indentation for a very wide range of material properties, and the inverse problem (i.e. how to extract the elastic modulus, the compressive yield strength and the friction angle) from instrumented indentation is solved. The applicability and limitations of the novel approach are highlighted. Finally, the model is validated against experimental data in metallic and ceramic glasses as well as polymers, covering a wide range of amorphous materials in terms of elastic modulus, yield strength and friction angle.

Recent trends in linear and angular accelerometers

Accelerometers are usually classified as either open loop or closed loop types. They are the preferred technique of measuring and/or monitoring shock and vibration due to small size, light weight and improved dynamic response. They are commonly used in all engineering fields. Widely reported superior magnetic and mechanical properties of amorphous materials have given a fresh impetus to the development of accelerometers to measure acceleration or rate of change of acceleration both in linear and angular vibratory modes. Stress-magnetic effects in amorphous wires may be harnessed to design linear or angular accelerometers. Constructed transducer characteristics were compared to a reference accelerometer. It is shown in this investigation that rugged, inexpensive magnetostrictive accelerometers may be developed with superior low frequency characteristics.