Thomas Antretter

Size effects on martensitic phase transformations in nanocrystalline NiTi shape memory alloys

Abstract Results of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The nanograins significantly impact the morphology of B19′ martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the nanograins.

Martensitic phase transformations of nanocrystalline NiTi shape memory alloys processed by repeated cold rolling

Abstract The impact of grain size on the martensitic phase transformations of bulk nanocrystalline NiTi shape memory alloys processed by repeated cold rolling was systematically studied by differential scanning calorimetry and transmission electron microscopy. With decreasing grain size, the formation of the martensite is strongly suppressed and its thermal stability decreases. The effect of grain size on the intermediate R-phase is much smaller than that observed in the case of the martensite. Reversible and irreversible contributions to the Gibbs free energy of the martensite were obtained that are larger than those arising from the formation of martensite in coarse grains. Considering the dependence of the energy barrier on the transformation eigenstrain and the grain size, the experimental results were modelled within the general thermodynamic framework of the martensitic phase transformation.

Simulation of the Roller Straightening Process with Respect to Residual Stresses and the Curvature Trend

The roller straightening process is a common method for straightening long products like beams after rolling and cooling. This process often causes an adverse residual stress state. All previous investigations operate only with the roller adjustment as correcting variable. However, this cannot properly describe the consequences on the cross section of the beam during bending. The present paper presents a concept to consider the development of curvature during the straightening process. In finite element analyses using Abaqus/Standard a beam with a rectangular cross section and simplified material properties is modeled for a fundamental and clear demonstration. The theo-retically determined residual stress state depends on the development of curvature during straight-ening. Vice versa it is possible to design a trend of curvature with the goal of tailoring the final re-sidual stress distribution to the desired optimum. The necessary roller adjustment is found in a simulation using the Abaqus user subroutine UAMP, where the curvature is permanently tuned by controlling the roller adjustment. The residual stress state resulting from these 1D considerations (“the theoretical stress state”) is verified in a subsequent 2D analysis giving the “actual stress state”. A comparison of the theoretical and the actual residual stress state illustrates the influence of the roller contact. The concept presented in this work can be applied to complex cross sections in com-bination with realistic material properties. However, for this purpose a large amount of calculation resources due to an extensive 3D modeling are needed.

Experimental characterization and modelling of triaxial residual stresses in straightened railway rails

Residual stresses in railway rails have a significant influence on the rail functional properties and reliability in service life. Already during the production, the roller straightening as the final production step removing the rail curvature causes the formation of complex stress fields. In this work, a complementary experimental characterization of longitudinal, transversal and normal residual stresses in an uncut straightened rail with a length of 0.5 m is performed using neutron diffraction and contour method. Additionally, the residual stresses are predicted numerically by means of an extensive three-dimensional finite element simulation taking into account the cyclic elastic–plastic material behaviour of the rail including combined kinematic–isotropic hardening. The very good agreement between the experimental and numerical data provides a basis for the understanding and predicting how the straightening procedure, that is, the positioning of the individual rollers and forces applied by the rollers, influences the triaxial stress fields at the rail cross-section.

Cyclic mechanical behavior of thin layers of copper: A theoretical and numerical study

In printed circuit boards, thin copper layers are used as current paths. During the thermal loading of printed circuit boards, stresses arise due to the different coefficients of thermal expansion of the used materials. To be able to model the mechanical behavior of printed circuit boards under cyclic thermal loads, cyclic mechanical tests of thin copper foils under changing tensile and compression loads at different temperatures were conducted. From these experiments, the isotropic and kinematic hardening parameters were determined serving as material input data for a nonlinear isotropic/kinematic hardening model in the finite element analysis-software Abaqus. The kinematic hardening parameters were fitted in an optimization process. The isotropic hardening variables were determined based on the stress versus plastic strain relationship that was constructed incrementally from the available individual cycles. The so-obtained curve was found to be not unique, but to depend on the loading situation. Hence, different approaches for strain range memorization were evaluated. Since these approaches were developed for modeling strain-controlled tests, whereas the experimental data were obtained in a force-controlled way, a phenomenological formulation was developed and applied. The results of curvature measurements during thermal cycling were used for model validation. The experimental results and the numerical predictions are in good agreement.

Calibration and Validation of an Elasto-Viscoplastic Material Model for a Hot Work Tool Steel Used in Pressure Casting Dies

Pressure casting dies are subjected to a large number of thermal as well as mechanical load cycles, which are leading to a characteristic thermally induced crack network on the die surface. As a typical representative for a die material the cyclic thermo-mechanical behavior of the hot work tool steel grade 1.2343 (X38CrMoV5-1) is investigated both experimentally as well as numerically. On the one hand the information from isothermal compression-tension tests is used in a subsequent analysis to calibrate a constitutive model that takes into account the characteristic combined isotropic-kinematic hardening/softening of the material. On the other hand the non-isothermal mechanical response of the material to thermal cycles is characterized by means of a periodic laser pulse applied to a small plate-like specimen which is cooled on the back. The residual stresses developing at the surface of the irradiated region of the specimen are determined ex-situ by means of X-ray diffraction. The obtained values agree well with the results of an accompanying finite-element study. This information is used to verify the calibrated constitutive model. The material law is finally used for the prediction of stresses and strains in a die.

Size Effects in Residual Stress Formation during Quenching of Cylinders Made of Hot-Work Tool Steel

The present work investigates the residual stress formation and the evolution of phase fractions during the quenching process of cylindrical specimens of different sizes. The cylinders are made of hot-work tool steel grade X36CrMoV5-1. A phase transformation kinetic model in combination with a thermomechanical model is used to describe the quenching process. Two phase transformations are considered for developing a modelling scheme: the austenite-to-martensite transformation and the austenite-to-bainite transformation. The focus lies on the complex austenite-to-bainite transformation which can be observed at low cooling rates. For an appropriate description of the phase transformation behaviour nucleation and growth of bainite are taken into account. The thermomechanical model contains thermophysical data and flow curves for each phase. Transformation induced plasticity (TRIP) is modelled by considering phase dependent Greenwood-Johnson parameters for martensite and bainite, respectively. The influence of component size on residual stress formation is investigated by the finite element package Abaqus. Finally, for one cylinder size the simulation results are validated by X-ray stress measurements.

A multi-block-spin approach for martensitic phase transformation based on statistical physics

Current strategies in modeling shape memory alloy (SMA) behavior follow either the concept of classical irreversible thermodynamics or the methodology of phenomenological approaches at the micro as well as at the macro space scale. The objective of the present study is to show a new approach in modeling SMAs by using a statistical physics concept without the requirement of evolution equations for internal variables. Thermodynamic principles in connection with the mathematical apparatus of statistical physics allow deriving relevant system properties in analogy to the formalism used for paramagnetic-ferromagnetic systems. As a result the macroscopic strains and the volume fractions of the martensitic variants and their rates are obtained. The multi-block-spin approach further maps the tension compression asymmetry of multivariant SMAs.

Interaction of Heat Checks in Aluminum Pressure Casting Dies and their Effect on Fatigue Life

Pressure casting dies are exposed to harsh service conditions consisting of cyclic thermal and mechanical loading and thus undergo thermo-mechanical fatigue. Due to cyclic plastic deformation of the material near the surface of the dies the loading conditions gradually change because of the formation of tensile residual stresses which add to the stress field from external loading. This change in the stress field influences the nucleation and the growth of cracks. Typically after a few thousand casting cycles a network of heat checks forms. In such a network crack shielding has a big influence on the evolution of the crack array. Firstly, it influences the propagation rates of the cracks and secondly it may change the propagation direction compared to the case where no neighbors are present. The crack growth rate and the length at which the cracks stop growing are also influenced by the thermo-physical and mechanical properties of the die material. It was found that the shielding effects of neighboring cracks are of equal importance. Crack deflection caused by the presence of neighboring cracks can lead to break-outs at the surface ensued by fast degradation eventually necessitating the replacement of the die. Consequently, the focus in this work is put on the investigation of the interaction of cracks in a network and their effect on the fatigue life. The problem is tackled by means of an automated strategy based on the finite element method.

Experimental validation of microstructure evolution in crystalline materials

The influence of initial grain orientation and geometrical restrictions (grain boundaries, sample geometry) on the microstructural evolution was investigated at ambient temperature. Single- and polycrystalline austenitic steel samples, strained under uniaxial tension, show pronounced orientation changes which are strongly dependent on initial orientation. The numerical simulation results from a crystal plasticity FEM model match the experimental ones.