
Citation: Maurizio Verri, Giovanna Guidoboni, Lorena Bociu, Riccardo Sacco. The role of structural viscoelasticity in deformable porous media with incompressibleconstituents: Applications in biomechanics[J]. Mathematical Biosciences and Engineering, 2018, 15(4): 933-959. doi: 10.3934/mbe.2018042
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Bainitic press hardening is a key production technology for advanced high strength steels, specifically in the automotive industry [1]. Structural parts like A- and B-pillars, roof rails or bumpers are produced based on this method. Currently, it is of specific interest to the industry to take this important technology to the next level in terms of process optimisation. This technique, using a combination of interrupted cooling and quasi-isothermal holding in the bainitic parameter regime, offers highly desirable strength-failure-strain combinations [2]. The possibility to control the process tool temperature offers a vast amount of possible advances in process technology, and work pieces with locally varying thermoelastic properties for a wide range of applications are one of the central goals in this context. A challenge of central importance to the bainitic press hardening process is the understanding of the bainitic transformation.
The mechanism of the bainite transformation is the subject of ongoing debate for decades, see e.g., [3,4,5,6]. The main controversy focuses on the following two arguments: (ⅰ) the nature of the bainitic ferrite/retained austenite interface and its capacity to support the displacive mechanism of bainitic ferrite growth and (ⅱ) the role of carbon transport during the bainite transformation, i.e., whether or not bainitic ferrite is initially supersaturated with carbon and then carbon partitioning takes place from the bainitic ferrite. Within this publication, we will focus on the transport-governed regime, i.e., the transformation is limited by carbon redistribution. The main branches of the scientific discussion about the details of the transport-governed regime can be summarised as follows.
Considerations about the carbon partitioning published in [7,8] suggest that the ferritic component of bainitic microstructures can form with maximal carbon supersaturation, while the escape of carbon from ferrite to austenite after transformation is the essential aspect of the carbon transport. Alternatively, the transport of carbon during diffusional transformation of austenite to ferrite under metastable equilibrium conditions at the interface is also considered (e.g., [5]). An intermediate scenario, where ferrite grows with a partial supersaturation of carbon, with the remaining carbon partitioning into austenite or forming carbides, is described e.g., in [9,10,11]. All of these hypothesises effectively consider the competition between carbon escape and interface migration at high supersaturations, thus the transformation might exhibit a higher velocity than is expected by equilibrium partitioning of carbon. For our investigations, we focus on a regime where the diffusional transport of carbon from the supersaturated austenitic phase at local equilibrium is the rate-limiting mechanism.
The following parts of this publication are organised in four sections; a basic theoretical treatment of solid-solid transitions in the displacive and the diffuse-displacive case is given in Section 2. We also introduce the basics of the boundary integral method, which is used for the investigations on the terminal subunit growth regime.
In Section 3.1, we discuss the influence of coherency stresses and capillarity stresses on the growth of banitic subunits of small radii in the initial growth regime of the diffusional-displacive transition.
In Section 3.2, we present calculations for the specific scenario of bainitic subunit growth frustration, which results form the decreasing temperature during the continuous cooling of the bainitic transformation.
In Section 4, we discuss the context of other related growth regimes also relevant to elastically influenced solid-solid transformations.
The work in this article is based on theoretical work and aims at elementary aspects of growth behaviour in bainitic subunits. As representative geometry, we present Figure 1.
The bainitic transformation is an example for diffusional or mixed diplacive-diffusional solid-solid transitions strongly influenced by elastic effects. An overview of elastical-diffusional transformations can be found e.g., in [12,13,14], in our studies reported in [15,16,17,18,19,20,21,22], we focused on transformations exhibiting coherent interfaces. Though the coupling of displacive and diffusional transitions has been investigated recently also emphasising engineering aspects, see [23], our focus in the course of this publication is on the possible dominance of elastic effects in the initial and terminal regime of subunit growth. When we refer to the terminal growth regime of a bainitic subunit, we consider the freezing-out carbon diffusion upon the continuous decrease of temperature during the progressing bainitic transformation.
Within this manuscript, we focus on two distinct scenarios. First, the initial growth regime of the subunit is considered. The corresponding model is a sharp interface model, which aims at the prediction of the geometric properties of the growing pattern. Second, we investigate the terminal regime of subunit growth, where we apply the boundary integral method. As the main part of the work is associated to the terminal growth regime, we present here the theoretical basics of the boundary integral method in the context of the physical model we assume for the terminal growth regime. As the boundary integral method is especially useful in the study of steady-state moving fronts, we focus on these steady-state scenarios, here, as systematically integrated in the non-isothermal context of the bainitic transformation. The growth predictions obtained from such steady-state scenarios can be interpreted as valid during the persistence time scale which is basically limited by the variation of the governing parameters of the process.
To introduce the boundary integral method, we focus on the diffusional aspect of the transformation. We represent the diffusion equation for a field
L[u]=∇2u+2lD∂∂yu=0, | (1) |
and use the Green's function
GL[u]−uL†[G]=∇ˉJ(u,G) | (2) |
where
For the terminal growth regime, the basis of our considerations is a coupled model of diffusional carbon transport and static elastic misfit stresses within the two adjoining phases, i.e., the parent austenite and the growing ferrite phase, together with the physically necessary boundary conditions at the progressing phase boundary. The diffusional transport is governed by a diffusion equation in the bulk, complemented by the mass conservation condition and the Gibbs-Thompson condition of local phase equilibrium. In total, we obtain the following three equations for the dimensionless carbon concentration field
D∇2u=∂u/∂t,υn=D→nΔ∇u|int,u|int=Δ−dκ+TeqδFel/LmΔc. | (3) |
By
δFel=σ(0)ikϵ(β)ik−E[(ϵ0ττ)2+(ϵ0ss)2+2ν(ϵ0ττ)(ϵ0ss)+2(1−ν)(ϵ0sτ)2]2(1−ν2). | (4) |
Here the indices
The physical picture emerging from these equations is clear. The propagation of the interface releases carbon proportional to the normal component of the growth velocity
The transformation of austenite to bainite involves displacive deformations of the matrix, diffusion controlled growth of bainitic sheaves and the precipitation of carbides inside the ferrite (lower bainite) or inside the austenite (upper bainite). A successful prediction of the transformation kinetics requires consideration of the complex interaction of these processes: elastic/plastic deformation, carbon partitioning and carbide precipitation. The local driving force for the transformation depends sensitively on the carbon concentration, the local misfit-stresses and the temperature. During the transformation, the carbon in the supersaturated bainitic-ferrite can either be released in the surrounding austenite phase or be captured by the embedded carbides, which are competitive for the solute atoms.
When we focus on the very initial regime of the solid solid transformation, involving a nucleus of nanometer size, the size dependence of the elastic parameters in this stage can lead to substantial elastic softening of the growing phase. Furthermore, on this scale, the bulk crystal symmetry structure can be suppressed due to the dominance of interfacial effects on the overall energy of the nucleus. Therefore, we approximate the growing inclusion by concerning only the hydrostatic, isotropic elastic response to stresses from the surrounding bulk phase. The assumption of isotropic elasticity exhibits the lowest expected error on the real elastic behaviour of the nucleus prior any information on the early
We assume that the new phase, which contains N particles and occupies a volume W, is homogeneous with chemical potential
W=Nvα=Nvγ+∫unds | (5) |
where
μ(P)=vα[f0γ+P+ˉσ2rr+ˉσ2ϕϕ−2νˉσrrˉσϕϕ2E+γκ] | (6) |
Here
Following the argumentation in [29] and [12], the radius
h=√ρR | (7) |
ρ=128γ(1−ν2)v2γ9πE(vα−vγ)2 | (8) |
Apparently, these expressions remain valid only when the elastic softening is pronounced and the assumption that neglecting the nonydrostatic aspect of the elastic behaviour in the inclusions is acceptable. However, within the approximative validity of these assumptions, we can consider the onset to be elastically dominated. The crucial point is the absence of any thermal quantities in
Within this section, we focus on the elasto-chemical diffusion-limited growth of a subunit in the terminal regime of carbon transport frustration, i.e., where an effective reduction of the thermodynamic driving force due to the size of the grown subunit can be assumed. The frustration of the carbon transport within the retained austenite is related to the continuous decrease of the overall temperature during the progression of the bainitic transformation. Upon a linear temperature decrease the carbon diffusion coefficient undergoes an exponential decay, as it obeys an Arrhenius-type temperature dependence. The combination of a reduced driving force and an exponentially decreasing diffusion coefficient suggest to study the zero velocity or zero diffusion limit.
To take full advantage of our assumptions, the boundary integral method, as introduced in the methods section, see Eq 3 and in [19], is most suitable. The resulting formulation of the problem combines the governing equations of the elastostatic and transport problem into a closed representation for the unknown boundary,
Δ−dκR+TeqcpδFelL2=pπ∫+∞−∞dx′exp[−p(y(x)−y(x′))]K0(pη(x,x′)). | (9) |
To simplify the representation, we introduce a splitting of the elastic contribution to the local equilibrium into a constant, hysteresis contribution
−σκ+ΔelpΦ[σϵiku(α)ik−Eϵ2B(→x)2(1−υ2)]=−1π∫+∞−∞dx′log[(x−x′)2+(y(x)−y(x′))2(x−x′)2+(−x22+x′22)2]1/2. | (10) |
Here we define
υdD=2π2σ˜Δ4, | (11) |
Rd=πσ˜Δ2. | (12) |
As mentioned, the specific scenario we apply the boundary integral formulation for is a scenario of very slow diffusion, so that the interface propagates slowly. Then, the diffusion is sufficiently low to have the elastic effects dominate the transition. In the case of very small
Therefore, we approximate this frustrated transformation regime via the limit of zero velocity and diffusion. Formally, we can reach the regime of a possibly negligible release of latent heat at the interface when
υR2D=p∼˜Δ2, | (13) |
which shows that for a given radius of the subunit, i.e., if
We introduce a representation of the problem which suits this approach, namely
−dΔelRκ+Φ[σϵiku(α)ik−Eϵ2B(x,x′)2(1−ν2)]=−1πpΔel∫+∞−∞dx′log[(x−x′)2+(y(x)−y(x′))2(x−x′)2+(−x22+x′22)2]1/2. | (14) |
Φ=2(1−ν2)Eϵ2(ˉϵ2yy+ˉϵ2zzˉϵyyˉϵ2zz) |
and the corresponding solvability parameter reads as
dΔelR=σel=1R2γ(1−ν2)Eϵ2(ˉϵ2yy+ˉϵ2zzˉϵyyˉϵ2zz) | (15) |
We note that we have introduced a solvability parameter which describes the growth regime without involving any diffusional transport parameters. This reflects the possible elastic selection on the formal level. We implemented a symmetric diffusion model instead of the one-sided model which is typically used for highly asymmetric carbon transport. Therefore, we note that the dependence of the stability parameter on the control parameter in the symmetric and one-sided model is qualitatively identical. The symmetric model implementation is numerically easier to handle in the regime of
A finite value of
R=dΔelσel∼1 | (16) |
υ=2ΔelσelπDd˜Δ2∼˜Δ2, | (17) |
where we note that the net driving force relative to the elastic hysteresis in our model is assumed to be reduced by the frustration of the transformation. A possible square-root dependence of the solvability parameter on
R∼1˜Δ, | (18) |
υ∼~Δ3. | (19) |
Despite of the inconclusive numerical results for the zero velocity limit concerning a finite
We presented our recent work on the possibilities of elastically dominated regimes in diffusion-limited solid state transformations. Referencing the bainitic transformation, specifically the scenario of sheaf growth via subunit growth into the retained austenite, the very initial and the terminal growth regime are considered. For the early growth regime, we find that under strong assumptions for the elastic softening and effective isotropy of the onsetting phase, the problem can be considered analogous to a crack formation problem. This is in line with previously published investigations on melt inclusion growth published in [29]. When the Mullins-Sekerka instability sets the transition from this early, elastically governed growth regime to the coupled diffusive-displacive growth regime, described in [15,16,18,19], the transport of carbon in the retained austenite is assumed to be the limiting factor of subunit growth. Taking into account the decrease of the carbon diffusion coefficient with decreasing temperature suggests to consider the effect of a reduced diffusion coefficient. In combination with elastically frustrated growth, i.e., reduction of the effective driving force, the reduced diffusion coefficient suggests to study a zero velocity limit. The presented numerical results show the possibility of an elastically dominated terminal growth regime of subunits.
N. T. would like to acknowledge financial support from Innovation Foundation For Postgraduate and Fundamental Research Funds of Central South University (Grant No. 1053320171074), Changsha, China. K. W. would like to acknowledge financial support from Chinese Scholarship Council (Grant No. 201706370221), China. X. Y., M. F. and C. H. acknowledge the support of the Deutsche Forschungsgemeinschaft under Project No. SPP 1713.
All authors declare no conflict of interest in this paper.
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1. | Mingxuan Lin, Carina Zimmermann, Kai Wang, Martin Hunkel, Ulrich Prahl, Robert Spatschek, Modeling Bainitic Transformations during Press Hardening, 2021, 14, 1996-1944, 654, 10.3390/ma14030654 |