Unraveling Dislocation Mediated Plasticity and Strengthening in Crack-Resistant ZnAlMg Coatings

Unraveling Dislocation Mediated Plasticity and Strengthening in Crack-Resistant ZnAlMg Coatings

Introduction

• ZnAlMg coatings are used for steel sheet protection in corrosive environments.
• Mg-alloyed zinc coatings have low cracking resistance during deformation.
• Detrimental phases and incompatible plastic deformation lead to early cracking.
• Aim: To overcome cracking by creating a microstructure with good strength and ductility.
• Previous research focused on microstructure, chemical composition, phase formation, oxidation, and corrosion.
• There is a need to thoroughly investigate ZnAlMg coatings for improved ductility and crack-resistance.
• Deformation behavior of ZnAlMg coatings is more complex than that of galvanized pure zinc (GI) coatings due to multiphase and anisotropic microstructure.
• Mechanisms of plasticity and cracking resistance improvement need understanding.
• Binary eutectic (BE) constituent (Zn and MgZn2 platelets) is a weak link, resulting in early-stage cracking.
• Primary zinc grains and ternary eutectic exhibit higher micro-ductility than binary eutectic.
• Primary zinc grains with low local strain hardening exponent (n) and low Schmid factor (m) have high cracking tendency.
• Early-stage cracks significantly change the local stress/strain state within the coating.
• This work unveils the plastic deformation performance of a tailored BE-free ZnAlMg coating.
• Deformation mechanisms, microstructural scale plasticity, strain/slip transfer, and strengthening mechanisms are thoroughly investigated.
• The study aims to show how microstructure control can result in crack hindering and improve the overall plasticity of ZnAlMg coatings on steel substrate.

Materials and Methods

• Two types of ZnAlMg coatings were produced using a hot-dip annealing simulator (HDS) on interstitial-free (IF) steel substrates.
• Reference ZnAlMg coating (1.7-2 wt.% Al, 1.7-2 wt.% Mg) containing binary eutectic (ZnMgAl-ref).
• BE-free ZnAlMg coating (2.9 wt.% Al, 1.9 wt.% Mg).
• Mean thicknesses of the coatings were measured as 13 µm and 15 µm, respectively.
• Tensile test samples were mechanically polished using 1 µm diamond suspension.
• Surface quality was improved using a JEOL IB-19520CCP ion polisher for high-quality EBSD maps.
• Surface microstructures were examined by scanning electron microscopy (SEM, Philips XL30-FEG ESEM).
• In-situ tensile tests were performed using a Kammrath & Weiss tensile module in a Tescan LYRA SEM–FIB dual beam microscope.
• Tensile samples were prepared with the same technique for precise DIC study.
• Yttria-stabilized zirconia nano-particles were sprayed on the surfaces as nanometer-sized markers for image contrast.
• Post-processing of DIC data was conducted using GOM Correlate software.
• OIM analyses were carried out before straining and after straining during in-situ tests using a Philips XL30 ESEM with an EBSD detector.
• Acceleration voltage of 30 kV and scanning step size of 150 nm were applied for high-resolution EBSD patterns.
• EDAX-TSL OIMTM Analysis 8 software was used to analyze EBSD data.
• Taylor factor maps were obtained using Zn slip systems (basal, prismatic, and pyramidal) with CRSS ratios and the deformation gradient as input into OIM Analysis software.
• STEM characterizations were conducted using a Thermo Fisher Scientific Themis ZTM scanning transmission electron microscope (S/TEM) at 300 kV.
• A 100 nm thick TEM lamella was extracted from the BE-free ZnAlMg coating after true strain of \epsilon = 0.1 using focused ion beam (FIB) in an FEI Helios G4 CXTM dual beam microscope.

Results and Discussion

Microstructure Control

• Microstructure knowledge is important for analyzing mechanical performance.
• ZnMgAl-ref coating comprises primary zinc, binary eutectic, and ternary eutectic.
• Binary eutectic consists of coarse lamellae of zinc and MgZn2 with a dendrite structure.
• Binary eutectic is identified as detrimental for compatible plastic deformation and crack initiation.
• BE-free coating consists solely of primary zinc and ternary eutectic.
• Ternary eutectic incorporates very fine grains of zinc (Zn), aluminum (Al), and ZnxMgy intermetallic.
• Binary eutectic was found less than 1% in the BE-free coating, in contrast to almost 20% in the ZnMgAl-ref coating.

Deformation and Cracking Behavior

• Interrupted in-situ SEM tensile tests were conducted to examine deformation/cracking behavior.
• Early micro-cracks nucleate in the binary eutectic of the ZnMgAl-ref coating at a strain value as low as 0.05.
• The number and size of cracks significantly increase with increasing global strain in the reference coating.
• The number and density of cracks are significantly reduced in the BE-free coating.
• Hardly any cracks with a large opening can be observed in the BE-free coating, even at high strain values.
• The presence of tiny micro-cracks (<5 µm in length) is mostly attributed to the retained binary eutectic (less than 1%).
• By eliminating the detrimental binary eutectic, the ductility of the coating is substantially improved due to very low crack density and small crack openings.
• Primary zinc and ternary eutectic exhibit a higher micro ductility and deliver an enhanced collective response in terms of plastic deformation and crack resistance compared to the reference ZnAlMg coating containing binary eutectic.
• Finer fiber-like ternary eutectic, compared to the coarse lamellar-shaped binary eutectic can accommodate the applied strain and results in additional strengthening.
• Detailed EBSD and DIC analyses were conducted on the BE-free coating to further analyze the plasticity mechanism and scrutinize the phenomena of ductility enhancement.

Identification of the Deformation Mechanisms

• Understanding the role of crystallographic orientation illuminates the mechanical behavior of a hexagonal close packed (HCP) polycrystalline metal.
• The crystallographic orientation of the ternary eutectic is normally aligned with that of the adjacent primary zinc grain.
• The coating exhibits a relatively random texture and several twins.
• All possible slip plane traces of the HCP zinc grains are defined and categorized with colored lines (i.e. basal, prismatic, 1st order pyramidal and 2nd order pyramidal).
• Identification of the activated slip systems as the function of the orientation of primary zinc grains can shed light on understanding the plasticity mechanism of the BE-free coating.
• The BE-free coating was subjected to uniaxial tensile test, and slip traces were detected by comparing EBSD results and SEM images.
• Five types of deformation systems are activated within the coating microstructure.
• Based on the von Mises criterion, the coating exhibits compatible plastic deformation because at least five independent deformation/slip systems are activated.
• Twinning is detected as the most abundant deformation event within the deformed BE-free coating.
• The dominant activation of non-basal slip systems, including 2nd order pyramidal
• By preventing the formation of early-stage cracks that can cause stress relaxation, the coating has managed to bear the plastic deformation without undergoing cracking, which boosts the probability of activation of abundant deformation mechanisms.
• Primary zinc grains surpassing the required critical resolved shear stress (CRSS) to commence the slip have accommodated the imposed plastic deformation.
• Homogenous deformation is accomplished in the BE-free coating since the primary zinc grains and ternary eutectics possess higher micro-ductility.
• It is essential to evaluate the collective response of the microstructure after deformation using an in-depth EBSD analysis.

GND and Taylor Factor Quantifications

• EBSD technique can assist in estimating the dislocation density distribution within deformed polycrystalline metals.
• [001] IPF plus image quality, geometrically necessary dislocation (GND) density map, Taylor factor map, SEM image, and GND distributions were obtained.
• GND density has been calculated based on local misorientations considering both screw and edge dislocation types on all available slip systems of HCP zinc.
• Some grains exhibit high GND density, while a few grains have low GND density, which is mostly attributed to the crystallographic orientation of the grains with respect to the applied load direction.
• The Taylor factor (M) is a useful parameter for determining the extent of microplastic deformation within the grains in a polycrystalline metal.
• The Taylor factor (M) is described as: M = \sum \Delta \gamma \Delta \epsilon
• where \Delta \gamma is the summation of the slip shears on all the available slip systems and \Delta \epsilon is the applied microscopic strain increment.
• Grains (G1 and G4) with a higher Taylor factor (close to 2.4) deliver a lower in-grain GND density, while grains (G2 and G3) with a lower Taylor factor (close to 2) deliver a higher extent of GND density.
• The in-grain GND density is found almost 4-5 times higher for G2 and G3 compared to G1 and G4.
• G1 with an orientation close to [1010] exhibits a higher Taylor factor but lacks the activation of the slip systems.

STEM Characterization

EDS Elemental Mapping

• A TEM lamella containing G3 and G4 was sliced using the FIB method.
• The ternary eutectic is composed of three elements: Zn, Al, and Mg.
• Fibrous nanostructure in the ternary eutectic is composed of Mg2Zn11 and Al in the matrix of zinc.
• Nano-sized Al precipitates are present within the primary zinc grains (G3).
• Selected area diffraction pattern (SAED) confirms that the FCC Al nano-sized precipitates are dispersed in the primary zinc matrix.
• Due to the higher Al content in the BE-free coating, Al is supersaturated in the liquid zinc, resulting in nano precipitation in the final coating.

Dislocation Density Determination

• Scanning transmission electron microscopy (STEM) has been utilized to explore dislocation characteristics in polycrystalline materials.
• Defect characteristics in polycrystalline materials delivers the possibility to capture the dislocations in a few microns field of view.
• [001] IPF combined with image quality map attained by conducting transmission electron backscatter diffraction (t-EBSD) on the same foil sample.
• Lattice orientation of each grain acquired by t-EBSD are given in the upper right corner of the BF-STEM images.
• Dislocations in the G3 and G4 primary zinc grains can be observed with good contrast in the BF-STEM images.
• G3 and G4 exhibit quite different dislocation patterns in terms of spatial density, size, and structure.
• Abundant and quite dense dislocations are revealed in G3, whereas the dislocations in G4 are sparse and mostly isolated with less density.
• Two beam condition in STEM imaging was performed by tilting the lamella to image the dislocations in G3 and G4.
• g.b (g and b are diffraction and Burgers vectors) was applied on the studied lamella to assure that the observed contrasts are attributed to the dislocations.
• In a quantitative manner, the intersection method can be used to evaluate the dislocation density using TEM (\rho{TEM}) as follows: \rho{TEM} = \frac{N}{L_{d}t}
• where N is the number of intersections of arbitrary drawn lines with the dislocations in a micrograph, L_d is the total length of these lines, and t represents TEM foil thickness.
• The average \rho_{TEM} values are found as 112 \times 10^{12} m^{-2} and 26 \times 10^{12} m^{-2} for G3 and G4, respectively.

Strengthening Mechanisms

• Additive strengthening is used to quantify the plasticity and strengthening of the coating materials.
• The contribution of each strengthening mechanism in the flow stress (\sigma{\epsilon}) can be expressed as follows: \sigma{\epsilon} = \sigma{0} + \Delta \sigma{GB} + \Delta \sigma{dis} + \Delta \sigma{pr}
• where \sigma{0}, \Delta \sigma{GB}, \Delta \sigma{dis} and \Delta \sigma{pr} are friction stress, grain boundary strengthening, dislocation strengthening, and precipitation hardening.
• \Delta \sigma{GB} can be described as the classical Hall-Petch model for the dominant zinc phase: \Delta \sigma{GB} = Kd^{-1/2}
• where K is a constant and found to be 8.5 MPa mm1/2 for Zn. d represents the grain size which is measured by the EBSD result as 13 µm on average for the Zn grains.
• The contribution of grain boundary strengthening is found to be 74.5 MPa.
• \Delta \sigma{dis} can be achieved using Taylor model: \Delta \sigma{dis} = \alpha MGb\rho^{1/2}
• where \alpha is a constant = 0.2, M is Taylor factor = 2.32, G represents the shear modulus equals to 40 GPa for Zn.
• \rho = 81 \times 10^{12} m^{-2} was attained and used in the Eq. 5. The contribution of dislocation strengthening is therefore revealed as \Delta \sigma_{dis} = 86.8 MPa.
• Orowan mechanism is employed to compute the precipitation hardening (\Delta \sigma_{pr}):

\Delta \sigma{pr} = \frac{MGb}{2\pi \bar{\lambda} \sqrt{1 - \nu}}ln \frac{dp}{r_0}

• where \lambda is the effective inter-precipitate spacing which is measured as 340 nm by using the STEM micrographs. Poisson’s ratio \nu is 0.25 for Zn. dp is the mean precipitate diameter that accounts to 72 nm and r0 is the dislocation core radius that will be assumed to be equal to the magnitude of Burgers vector (Hidalgo-Manrique et al., 2017), i.e. r0 = b = 0.52 nm.
• \
• By inserting all the known parameters into Eq. 6, \
• \Delta\sigma_{pr}
  is found to be 127.8 MPa.
• Hence, by knowing all the strengthening contributions required for the Eq. 3 and when \sigma0 = 23 MPa for Zn (Dirras et al., 2011), the flow stress \\sigma{\epsilon}
• is then calculated as 312.1 MPa.
• Precipitation hardening of Al nano-sized precipitates with a dominant contribution of 41%, followed by 28% and 24% contributions of dislocation and grain boundary strengthening mechanisms.
• The coating flow stress found by additive strengthening mechanism is quite close to the overall stress of the coating and the substrate.
• The plasticity of the coating is comparable to that of the steel substrate, which means that the coating and substrate would deform homogeneously.
• The ternary eutectic can contribute more to the total strengthening of the coating.

Grain Boundary Slip Transfer Analysis

• Grain boundary characteristics can elaborate further on the plasticity of a polycrystalline material.
• Grain boundary characteristics can be generally categorized into two different cases:
  • The impenetrable boundary
  • The transparent boundary
• Identifying the implication of the interface features within the coating microstructure can further illuminate the corresponding ductility behavior.
• Slip transfer along the grain boundaries and defined a quantitative/geometrical parameter:

m' = cos\kappa \cdot cos\psi

• where m' is slip transfer parameter, \kappa is the angle between slip directions and \psi is the angle between plane normals of the two neighboring grains.
• By implementing the EBSD data in a modified Matlab code for HCP zinc crystals, grain boundary slip transfer parameters were calculated.
• The grain boundaries slip transfer for basal slip systems is quite low, exhibiting the average value of m′ = 0.31.
• In the case of prismatic slip transfer, the boundaries also show a low average value (m′ = 0.52).
• For the sets of 1st order and 2nd order pyramidal slip systems, grain boundaries deliver pretty high m′-values (m′ =0.91 and m′ =0.87 in average, respectively), signifying transparent grain boundary behavior.
• The ductility enhancement of the binary eutectic-free coating is promoted by the easy slip transfer through grain boundaries mostly governed by 1st and 2nd order pyramidal slip transfer mechanisms.

Local Deformation Mapping and Strain Transfer by DIC/EBSD

• Full field strain mapping via DIC accompanied by EBSD analysis can cast light on the deformation state and accurate determination of the local strain distribution at the individual grain level.
• The interrelationship between crystallographic orientation and strain accumulation within the coating is assessed.
• EBSD analysis was performed on a selected region prior to the tensile test, followed by the decoration of the same region with zirconia nanoparticles for µ-DIC.
• The measured value of actual (local) strain within the coating is quite comparable to the global strain.
• The primary zinc grains carry the majority of the applied strain within the coating.
• In contrast to the reference ZnAlMg coating containing binary eutectic, the BE-free coating accommodates the applied strain without early severe localization and cracking.
• The orientation of the primary zinc grains determines the extent of plasticity and the tendency to carry the local strain.
• Grain boundary slip transfer parameter (m′) of the indicated areas are calculated.
• A high strain accumulation exists along GB1 and exhibits very high m′ values can transmit the incoming dislocations leading to easy slip and strain transfer.
• The strain is trapped within the grain and almost no significant strain transfer has occurred along the boundary and measured strain in the vicinity of the grain boundary (GB2) is found quite low.
• the findings of this work deliver insightful approaches on unravelling the mechanisms of plastic deformation and crack resistance in zinc alloy coatings.
  Proper microstructure control has been shown to induce dislocation driven plasticity and strengthening and consequently reduce undesirable crack nucleation and propagation significantly.

Conclusions

• By eliminating the detrimental binary eutectic, the ductility and formability of ZnAlMg coatings can be significantly improved.
• Slip trace analysis using correlative SEM-EBSD indicates the activation of five independent slip/twinning systems leading to compatible deformation within the BE-free ZnAlMg coating.
• Twining and pyramidal slipping are revealed as the dominant deformation mechanisms of the BE-free ZnAlMg coating.
• Non-basal activated slip systems, including 2nd order pyramidal
• It has been unveiled that the grains with the lower Taylor factor are deformed more easily by slip mechanisms generating higher dislocation densities.
• Precipitation hardening is the dominant contribution, followed by dislocation strengthening and grain boundary strengthening, respectively.
• DIC analysis denotes that primary zinc grains carry the majority of the microscopic strain compared to ternary eutectic.