Microscopy and Nanoindentation. Combining Orientation Imaging. to investigate localized. deformation behaviour. Felix Reinauer

Transcription

1 Combining Orientation Imaging Microscopy and Nanoindentation to investigate localized deformation behaviour Felix Reinauer René de Kloe Matt Nowell Introduction Anisotropy in crystalline materials Presentation outline Local characterisation techniques Nano-indentation Electron BackScatter Diffraction Combining EBSD and nanoindentation data Summary 2

2 Crystallographic anisotropy The strength of wood varies with the direction of the grain A similar orientation dependence is valid for many crystals Weak Orientation Strong Orientation 3 Isn t anisotropy averaged out in a polycrystalline material? Not necessarily A material will be isotropic if all of the grains have random orientations. If the grains have similar orientations then the bulk material will exhibit anisotropy similar to the constituent crystals. The distribution of crystal orientations is called texture. Most forming processes produce materials with some texture and property anisotropy. 4

3 Why is crystallographic orientation important in materials? Materials properties are mainly anisotropic. For example, graphite pencils are strong in one direction and the weak in the other to allow writing and drawing. Materials scientists can change properties by controlling orientation and anisotropy. Adding a filler into the graphite core of the pencil can make it harder. EBSD can be used to measure orientation and anisotropy to optimize the processing. hard orientation Weak orientation 5 EBSD map on brass sample Orientation might look random Anisotropy example brass 6

4 Introduction Anisotropy in crystalline materials Local characterisation techniques Nano-indentation Electron BackScatter Diffraction Presentation outline Combining EBSD and nanoindentation data Summary 7 Introduction to nanoindentation Tool to measure nanomechanical properties by pushing a small probe into the surface with controlled force The size of the probe allows positioning inside individual grains so that possible orientation or phase effect can be characterised This allows local quantitative measurement of properties like: hardness, stiffness, elastic modulus Indentation may be combined with compression, bend, and tensile tests The nanoindenter can be mounted inside the SEM chamber for direct analysis without breaking vacuum Sample is mounted on a stage that can be oriented to face any detector in the SEM chamber 8

5 Introduction to nanoindentation Shear band formation resulting from a 120 mn indentation on a bulk metallic glass (Cu 45 Zr 45 Al 10 ) using a cube corner probe 9 Nanoindentation example Regular grid of nanoindents on a multiphase material Indentation depth (at constant force) depends on phase, deformation state, crystal orientation The reduced modulus is constant in each phase The hardness shows variations inside the layers suggestion an orientation dependence multiphase structure 720 nm distance, 50 nm depth reduced modulus, GPa hardness, GPa 10 Data courtesy of Ude Hangen, PhD

6 11 12 Orientation Imaging Microscopy (OIM) Automated EBSD mapping ϕ1, Φ, ϕ2 In an OIM scan the beam is stepped across the sample surface in a regular grid. At each point the EBSP is captured and automatically indexed and the orientation and other information recorded (such as the quality of the EBSP, an indexing reliability factor, the secondary detector intensity and EDS data.) Analysis results

7 Using EBSD to confirm material performance The point-by-point orientation measurements recorded using EBSD can be used to predict and understand materials properties and performance by calculating key microstructural parameters. Here the size (width/depth) of a microhardness indent (black diamond) in various alloys varies as expected with grain size and plastic strain (Orientation Deviation) measurements. Orientation Deviation Maps 149µm 135µm 118µm Inconel µm Grains Inconel µm Grains 304 Stainless Steel 2µm Grains 13 Mechanical response maps Taylor Factor Map Indicates resistance to permanent deformation Large contrast along grain boundaries indicate potential misfit areas that will develop during deformation Elastic Moduli Map Indicates magnitude of potential deformation that will revert to the original state when the load is removed Such maps are based on model values Correlation with quantitative local mechanical data is required for a full understanding 14

8 Coupling orientation and mechanical response Combining the orientation measurements from OIM with models of how a material responds to an external force allows a prediction of material response and behavior Slip on {110 } < 1 11 > Orientation data + Deformation conditions + slip system = Taylor factor map 15 Presentation outline Introduction Anisotropy in crystalline materials Local characterisation techniques Nano-indentation Electron BackScatter Diffraction Combining EBSD and nanoindentation data Summary 16

9 Correlating OIM and Nanoindentation Step 1 Measure Orientations Titanium [0001] Step 2 Measure Hardness Step 3 Plot Hardness by Orientation [2110] _ [1010] 17 Correlated results Correlation results shown as inverse pole figures in hardness (IPFH) Good correlation of hardness with orientation Basal plane orientations harder than prismatic plane orientations 18 Fizanne-Michel et. al., Materials Science & Engineering A 613(2014)

10 Measuring plastic deformation 5mN Load 15mN Load 25mN Load Image quality maps plotted on same spatial scale with increasing indentation load Plastic deformation visualized through lower IQ values Slip bands visible Grain boundary precipitates also visible 19 Measuring plastic deformation 5mN Load 35mN Load Image quality maps of smallest and largest indent load maps. 20

11 Unit cell overlap shows 3D crystal 25mN Load orientation of indented grain Slipplanesremain parallel to Plane traces (111) show plane traces the position of the (111) planes These planes are the active slip plane for the FCC Nickel material Visualising slip systems 35mN Load Slipplanes appear parallel to indent edges 21 Value of orientation precision Having good orientation precision performance captures the true representative deformation structure without requiring data cleanup EDAX provides orientation precision values < 0.1 in the course on normal acquisition with no special mode required The effects of superior orientation precision performance difficult to observe in general orientation maps 22

12 Measuring plastic deformation 5mN Load 15mN Load 25mN Load 23 Orientation maps of indented regions Plastic deformation observed as small changes in orientation color adjacent to indent position Blue-green 16 Red-yellow 16 Brown-black 12 Measuring plastic deformation 5mN Load 35mN Load Orientation maps of indented regions Plastic deformation observed as small changes in orientation color adjacent to indent position 24

13 Interactive measurements of orientation changes 35mN Load 25 Measuring plastic deformation 5mN Load 15mN Load 25mN Load Grain reference orientation deviation (GROD) angle maps show the local plastic strain fields created by the indentations 26

14 5mN Load Measuring plastic deformation Grain reference orientation deviation (GROD) angle maps show the local plastic strain fields created by the indentations 35mN Load 27 Indents near grain boundaries allow investigation of how plastic deformation interacts with specific grain boundaries. Grain boundary effects 28 Kacher et.al., Current Opinion in Solid State and Materials Science 18 (2014)

15 Summary Coupling Orientation Imaging Microscopy and nanoindentation allows for correlation of local mechanical response with crystallographic orientation Plastic strain fields created by the nanoindentations can be visualized when measured with high orientation precision Good correlation between hardness and orientation has been shown 29 Thank you for your attention

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