Aluminum 7075 Microstructure and Current

Research through the use of In-situ X-ray Diffraction

By: Jay Schuren

Outline

• Why Al 7075?

• General Aluminum Overview

• Microstructure of 7075

• Current Diffraction Research on Al 7075

Why Al 7075?

• Aluminum is an abundant resource

• Relatively cheap

• High stiffness/density and strength/density ratios

• Damage tolerant

• Corrosion resistant compared with conventional alloys

Uses of Al 7075

• Gears and shafts

• Aircraft

• Other Aerospace and defense applications

General Aluminum Alloy Overview

Principal Aluminum Alloys

Wrought alloys are divided into seven major classes

Classes set by their principal alloy elements

• Strengthened by work hardening

– 1XXX, 3XXX, 4XXX, 5XXX

• Strengthened by heat treatment (precipitation hardening)

– 2XXX, 6XXX, and 7XXX

The seven classes can be subdivided:

Overview

• 1XXX -Commercially Pure Al.

• 3XXX - Al. Manganese Alloys

• 4XXX - Al. Silicon Alloys

• 2XXX - Al. Copper Alloys

• 6XXX - Al. Mg. Si. Alloys

• 7XXX - Al. Zinc Alloys

Work Hardened Precipitate Hardened

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7075 Microstructure

• Ingot can form (Fe,Cr)3SiAl12, Mg2Si and/or a pseudobinary eutectic made up of Al and Mg(Zn,Cu,Al)2.

• Heating causes iron rich phases to transform to Al7CuMg precipitates. Chromium is precipitated from supersaturated solution as Cr2Mg3Al18 dispersoids, concentrated heavily in the primary dendrite region.

• Recrystallized grains are extremely elongated or flattened because of dispersoid banding, and unrecrystallized regions are made up of very fine subgrains in which boundaries are decorated by hardening precipitates

7075 Microstructure

• Stable properties

• Higher strengths

• Improved corrosion resistance

• Lower rate of growth of fatigue cracks are.

Aging at elevated-temperature can provide:

Diffraction Applied to 7075

• Approach– Measure the changes in lattice spacing of the

aggregate as the specimen is under load– Use X-ray diffraction (XRD)

ApproachApproach

ßLaue (transmission) geometryto maximize statistical relevanceof diffraction volumeßLarge area detectorßNormal incidence at detector isdesiredßMonitor the azimuthal changein radius of the Debye rings

Bragg Diffraction and XRD GeometryBragg Diffraction and XRD Geometry

ßMeasure the changes in lattice spacings of the aggregateas specimen is under cyclic loading conditionßUse x-ray diffraction (XRD)

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€

λ = 2 dhkl

sin θhkl

Bragg’s Law

€

ρhkl

= D tan 2 θhkl

Geometry

Lattice strain

€

εhkl

=

dhkl

− dhkl

0

dhkl

0

Experimental Setup

ßCircumferential integration (caking) generates spectrafor azimuthal angles, ηß {An hkl} peak location from spectrum atη and recorded

{at N is compared to the samehkl}’ s peak location from spectrum at the sameη recorded at virgin state to calculate lattice strain

ß {Repeat for allhkl} s for allη at N

Data Reduction Data Reduction

Actual Al 7075 T6 DataStrain Pole Figures

Stress-Strain Curve for 7075 T6

• In-situ X-ray diffraction provides a “snap shot” of the aggregate lattice strain

• Can invert lattice strain to find full strain tensor

• Validates micromechanical models

What In-situ X-ray Diffraction gives us

References

• Aluminum: Properties and Physical Metallurgy by John Hatch• Experimental measurement of lattice strain pole figures using synchrotron

x rays by M. P. Miller• Measuring crystal lattice strains and their evolution in cyclic loading by J-

S. Park• On the mechanical behaviour of AA 7075-T6 during cyclic loading by

Turkmen• Influence of modelling variables on the distribution of lattice strains in a

deformed polycrystal, with reference to neutron diffraction experiments by Loge

• Elements of X-ray Diffraction by Cullity• http://www.sintef.no/static/mt/norlight/ProjectPortfolio/HeatTreatmentFun

damentals/dispersoids.htm• http://www.alcoa.com• http://www.msm.cam.ac.uk/phasetrans/2002/robson/img4.htm• Electrochemical Characterization of 7075 Aluminum Alloys Using The

Microcell by Barbara N. Padgett