Ultra-Fast EBSD for Steels and Additive Manufacturing

Electron backscatter diffraction (EBSD) maps the crystal orientation of every point on a polished surface. From those maps come grain size, texture, phase fractions, grain-boundary character and local strain: the microstructural facts that explain why a weld cracked, why a printed part is anisotropic, or why one heat of steel is tougher than another.

For most of its history, EBSD has been slow. A detailed map could take hours, so users chose between resolution, area and time. Detectors built on direct electron detection change that trade-off. This article looks at what the speed means in practice, using Bruker’s published examples for its eWARP detector, part of the QUANTAX EBSD system.

Why direct detection is faster

A conventional EBSD camera turns backscattered electrons into light with a phosphor screen and then images that light. Each step loses signal and adds noise. A direct detector counts the electrons themselves on a pixelated sensor, so far more of the available signal ends up in each pattern. That lets the detector work at lower beam current, lower accelerating voltage, or much higher frame rate, or a mix of the three.

Bruker quotes up to 14,400 patterns per second at 10 kV and 12 nA, and up to 350,000 patterns per second in a binned mode used for fast forescatter and backscatter imaging.

Duplex steel in under a minute

Duplex stainless steels, widely used in chemical processing, desalination and offshore work, owe their properties to a balance of ferrite and austenite. Measuring that balance, and how each phase is distributed, is a routine EBSD task.

On its eWARP page, Bruker shows a duplex steel map of about 666,000 points collected in 47 seconds at 10 kV and 12 nA, with more than 99.5% of points indexed. A map like that once took the best part of an hour. At these speeds, phase balance can be checked on several fields or several samples in the time one used to take.

A companion note on routine low-kV EBSD mapped another duplex stainless steel in 18 seconds at the same settings, with 98.5% of points indexed before any data cleaning and 1,525 grains measured.

Resolving ultra-fine austenite

Speed also buys resolution. Retained austenite in martensitic and advanced high-strength steels transforms to martensite under load. This transformation-induced plasticity (TRIP) effect raises strength, ductility and energy absorption, so the amount, size and shape of the retained austenite matter. The grains are often only tens of nanometres wide, which needs step sizes below about 40 nm. In the past that often meant switching to transmission Kikuchi diffraction (TKD) on a thin foil.

Bruker’s note on ultra-fine retained austenite in a martensitic steel mapped a bulk sample at 10 kV with a 30 nm step, at close to 5,000 frames per second. The 2.68-megapixel map indexed more than 90% of points and measured about 86% martensite and 4.6% austenite. It counted around 1,300 austenite grains averaging about 300 nm, and resolved needle-shaped austenite only 60–90 nm wide. The phase map was shown without data cleaning.

Low accelerating voltage helps here: at 10 kV the interaction volume is smaller than at the usual 20 kV, so spatial resolution improves. Direct detection supplies enough signal to index at that lower voltage.

Additive manufacturing

Metal additive manufacturing is growing in Australia, from defence and aerospace components to medical implants and mining spares. Printed parts have microstructures quite unlike wrought or cast metal: columnar grains that grow along the build direction, strong texture, fine cellular structures, and residual strain from repeated melting. All of these affect mechanical properties and how a part responds to heat treatment.

EBSD is the natural tool for qualifying printed material, but printed microstructures are often both fine and large in extent. A map has to cover several melt pools to be representative and still resolve the fine features inside them. Fast acquisition makes that practical. Bruker’s examples include high-resolution maps of additively manufactured steel and titanium alloys (see its eWARP video on AM steel).

Results from less-than-perfect samples

Sample preparation is the usual bottleneck in EBSD. The patterns come from the top few tens of nanometres, so any polishing damage weakens them. Bruker tested this deliberately in a note on imperfect sample preparation: a roughly 400-year-old forged iron dagger, full of slag inclusions and given only a coarse mechanical polish. Compared with a conventional camera, the direct detector indexed about five times more points and acquired patterns about 24 times faster (800 versus 33 patterns per second), at a lower beam current.

Good preparation still pays off, of course. Broad-beam argon ion milling removes the damaged layer left by mechanical polishing, and we covered that workflow in our article on argon ion milling for high-resolution EBSD with the COXEM IP-10K. But a detector that copes with imperfect surfaces makes difficult materials, such as soft metals, archaeological artefacts and beam-sensitive phases, much more accessible.

Green steel and beyond

Speed opens other doors. Bruker describes EBSD of hydrogen-reduced iron ore, mapping ferrite and magnetite phases, which is relevant to Australia’s interest in low-emission iron and steel. Fast mapping also makes it realistic to follow a microstructure as it changes during heating or loading inside the SEM. We look at that in our article on in-situ tensile testing with EBSD.

For SEMs with less space or a smaller budget, including tabletop and tungsten-filament instruments, Bruker’s QUANTAX ED-XS combines EDS and EBSD in a smaller package.

If you are planning an EBSD upgrade or a new system, we can talk through your materials, your SEM and the speed and resolution you need. Contact us to start the conversation.

Application examples summarised from Bruker’s published notes, linked above. Figures are reproduced as Bruker published them, with their credits.