Kyushu Synchrotron Light Research Center（ Form: No.5）

Experiment Report for Prefectural Beamline
Proposal no. : 1709091P
Beamline no.: BL7
Report date

: 2018/2/20

Synchrotron diffraction of nanostructured magnesium alloy WE43
prepared via cryomilling and spark plasma sintering
Jake Stremfel 1 , Jenn-Ming Yang 1 , Katsuhiko Saito 2 , and Qixin Guo 2
1

University of California at Los Angel, USA
2
Saga University, Japan

１ ． Summary
The intermetallic phases present in the magnesium matrix were studied by synchrotron x-ray
diffraction (XRD). The obtained data has given additional information that traditional XRD
could not, which will provide insight into what other phases are present in this material. In
addition to phase identification, this data has also given us a precise measurement of the la ttice
parameters of magnesium for all the samples which shows that the lattice parameters only varyslightly.

２．Purpose of experiment and background
Magnesium (Mg) alloys have long been an area of research due to the low density and highstrength-to-weight ratio, but have yet to reach widespread adoption due its relatively low strength compared toother alloy systems such as aluminum. However, recent improvements in strength and creep resistance havebeen achieved with the addition of yttrium (Y) and rare earth (RE) elements such as neodymium (Nd) andgadolinium (Gd). One of the most popular of these commercial alloys is WE43 (4 wt. % Y, 3 wt. % RE, Mgbalance) which achieves its maximum strength from traditional casting with the additional of a T6 temper (525ºC for 8 hrs solid solution treatment + water quench + 250 ºC for 16 hrs aging). The majority of the improved
strength and creep resistance compared to traditional aluminum-based magnesium alloys is attributed toprecipitation hardening from Mg-RE intermetallic compounds. Due to the casting process and subsequent
solution treatment, small grains are not achieved in this material and therefore this material traditionally does
not see any enhanced mechanical properties due to grain refinement.
Our current research takes this WE43 alloy and applies special processing to achieve nanocrystalline
grains in order to achieve additional strength improvements. This process involves taking WE43 gas atomizedpowders (“as-received powder”) and cryomilling (mechanical ball billing in liquid nitrogen) for 8 hours

which results in a powder with a final grain size of 20 nm (“cryomilled powder”). These nanocrystalline
powders are then consolidated in a process known as spark plasma sintering (SPS) which applies pressure (100MPa) and heat (400 ºC) in the form of a pulsed DC current for ~5 min in order to achieve a bulk material(“as-SPS’ed”) while retaining most of the nanocrystalline structure.
However, due to the nanocrystalline nature of this material and unique processing route, it is not possible
to characterize the phases present in the cryomilled powder and as-SPS’ed bulk sample by traditional means. It
is unclear in what way the precipitation formation/sequence of the intermetallic compounds differs from thetraditional WE43 aging process and in what way the large grain boundary volume fraction influences this. If alarge majority of the RE elements are segregated to the grain boundaries, this could affect what precipitates canform and in what relative quantities.
The aim of this study is to use synchrotron x-ray diffraction to identify the intermetallic phases present in
the alloy as well as to measure the lattice parameter of the magnesium matrix. Identifying the intermetallic
phases of the precursor powders (as-received and cryomilled) will show the effect of the mechanicalprocessing before consolidation and whether or not any intermetallic compounds are present before
consolidation. It is hypothesized that the cryomilling process pushes the RE elements into a super saturated
solid solution due or forces them into the high volume of grain boundaries and therefore does not contain any
intermetallic compounds. Measuring the lattice parameter will help determine where the RE elements arelocated if not located in intermetallic compounds.
Additionally, identifying the intermetallic phases and measuring the lattice parameter of the magnesium
matrix after consolidation (as-SPS’ed) and subsequent heat treatment will show what intermetallic compounds
are formed and how stable these compounds are during post-consolidation heat treatments. This will also
provide information of the aging response of the material (if any) as well as the thermal stability. With thisinformation, we will be better able to predict the mechanical deformation mechanisms as well as routes for
future improvements to the mechanical properties of future alloy developments.
３．Experimental
The XRD experiments were carried out using a standard diffractometer installed at BL07. The experiments
will be conducted at incident x-rays energies of 25 keV at room temperature. Debye-Scherrer rings will berecorded on the 2D detector (PILATUS 100K) with an area of 487 × 195 pixels with a pixel size of 172 × 172µm2.

Fig 1: Schematic setup for the XRD experiment. The 2D detector (PILATUS) was used to collect thediffraction intensity in a 2θ range.

４．Results and Discussions
The data acquired was for the as-received powders (Sample 01), the cryomilled powders (Sample 02), theas-SPS’ed sample or as-compacted/bulk sample (Sample 03), and four heat treatments of the as-SPS’ed sample(Samples 04-07). All samples show six characteristic diffraction peaks from Mg, and two characteristic
diffraction peaks from magnesium oxide (MgO) with the exception of Sample 01 which does not contain theoxide. Sample 01 shows the presence of an intermetallic containing Mg and rare earth (RE) elements (Fig. 2).
Besides these three phases just mentioned, the other phases (below 10 degrees) have yet to be identified. Thesephases do not show up in traditional XRD (Cu K-alpha radiation) that we have run, so it is still unclear what
they could be. Possible candidates for these phases include other Mg-RE intermetallic compounds,
intermetallic compounds not containing Mg, or impurities from the cryomilling process. It is interesting to notethat Sample 02 contains a phase(s) which is/are not detected in any of the other samples as shown in Fig. 3.
Additionally, there seems to be a phase (around 5 degrees) that is present in all of the samples.
In addition to phase identification, this data has also given us a precise measurement of the lattice
parameters of magnesium for all the samples which shows that the lattice parameters only vary slightly. We
also can see that this material is thermally stable with peak intensities not changing, with the exception ofSample 07 which shows the first MgO peak slightly collapsing which most likely is a result of this phasecoalescing into larger grains. However, despite this, the samples show remarkable thermal stability.

Fig 2: XRD pattern for Sample 01.

Fig 3: XRD pattern for Sample 02.

５．Future issues

This synchrotron data has given additional information that traditional XRD could not, which will provide
insight into what other phases are present in this material. Additional analysis of the XRD diffraction results
will need to be performed to reveal which phases these are, as there is limited information on such phases.
６．References
７．Publications, patents
８．KeywordsMagnesium matrix, XRD, Nanocrystallin
９．About the publication of research results
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① Publication of a refereed journal article② Submission of a manuscript for Research Report(the date of publication：）(the date of submission：March 31, 2019）