(a) Compression Axis (b) ++ -- (c) ++ -- Compression Axis = 1.96 = 0.24 Measurement of Twinning Angle with Strain: New finding on slip-twin.

(a)(a)

CompressionAxis

(b)(b)+-

(c(c)) +-

CompressionAxis

= 1.96 = 0.24

Measurement of Twinning Angle with Strain:New finding on slip-twin interaction in primary Mg

Amit Ghosh, University of Michigan, DMR 0314218

Spread of Twin Angle with Strain must be accommodated by Dislocation Plasticity in the

volume between twins


•Twins undergo significant rotation due to slip

•Research relating stress direction to twin direction is likely to produce questionable results without considering rotation effects


• (a) Discovery - Fundamentals and Applications• It was first demonstrated that in coarse grain Mg alloys twinning deformation mechanism occurs

to create a very large degree of anisotropy of flow stress in different directions of the sheet, making low temperature forming of wrought Mg sheet unpredictable and unreliable as a robust engineering process.

• This finding gave a new direction for exploring alternate approaches for low temperature forming process for Mg alloys for which forming temperatures lower than superplastic temperature is sought. Low forming temperature as in warm forming minimizes oxidation problem, reduces cost and health hazard associated with surface cleaning, lowers energy and equipment costs and operator discomfort associated with high temperature forming operations.

• This new direction relied on creating ultrafine grain (UFG) size in Mg that enhances plasticity at relatively low elevated temperatures by suppressing twinning phenomenon.

• Deformation imparted to coarse grain plates by alternately corrugating in two biaxial directions (ABRC process [R3]), large plastic deformation was shown to create submicron grain size (near-nano) in a simple, automatable process capable of creating sheets or plates rather than relying on complex die process such as ECAP for grain refinement, requiring very high press loads, large friction effect, creating product that is not a sheet.

• This new discovery has forged strong industrial partnership with Thixomat, Inc. to develop a rapid manufacturing process for “nanostructuring” rapidly injected thixomolded Mg alloys to produce complex components by warm forming processed sheets for commercial and defense applications ([R13], new Patent).


• (b) Fundamental Scientific Knowledge gained• Pure compression deformation of coarse grain Mg plate increases hexagonal basal pole intensity

normal to plate irrespective of starting orientation, but repeated corrugation creates a deformation state different from compression showing texture transition during plastic deformation.

• Increase in basal pole intensity was found to be not due entirely to basal slip, rather non-basal slip is activated in a cooperative manner to increase basal pole intensity.

• Twinning mechanism has shown to operate in coarse grains but suppressed in fine grains because twin nucleation stress increases with decreasing grain size.

• A grain size dependence of twinning stress was derived from this research.• Grain refinement is more efficient with repeated changes in deformation direction due to activation of

basal slip in more grains not possible by having a single deformation axis.• Single twins and intersections of multiple twins were shown to cause subdivision of coarse grains,

but after significant amount of subdivision by twinning, twinning efficiency drops and slip intersection and recovery are more dominant in the creation of new grain boundaries.

• Below a grain size of 3 mm, twinning within the grains was found to disappear because weaker deformation mechanisms become available.

• Below a grain size of 3 mm, deformation is accommodated by dislocation slip processes, sliding/shearing near grain boundaries and soft twinning,, only near grain boundaries.

• Experimental evidence of shearing and sliding near grain boundaries of UFG Mg has been obtained in support of deformation mechanisms that are proposed.

• At room temperature, these grain boundary-based deformation processes enhance strain rate sensitivity (m) of UFG Mg from 0.008 to 0.029, the former value seen when twinning mechanism dominates.

• Higher m increases post-uniform elongation of UFG Mg in a tensile test by delaying localized thinning

• Ref. [4](a)Fig. P-1(a) UFG Mg alloy microstructure prepared by ABRC process, (b) and (c) computed grain size dependence of flow stress and m, based on a composite model of grain mantle deformation


• within diffusely necking region in sheet specimen. The mechanics of this process is well established and UFG Mg is found to follow established relationships on ductility as most other metals. Grain refinement to 1 mm and below (in the submicron size range) causes significant increase in the yield strength of AZ-31 Mg but high tensile elongation is achieved without failure.

• Grain size strengthening follows a near Hall-Petch relation but plastic anisotropy saturates at large processing strain (i.e. smaller grain size) to produce a non-Hall-Petch response.

• An important clarification is that the area-based average grain size (or volume-based average) is different from average linear intercept for size distributions encountered in Mg alloys, making plots such as Hall-Petch equivocal.

• It has been found that near room temperature, values of m in h.c.p. Mg are not much different from that of b.c.c. Fe for similar grain sizes in the UFG range determined by quasistatic tests.

• To soon appear in Mat. Sci. Eng. A [R5], it has been demonstrated that high strain rate tests (performed by certain investigators) are unsuitable for discerning deformation mechanism because those tests are adiabatic and dislocation structure and temperature cannot be maintained constant during those tests. Adiabatic thermodynamics violate infinitesimal changes inherent in the definition of m.

• An initial model for m based on grain boundary processes [R4] has been shown to predict the right range for observed m. Proposed project will build on this framework.

• Did not fail(a)(b)Fig. P-2. (a) Biaxial dome formability is improved at low forming temperature after ABRC process; (b) strengthening is improved by second phase particles in of UFG AZ91D Mg (b)

• At a slightly elevated temperature of 200°C, UFG Mg has shown m values smoothly rising to 0.3-0.45, approaching that for grain boundary sliding mechanism with tensile elongations of 300%.

• This result is remarkable as it meets the objective of warm forming of UFG Mg as a viable process at relatively low level of elevated temperature.

• Thixomolded alloys can improve on these capabilities because in addition to a primary solid phase content of 10-20%, they contain a rapidly cooled eutectic phase containing fine distribution of beta phase precipitate particles to pin grain boundaries, with combined effects of UFG and particles.

• Work on thixomolded alloys show improved strength and high efficiency of grain refinement due to the presence of the pinning particles.


• (c) Broader Impacts: Organized seminars on Mg alloys for general education of undergraduate students with Industry speakers from DaimlerChrysler, General Motors and Thixomat, Inc. increasing awareness of Mg alloys in automobiles and industry; Several presentations in National meetings, and information disseminated through DoE workshop on Mg in Dearborn, MI, October 2005; Plant visits organized for students; student internships created in Industry; UROP students exposed to technology; hosted Camp for teachers involved in K-12 education; Human Resources generated (below); Publications and patents; New company named Nanomag LLC founded; Alumni pool briefed of New Technology and Newsletter in Preparation. New knowledge regarding twinning and fine grain size is being incorporated in the 2006 edition of textbook on “Mechanical Behavior of Materials” by W.F. Hosford who is listed as co-P.I.

• Development of Human Resources• PhD degree awarded to Qi Yang, currently member of Technical Staff, Hitachi America,

Farmington Hills, MI • Post-doctoral award certificate to Yi Liu, currently Manager of Microscope Facility, Wayne State

University, Detroit, MI • M.S. degree awarded to Xiang Li, currently PhD candidate, University of Michigan• B.S. degree, awarded to David Alberts, assisted Qi Yang in his research, presently graduate

student, University of Michigan• PhD student Bilal Mansoor, grad student supported by Thixomat Inc. partner in GOALI project, in

a new project• New start up company Nanomag LLC developed based on NSF research
