| Project Team | Institution | Role | ||
| Professor Matthew Barnett | Deakin University | Chief Investigator | ||
| Dr Young Bum Chun | Monash University | Research Fellow | ||
| Dr Pavel Cizek | Deakin University | Senior Research Fellow | ||
| Mr Daniel Curtis | Monash University | Research Assistant | ||
| Assoc Professor Chris Davies | Monash University | Chief Investigator | ||
| Dr Jie Geng | Monash University | Research Fellow | ||
| Dr Ross Marceau | The University of Sydney | Research Fellow | ||
| Professor Jian-Feng Nie | Monash University | Chief Investigator | ||
| Professor Simon Ringer | The University of Sydney | Chief Investigator | ||
| Dr Gang Sha | The University of Sydney | Senior Research Fellow | ||
| Dr Nicole Stanford | Deakin University | Senior Research Fellow | ||
| Mr Zhou Xu | Monash University | Postgraduate Student | ||
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Executive Summary
Significant inroads have been made in our understanding of the effect of rare-earth elements on the texture development and deformation behaviour of magnesium alloys. We have found that only a few hundred parts per million of these elements are required to significantly modify the texture and refine the grain size of extrusions by almost an order of magnitude. The effect of rare-earth elements on the strain rate sensitivity has also been investigated, and it was found for the first time that a negative rate sensitivity can be exhibited by alloys containing Gd. Atom probe tomography confirmed that this was the result of solute segregation to dislocations, suggesting that the reason these elements have such a strong effect is that they exhibit strong segregation behaviour.
An investigation of the effect of precipitates on the mechanical behaviour of magnesium-based alloys revealed some fascinating information on the behaviour of the operative slip and twin systems. It was observed that the particle morphology can change the relative hardening between prismatic slip and twinning, and that precipitates can be selected to either increase or decrease the tension compression asymmetry.
Alloy AZ31 was found, from a series of mechanical tests, to display auxetic behaviour when highly textured and in the presence of extensive twinning.
Significant inroads have been made in our understanding of the effect of rare-earth elements on the texture development and deformation behaviour of magnesium alloys. We have found that only a few hundred parts per million of these elements are required to significantly modify the texture and refine the grain size of extrusions by almost an order of magnitude. The effect of rare-earth elements on the strain rate sensitivity has also been investigated, and it was found for the first time that a negative rate sensitivity can be exhibited by alloys containing Gd. Atom probe tomography confirmed that this was the result of solute segregation to dislocations, suggesting that the reason these elements have such a strong effect is that they exhibit strong segregation behaviour.
An investigation of the effect of precipitates on the mechanical behaviour of magnesium-based alloys revealed some fascinating information on the behaviour of the operative slip and twin systems. It was observed that the particle morphology can change the relative hardening between prismatic slip and twinning, and that precipitates can be selected to either increase or decrease the tension compression asymmetry.
Alloy AZ31 was found, from a series of mechanical tests, to display auxetic behaviour when highly textured and in the presence of extensive twinning.
Project Aims/Targets
To improve the low temperature plasticity of magnesium alloys without impairing strength so that they can occupy regions of specific strength – ductility property space traditionally occupied by titanium and aluminium. Targets include magnesium alloys that can undergo useful forming at temperatures approaching room temperature and levels of yield asymmetry >0.8. To achieve this goal the deformation mechanisms of magnesium alloys and how these mechanisms relate to the physical metallurgy of the material have been studied.
To determine the extent to which precipitate structures can be used to “harden” against twinning. This understanding will be captured in mathematical models of precipitate-twin strengthening that aid design and control.
To improve the low temperature plasticity of magnesium alloys without impairing strength so that they can occupy regions of specific strength – ductility property space traditionally occupied by titanium and aluminium. Targets include magnesium alloys that can undergo useful forming at temperatures approaching room temperature and levels of yield asymmetry >0.8. To achieve this goal the deformation mechanisms of magnesium alloys and how these mechanisms relate to the physical metallurgy of the material have been studied.
To determine the extent to which precipitate structures can be used to “harden” against twinning. This understanding will be captured in mathematical models of precipitate-twin strengthening that aid design and control.
Project Progress: Technical Details and Research Outcomes
Precipitate Interactions with Twins
The yield strength anisotropy (the ratio between compressive and tensile yield strengths of extruded products), which ranges from 0.3 to 0.6, is one of the major drawbacks preventing more widespread use of wrought Mg alloys. Such yield strength asymmetry is ascribed to the activation of twinning during compression which, however, is rarely activated in tension due to the basal texture in Mg alloys. Significant advances have been made in the understanding of the effect of precipitates on the mechanical behaviour of magnesium alloys. Our first finding on alloy Z5 showed that needle shaped precipitates (Figure A6a) can harden both the twinning and prismatic slip systems. The precipitation also produced a dramatic increase in the tensile strength of over 100 MPa, Figure A6b. This was concurrent with an increase in the tension-compression asymmetry. The yield strength anisotropy has been nearly overcome in an extruded Mg-6Gd-1Zn-0.6Zr (wt.%) alloy in which the ratio between compressive and tensile yield strengths was approximately 0.95. The twinning activities in tension and compression have been observed using EBSD ((a) and TEM (Figure A7(b) and (c)). Low yield asymmetry was achieved in the Mg-6Gd-1Zn-0.6Zr alloy even with the activation of twinning during compression, suggesting the CRSS for onset of tension twins could be increased in this alloy by means of precipitate dispersion. From these experiments it is proposed that the change in the tension-compression asymmetry after ageing is dictated by the relative hardening of the twin and prismatic systems, rather than by twinning activity alone.
Precipitate Interactions with Twins
The yield strength anisotropy (the ratio between compressive and tensile yield strengths of extruded products), which ranges from 0.3 to 0.6, is one of the major drawbacks preventing more widespread use of wrought Mg alloys. Such yield strength asymmetry is ascribed to the activation of twinning during compression which, however, is rarely activated in tension due to the basal texture in Mg alloys. Significant advances have been made in the understanding of the effect of precipitates on the mechanical behaviour of magnesium alloys. Our first finding on alloy Z5 showed that needle shaped precipitates (Figure A6a) can harden both the twinning and prismatic slip systems. The precipitation also produced a dramatic increase in the tensile strength of over 100 MPa, Figure A6b. This was concurrent with an increase in the tension-compression asymmetry. The yield strength anisotropy has been nearly overcome in an extruded Mg-6Gd-1Zn-0.6Zr (wt.%) alloy in which the ratio between compressive and tensile yield strengths was approximately 0.95. The twinning activities in tension and compression have been observed using EBSD ((a) and TEM (Figure A7(b) and (c)). Low yield asymmetry was achieved in the Mg-6Gd-1Zn-0.6Zr alloy even with the activation of twinning during compression, suggesting the CRSS for onset of tension twins could be increased in this alloy by means of precipitate dispersion. From these experiments it is proposed that the change in the tension-compression asymmetry after ageing is dictated by the relative hardening of the twin and prismatic systems, rather than by twinning activity alone.
(b) Effect of precipitation in Z5 on mechanical properties showing a large increase in tensile strength.
Texture development in wrought magnesium alloys is receiving significant attention worldwide because it is an excellent method for improving the room temperature ductility of magnesium-based alloys. An extensive study was carried out that examined four known texture modifiers: Y, Gd, La and Ce. It was found that a remarkably small concentration of these elements is required to modify the texture, only a few hundred parts per million. These small concentrations are also sufficient to reduce the grain size by almost an order of magnitude, see Figure A8. High resolution transmission electron microscopy showed extensive segregation of these elements to the grain boundaries after processing, and this segregation is thought to be largely responsible for the strong effects that rare-earth elements have on the behaviour of magnesium.
Pure magnesium is also shown as a benchmark.
Strain Rate Sensitivity
An extensive array of mechanical testing has been carried out on a group of alloys to examine their strain rate sensitivity (SRS). This parameter has received significant research attention in aluminium-based alloys because a negative SRS can significantly reduce ductility and consequently impair sheet formability. This group of experiments revealed for the first time that alloying additions may produce a negative SRS in magnesium. Despite this observation, within the regime of the negative SRS, there was no observed reduction in the ductility of the alloy. It appears from this work that the ductility of magnesium is dominated by the effects of mechanical twinning at low temperature and dynamic recrystallisation at high temperature. Therefore, even though a negative SRS can be found in magnesium alloys under certain conditions, this is not necessarily detrimental to sheet formability.
One alloy found to exhibit a negative SRS is a binary alloy containing the rare-earth element Gadolinium. Elements within the same group are known to cause interesting effects when added to magnesium, such as texture modification and improvements in ductility. Atom probe tomography on this alloy has revealed that Gadolinium segregates to dislocations, see Figure A9. A trace amount of Al was found in that particular sample, and, interestingly, this showed no segregation behaviour, so it appears that strong segregation may be unique to the rare earth elements, and those with large atomic radii.
Auxetic materials, that expand laterally when stretched or shrink laterally when compressed (Figure A10a), are of great interest both from the fundamental and the practical points of view. It was found from a series of mechanical tests conducted on AZ31 alloy that extensive twinning in highly textured AZ31 can result in auxeticity (Figure A10b). A theoretical calculation of twinning-induced strain also confirmed the likelihood of twinning-induced auxeticity. The present finding may be useful in the design of functional devices, since the extent of auxeticity can be manipulated by various (but simple) processing methods that are commonly used in conventional metallurgy.
Dashed and solid rectangles represent un-deformed and deformed states, respectively.
(b) Variation of lateral strain with compressive strain of AZ31 alloy deformed in compression.
As a result of the finding that precipitates can alter the behaviour of mechanical twins during room temperature deformation, a very focused research activity designed to fully characterize this behaviour is planned. For this work three magnesium-based alloys with divergent precipitate morphologies have been chosen for study. These will be used to produce a full yield locus describing the effect of the different precipitate types on the yield strength of the alloy. This will be used in conjunction with mathematical modelling to gain accurate values for the critical resolved shear stress for the three dominant deformation modes: basal slip, prismatic slip and twinning. It is hoped that this data will allow us to engineer the precipitate morphology so that the material properties suit the desired application, specifically in respect to the strength and tension-compression asymmetry.