Interaction Between Precipitate Basal Plates and Tensile Twins in Magnesium Alloys
- PDF / 4,081,866 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 75 Downloads / 218 Views
TRODUCTION
WROUGHT Mg parts usually exhibit a remarkable yield stress asymmetry at room temperature (RT), with a factor 2 difference in strength not uncommon when tested in compression and tension along the same axis.[1–4] This asymmetry hinders the wider utilization of Mg in structural applications and means that the potential of Mg to greatly reduce weight and increase fuel efficiency in transport applications is not fully exploited. Mechanical asymmetry is mainly associated with the strong crystallographic texture developing in Mg during the most common processing techniques[5,6] and the polar nature of twinning, since the easy twinning mode is active only when there is an extension component parallel to the c-axis.[7] Therefore, at RT, in textured rolled sheets, where the basal planes preferentially orientate parallel to the rolling plane, when testing along the in-plane directions, extension twinning is active under compression, whereas prismatic slip is active under tension.[8] The critical resolved shear stress
P. HIDALGO-MANRIQUE, and J.D. ROBSON are with the School of Materials, University of Manchester, MSS Tower, Manchester M13 9PL, UK. Contact e-mail: [email protected] Manuscript submitted July 18, 2018. Article published online May 29, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
(CRSS) for both deformation systems is different,[9,10] which results in the above-mentioned yield stress asymmetry, with the yield stress being higher in tension than in compression. The same phenomenon is also observed in extrusions when loading along the extrusion direction, and has the same fundamental origin.[11,12] One approach to overcoming this problem is to modify (weaken) the texture by adding certain elements,[13–17] changing the conditions of classical thermomechanical processing techniques[18–20] or using more complex strain paths like equal channel angular extrusion (ECAE).[21,22] However, these approaches also add considerably to the cost of the product, either through the introduction of expensive elements (e.g., rare earths) or more complex process pathways (e.g., ECAE). Moreover, texture weakening can only lead to a reduction in asymmetry by reducing overall strength as it works by enabling yield to be controlled in all orientations by low CRSS modes (e.g., basal slip). Another way to attenuate the RT yield stress asymmetry of textured Mg consists in decreasing the CRSSprismatic/CRStwinning ratio, so that the activity of prismatic slip is enhanced at the expense of twinning. Since the sensitivity to grain size is higher for twinning than for slip,[23,24] grain refinement is one possibility. However, optimizing the processing conditions in order to attain fine enough grain sizes may be very time-consuming and not practical. Solute additions and precipitation are other possibilities as it is known that solutes[25,26] and precipitates[27–43] do not strengthen all
VOLUME 50A, AUGUST 2019—3855
the deformation mechanisms equally and indeed, solute atoms may even soften prismatic slip.[25,44]
Data Loading...
