Warm-temperature tensile ductility in Al-Mg alloys

Abstract

Several binary and ternary Al alloys containing from 2.8 to 5.5 wt pct Mg were tested in tension at elevated temperatures (200 ЊC to 500 ЊC) over a range of strain rates (10 Ϫ4 to 2.0 s Ϫ1 ). Tensile ductilities of up to 325 pct were obtained in binary Al-Mg alloys with coarse grains deformed in the solute-drag creep regime. Under test conditions in which solute-drag creep controls deformation, Mg in concentrations from 2.8 to 5.5 wt pct neither affects tensile ductility nor influences strain-rate sensitivity or flow stress significantly. Strength is shown to increase with increasing Mg concentration, however, in the power-law-breakdown regime. The solute-drag creep process, which leads to superplastic-like elongations, is shown to have no observable grain-size dependence in a binary Al-Mg material. Ternary alloying additions of Mn and Zr are shown to decrease the strain-rate sensitivity during solute-drag creep, negatively influencing ductility. An important cause of reduced ductility in the ternary alloys during creep deformation is found to be a transition from neckingcontrolled failure in the binary alloys to cavitation-controlled failure in the ternary alloys investigated. An increase in ternary element concentration, which can increase the relative volume percentage of proeutectic products, increases cavitation.

Characteristics of the Transition from Grain-Boundary Sliding to Solute Drag Creep in Superplastic AA5083

“…These data show that the value of m decreases from a maximum 0.45 at a strain rate of~5 · 10 -4 s -1 , corresponding to deformation by GBS, to £0.25 at a strain rate of 5 · 10 -2 s -1 , and deformation by SDC. The combination of flow stress and strain rate for the highest strain rate test here corresponds to the onset of power law breakdown (PLB) for binary Al-Mg alloys, [28,[31][32][33] and this is reflected in an apparent m value <0. 25.…”

“…[24][25][26][27][28] However, the addition of Mn to Al-Mg materials has been shown to alter alloy constitutive behavior, in that ternary Al-Mg-Mn compositions exhibit lower strain-rate sensitivity coefficient values of m ' 0:25 (n ' 4) as Mn content increases above approximately 0.25 wt pct. [29][30][31][32][33] Nevertheless, the persistence of inverse stress transients following strain-rate changes in the ternary Al-Mg-Mn alloys demonstrates that they still deform by SDC. Inverse stress transients involve an abrupt stress increase upon a step increase in strain rate, followed by a subsequent decay toward a relatively steady stress at the new strain rate.…”

Achieving a Superplastic Forming Capability through Severe Plastic Deformation

“…Thus, whereas the unpressed material exhibits elongations to failure of < 200 % at all testing strain rates, the samples processed by ECAP give maximum elongations of~460 % and 500 % at strain rates of 1.0 10 ±3 s ±1 and 1.0 10 ±2 s ±1 after pressing at RT and 373 K, respectively. The low elongations attained in the unpressed material are characteristic of the enhanced but non-superplastic ductilities observed in many Al± Mg alloys [29] and the high elongations achieved after ECAP demonstrate the potential for developing superplastic characteristics in aluminum-based alloys even in the absence of Sc and Zr additions.…”

Evolution of texture and grain misorientation in an Al-Mg alloy exhibiting low-temperature superplasticity

“…(1) Can grainals to exhibit high-rate superplasticity (HRSP) and/or low boundary sliding (GBS) still proceed predominantly at a low temperature superplasticity (LTSP) by using thermomechantemperature of 250 ЊC for Al base alloys? (2) Is there a ical treatments (TMTs), equal-channel angular (ECA) extrucritical transition strain level at which the microstructure sion, multiple forging, cyclic extrusion, torsion under would transform from ill-defined subgrains, characteristic compression, or accumulative roll bonding [3][4][5][6][7][8][9][10][11][12][13][14] on commerof anisotropic slip deformation, into well-defined equiaxed cial or experimental alloys. In some cases, modified alloy grains favorable for GBS?…”

“…In some cases, modified alloy grains favorable for GBS? (3) What could be the most crucial compositions with an addition of a small amount of Fe, microstructural parameter that governs the LTSP behavior, Mn, Zr, Cr, or Sc [9,[13][14][15][16][17][18][19][20][21] have resulted in either superior or and would the coincident-site lattice (CSL) boundaries detrimental effects.…”