Al-Mg-Mn-Sc-Zr合金的断裂韧性

Fracture toughness of Al-Mg-Mn-Sc-Zr alloy

采用标准三点弯曲试样测试了Al-4.9Mg-0.6Mn-0.2Sc-0.1Zr合金热轧板L-T和T-L取向平面应变断裂韧度K IC,并对试样断口进行SEM和TEM分析。结果表明,Al-4.9Mg-0.6Mn-0.2Sc-0.1Zr合金热轧板L-T和T-L取向K IC值分别为32.66 MPa·m1/2和32.03 MPa·m1/2,两者相差2%,表明试验合金平面应变断裂韧性各向异性差别较小。L-T和T-L取向的断口表面主要由韧窝和撕裂棱组成,属于韧窝型塑性断裂,说明试验合金韧性比较高。T-L取向断面裂纹的扩展方向与轧制方向一致,裂纹沿着第二相粒子形成的断裂通道扩展,断口上韧窝排列具有明显的方向性;而L-T取向的裂纹扩展方向垂直于轧制方向,断口韧窝排列方向性不明显。粗大的Al6Mn相作为裂纹的萌生源对断裂韧性有不利影响,微米级Al6Mn相粒子也可以形成微裂纹,而纳米级Al3Sc相粒子通过钉扎位错和晶界可以稳定材料微观结构,减小亚晶界前沿位错堆积密度,故可以改善试验合金热轧板的断裂韧性。

Plane strain fracture toughness Ktc of Al-4.9Mg-0.6Mn-0.2Sc-0.1Zr alloy hot-rolled plate was measured by three points bending samples in L-T and T-L directions. Fracture morphology of the alloy was analyzed by SEM and TEM. The results show that the values of Klc in L-T and T-L directions are 32.66 MPa . m^1/2 and 32.03 MPa . m^1/2 respectively, which have a discrepancy of 2%, so the plane strain fracture toughness Klc of the alloy hot-rolled plate has weak anisotropy. The facture surfaces in L-T and T-L directions are composed by dimples and tear ridges. Dimple-type plastic fracture mode shows relatively high toughness of the alloy. Crack growth direction in T-L direction parallels to the rolling direction, and crack growths along fracture channel is formed by second-phase particles, so dimples arrangement on the fracture surface has obvious directionality. Crack growth direction in L-T direction is perpendicular to the rolling direction, so directionality of dimples arrangement in L-T direction is weak. Coarse Al6Mn as crack origins is detrimental for the fracture toughness. Micro-meter AI6Mn particles can cause micro-cracks as well. Nano-meter Al3Sc particles stabilize sub-structures by pinning dislocations and grain boundaries and reduce the dislocation density in front of the sub-grain boundaries, so the fracture toughness of the alloy hot-rolled plate is improved.