Mesoscopic origin of damage nucleation in dual-phase steels

The damage modes in ferrite-martensite dual-phase (DP) steels include ferrite grain boundary (F/F) decohesion, ferrite/martensite interface (F/M) decohesion and martensite cracking. To explore the mesoscopic origin for each damage nucleation mode, we investigated ferrite-martensite dual-phase (DP) steels with different volume fractions of martensite, and characterized mesoscopic strain and stress distributions using microscopic-digital image correlation in SEM (mu-DIC) and finite element (FE) calculations. We performed in-situ tensile testing in a SEM and observed that the dominant damage nucleation mode changed from ferrite grain boundary (F/F) decohesion to ferrite/martensite interface (F/M) decohesion and finally to martensite cracking, with increasing martensite volume fraction (V-m) and varying martensite distribution. Mesoscale stress and strain analysis based on mu-DIC and FE calculations clearly reveal mesoscale origins of these damage modes: 1) F/F decohesion is caused by high strain which promotes accumulation of regular dislocations near the grain boundary and residual dislocations in the grain boundary; 2) F/M decohesion is attributed to high strain gradient which is associated with the accumulation of geometry necessary dislocations; 3) Martensite cracking stems from high stress that is partitioned to martensite. Our work demonstrated the pivot of microstructure engineering to improve the damage resistance of composite-like alloys consisting of soft and hard constituent phases, such as dual-phase steels.

铁素体 - 马氏体双相（DP）钢的损伤模式包括铁素体晶界（F/F）分离、铁素体/马氏体界面（F/M）分离以及马氏体开裂。为了探究每种损伤形核模式的细观起源，我们研究了具有不同马氏体体积分数的铁素体 - 马氏体双相（DP）钢，并利用扫描电子显微镜中的微观数字图像相关（μ - DIC）和有限元（FE）计算来表征细观应变和应力分布。我们在扫描电子显微镜中进行了原位拉伸试验，观察到随着马氏体体积分数（V - m）的增加以及马氏体分布的变化，主要的损伤形核模式从铁素体晶界（F/F）分离转变为铁素体/马氏体界面（F/M）分离，最终转变为马氏体开裂。基于μ - DIC和FE计算的细观应力和应变分析清楚地揭示了这些损伤模式的细观起源：1）F/F分离是由高应变引起的，高应变促进了晶界附近常规位错的积累以及晶界内残余位错的积累；2）F/M分离归因于高应变梯度，它与几何必需位错的积累有关；3）马氏体开裂源于分配到马氏体上的高应力。我们的工作证明了微观结构工程对于提高由软质和硬质组成相构成的类复合材料合金（如双相钢）的抗损伤能力的关键作用。