Lopez-Covaleda Edwin A, Ghodrat Sepideh, Kestens Leo A I, Sacre Charles-Henry, Pardoen Thomas
Metals Science and Technology Group, EEMMeCS Dept., Ghent University; Technologiepark 903, 9052 Gent, Belgium.
Department of Materials Science and Engineering, Delft University of Technology; Mekelweg 2, 2628 CD Delft, The Netherlands.
Materials (Basel). 2018 Jul 7;11(7):1159. doi: 10.3390/ma11071159.
Compacted graphite iron is the material of choice for engine cylinder heads of heavy-duty trucks. Compacted graphite iron provides the best possible compromise between optimum mechanical properties, compared to flake graphite iron, and optimum thermal conductivity, compared to spheroidal graphite iron. The vermicular-shaped graphite particles, however, act as stress concentrators, and, as a result of delamination from the metal matrix, they are responsible for crack initiation during the thermomechanical fatigue cycles occurring through engine startup and shutdown cycles. Scratch tests driven over the matrix and into the graphite particles were performed in order to characterize the strength of the metal⁻graphite interface. Samples extracted from a cylinder head in as-cast condition were compared to samples subjected to a heat-treatment at 700 °C for 60 h. The former samples were composed of a primarily pearlitic matrix and graphite particles (~11.5 vol %), whereas, after annealing, a certain pearlite fraction decomposed into Fe and C, producing a microstructure with graphite⁻ferrite interfaces, exhibiting a partially spiky morphology. The scratch test revealed that the ferrite⁻graphite interfaces with spiky nature exhibited a stronger resistance to delamination compared to the ferrite⁻graphite interfaces with smooth morphology. One reason for the high interface strength is the mechanical interlocking between graphite spikes and ferrite, increasing the contact area between the two phases.
蠕墨铸铁是重型卡车发动机气缸盖的首选材料。与片状石墨铸铁相比，蠕墨铸铁在最佳机械性能方面提供了尽可能好的折衷方案；与球墨铸铁相比，它具有最佳的热导率。然而，蠕虫状石墨颗粒起到了应力集中器的作用，并且由于从金属基体上分层，它们在发动机启动和关闭循环中发生的热机械疲劳循环期间导致裂纹萌生。为了表征金属-石墨界面的强度，对基体和石墨颗粒进行了划痕试验。将铸态气缸盖中提取的样品与在700℃下进行60小时热处理的样品进行比较。前者样品由主要为珠光体基体和石墨颗粒（约11.5体积%）组成，而退火后，一定比例的珠光体分解为铁和碳，产生具有石墨-铁素体界面的微观结构，呈现出部分尖刺状形态。划痕试验表明，与具有光滑形态的铁素体-石墨界面相比，具有尖刺性质的铁素体-石墨界面表现出更强的抗分层能力。界面强度高的一个原因是石墨尖刺与铁素体之间的机械互锁，增加了两相之间的接触面积。
