In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320% and 520% with a drop in fracture toughness from 7.5 to 4 MPam^1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200 °C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500 °C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries.

原位微悬臂梁断裂试验用于证明纳米结构的，高度冷拔的珠光体钢丝的断裂行为随拉伸应变和退火条件的变化。结果表明，这些钢在拉伸应变为320％至520％之间表现出明显的断裂行为转变，断裂韧性从7.5降低到4 MPam ^1/2。从断裂的性质证实了这一点，该断裂的性质在320％以下的拉伸应变时具有一定程度的可塑性，并且在420％以上的拉伸应变时变为破坏性断裂。这种转变和相关的脆性归因于这些钢在较高拉伸应变下所经历的结构性转变（胶结体分解和应变引起的四方性增加）和微观结构性（纳米晶度和位错密度增加）的演变。在对420％应变的样品进行热处理时，低温（200°C）退火会继续发生脆性断裂，微观结构无明显变化，而高温（500°C）退火则会抑制裂纹扩展并恢复大规模塑性。伴随着晶粒的粗化