| dc.description.abstract | ABSTRACT
This doctoral thesis aims to make a thorough investigation on the matrix microstructure of ADI, hoping to understand the relations between phase transformation and mechanical performance via different approaches. The whole thesis is composed of three parts. In Part I, the discussion focuses on the effects of austempered temperatures and alloy addition on the phase transformation and mechanical properties of alloyed (0.9%Ni-0.6%Cu) and unalloyed ADI. When treated at the low temperatureat (320℃), slim acicular ferrite plates were densely distributed in matrix, so the volume fraction of string-type austenite was higher. By contrast, at the high temperature (360℃), ferrite plates were coarse and loosely distributed in matrix, resulting in the increase of more blocky austenite. Also, the stubbed ferrite plates were coarse and dense in unalloyed ADI. Ausferrite transformed with a great nucleation rate and/or a pronounced sidewise growth, so more apparent branches of ferrite plates and large amounts of string-type austenite emerged in unalloyed ADI. String-type austenite tended to generate carbide precipitation. So the specimens at 320℃ or unalloyed ADI exhibited higher strength and hardness, but lower impact energy and elongation. But there were less long slim ferrite plates with more blocky austenite evenly distributed in the matrix of alloyed ADI. More blocky austenite implies more austenite content. During plastic deformation, blocky austenite was prone to absorb energy accompanied with the formation of stress-induced martensite transformation, i.e. the TRIP effect. So the elongation and impact energy were higher in the specimens at 360℃ or alloyed ADI. Fatigue properties are also significantly affected by the austenite content. Specimens at 360℃ or alloyed ADI developed excellent high-cycle fatigue strength attributed to large amounts of retained austenite. After painting and baking, the fatigue strength, however, decreased because of (1) the precipitation of coarse carbides and/or (2) the micro-cracks caused by the uneven diffusion of hydrogen along the interface of ferrite and austenite. In short, the alloyed ADI treated at 360℃ developed an optimum strength and elongation as well as superior impact toughness and high-cycle fatigue strength due to large amounts of dispersed blocky austenite.
In Part II, an ultrasonic vibration treatment was adopted to investigate the effect of cavitation erosion (combining the effects of micro-jet impact and shock waves) on the phase transformation and micro-hardness changes in the matrix of alloyed/unalloyed ADI at 320℃/360℃ for 2hr. Results show that the matrix temperature would gradually increase along with the ultrasonic treatment and fostered the carbon diffusion to undergo the Stage II reaction. Specimens after subjected to the ultrasonic vibration treatment might develop phase transformation induced by plastic deformation. Some string-type austenite was found to precipitate carbides, while some island-like/blocky austenite was transformed into martensite. With increased cumulative ultrasonic time, carbide particles became coarser, and martensite might further precipitate to ferrite and carbides. However, the stress-induced martenite transformation in the austenite of alloyed ADI was obviously retarded due to the addition of Ni and Cu. The stabilized austenite behaved good work hardening. Detecting the stress-induced martensite areas after ultrasonic vibration by EPMA, we found that micro-cracks were rarely seen in the blocky austenite area with higher Mn content, but they appeared in the area with lower Mn content. After the treatment, the micro-hardness values increased with the increasing treating time, while the retained austenite content revealed a declination tested by X-ray diffraction. The occasional declination of micro-hardness in unalloyed ADI might be resulted from the decomposition of martensite into ferrite and carbides.
The aim of Part III is to elucidate the influence of inclusion particles on the microstructure and mechanical properties of high strength unalloyed ADI with different contents of Mn and Si. For a given C and Si content, the total particle count decreased with the increasing Mn content (0.25-0.48%). But under a given Mn content, increasing Si content from 2.45% to 3.10% contrarily enhanced the total particle count. In addition, within the colony of eutectic cell, a Mg-deficient inclusion tended to be surrounded by ferrite ring, while the vicinity of a Mg-enriched inclusion formed acicular ferrite or ferrite lath. Inclusion particles, especially when its size is less than 5µm, were mostly found in intercellular regions. Whether an inclusion particle can induce the formation of acicular ferrite lies in Mn segregation. In intercellular region, acicular ferrite was hard to form in the vicinity of inclusion particles due to (1) serious Mn segregation, and/or (2) the Mg-enriched inclusions here in halo-like. Consequently martensite tended to form in the blocky austenite that was unevenly distributed in intercellular region after austempering treatment. Under the plastic deformation, cracks would form and accelerated their speed along the inclusions, so more fine inclusions (＜5mm) accompanied with Mn segregation caused the deterioration of fatigue strength and ductility. | en_US |