Views
5 years ago

2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

eactions in the MMC

eactions in the MMC samples are indicated by positive peaks and negative peaks indicate exothermic reactions. The condition of equilibration of the system could not be fulfilled as a rather fast heating rate of 20 K/min was chosen. Therefore the SDTA investigations represented a comparative study between the different preform types only. Differential temperature T T -T (°C) ref – refTsample sample / °C 0.3 0.2 0.1 0.0 -0.1 MOPC20IS endothermal AOPC20IS 200 400 600 800 159 TOPC10IS Reference temperature T ref /°C (°C) ref Figure 4.61 Differential thermal analysis on MMCs. Differential temperature as a function of reference temperature. The curve of AOPC20IS MMC showed minor fluctuations in ΔT between 100°C and 575°C which were assumed to be within the range of the scatter of the measuring system. A sharp positive peak, indicating the endothermic melting of the alloy, occurred above 575°C. The curve for MOPC20IS exhibited a marginal endothermic reaction between 100°C and 300°C, which was attributed to the decomposition of magnesium hydroxide on the surface of the sample. The hydroxide resulted from the sample preparation which was wet ground and dried prior to the SDTA analysis. The fusion peak of the non-reactive AOPC20IS was reproduced and at higher temperatures no significant trend was observable. After the fusion peak TOPC10IS showed a significant exothermic reaction starting at 650°C and finishing at 720°C. After that point, no significant reactions were indicated.

In order to determine the effect of the SDTA thermal cycle on the microstructure of the MMC, the micrographs of the MMC samples were investigated as shown in Figure 4.62 a) and b) and in Figures 4.63 a) and b). Prior to the test the samples were ground and therefore the droplet on the edge of the sample of AOPC20IS must be a result of an extrusion of the alloy during remelting (Figure 4.62 a). This behaviour was not observed in the reactive MMCs TOPC10IS and MOPC20IS. The needle-like shape of the Si phase in the alloy was similar to that in the microstructure taken prior to the SDTA cycle presented in Figure 4.53. The coarse precipitates were a result of the rather slow cooling rate of 20°C/min after the test run was finished. Figure 4.63 a) shows that after the SDTA test, TOPC10IS had a significantly different microstructure compared to the initial MMC in Figure 4.55 a). A significant fraction of porosity (black) was formed which quantitative metallography showed to be 15%. Furthermore, the colour of the former alloy areas changed from white to a light greyish colour and the former ceramic grains were subdivided into greyish and brownish phases. (a) (b) Figure 4.62 Microstructure of MMCs after thermal analysis. a) AOPC20IS overview near the edge and b) centre of the sample. 160

PDF File 2 - University of Alabama at Birmingham
no· l'2-'1t1 - Repository - Texas A&M University
eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
eA - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
MODULE 2 - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
Caroline Winstanley - eTheses Repository - University of Birmingham
pains of sin - eTheses Repository - University of Birmingham
Witch Shoes - eTheses Repository - University of Birmingham
epinician precepts - eTheses Repository - University of Birmingham
1 - eTheses Repository - University of Birmingham
Cymbeline - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
here - eTheses Repository - University of Birmingham
J - eTheses Repository - University of Birmingham
View - eTheses Repository - University of Birmingham
5% - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
Engineering texts - eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
THE GOD–MAN - eTheses Repository - University of Birmingham
ford madox brown - eTheses Repository - University of Birmingham
Life on Wings - eTheses Repository - University of Birmingham
Table of Contents - eTheses Repository - University of Birmingham
ford madox brown - eTheses Repository - University of Birmingham
attending to others - eTheses Repository - University of Birmingham
Anne Bevins 606082 - eTheses Repository - University of Birmingham
Rewriting history - eTheses Repository - University of Birmingham