顯微結(jié)構(gòu)的量化處理及組織分析的教程lecturen(1)-wenkub.com
2025-01-05 08:37 本頁面
【正文】 m with 95% confidence limits. This pares very well with the value calculated with more steps in worked example 1. Other Measures of the Size of Features The linear intercept method works well if a large enough area can be imaged. However, we are sometimes faced with the need to determine the size of a relatively small number of second phase particles or inclusions. In this case, we can make individual measurements of the particles (although it must be remembered that if the number in the sample is small, the statistical significance of the result will be low). There are a number of ways of measuring the dimensions of isolated particles, which are really all different ways of deciding what to consider the characteristic size of a particle。m. This is slightly different from the value determined in worked example 1, as the way we are summing the values and their consequent weighting is slightly different. Using Eqn. (8) gives an estimated relative standard error of (% of the measured value), which corresponds to an absolute standard error of ( 181。 10% 169 177。m )(GBLL = 181。 in fact, the method of linear intercepts cannot capture information about the range of grain sizes present. ASTM Grain Size Number The ASTM1 grain size number, g, was originally defined from the number of grains per square inch at a magnification of 100, but is now related to the number of grains per millimetre squared (NA) measured at any appropriate magnification by the equation: ? ?ANg lo g3 2 5 ??? (2) Grain Aspect Ratio In cold worked metallic materials, it is mon for grains to be deformed, such that the average grain size measured varies with the direction in which is it measured, see for example Figure 3, an extruded sample of aluminium, showing grains that are elongated along the direction of extrusion. When making grain size measurements on such samples there are two options open to us. We could replace the straight lines used in the method described above by circles, which will sample all orientations equally (for this to be valid, the circle should be sufficiently large that on the scale of the grains it approximates to a straight line, something that may be difficult to achieve). A better option is to measure the degree of elongation of the grains (as this may have an important effect on the uniformity of properties, or tell us useful information about the process that led to the grains forming in this way). We can do this by measuring the grain aspect ratio。m) Difference from sample mean ? ?2LLi? 1 20 2 26 3 25 4 21 5 25 6 25 7 26 8 19 9 21 10 23 ?Li = ? ?122???? n LLs i = L ???Li / 10 = 181。m s = Table 1 – The calculation of grain size using the linear intercept method In the case of this example, we can therefore express the mean grain size of the microstructure in Figure 1 with 95% confidence limits as being 177。 the ratio between the largest and the smallest grain size when it is measured along three orthogonal directions. The closer this value is to 1 the more equaixed are the grains. In order to determine this parameter, polished sections should be prepared along at least 2 orthogonal directions in the sample. Lines are then placed at right angles on images taken from these surfaces, and used to determine L1, L2 and L3. Figure 3 – Extruded % pure aluminium, showing the elongation of the grains in the extruded direction (horizontal in the image). Polarised light microscopy shows the individual grains in different colours. 1 ASTM – The American Society for Testing and Materials, an anisation that publishes standard methods for a wide variet
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