Metallographic Analysis of PM Fracture Surfaces

Thomas F. Murphy and Bruce Lindsley

Hoeganaes Corporation 1001 Taylors Lane

Cinnaminson, NJ 08077 USA Abstract The fracture surfaces produced by breaking PM materials appear substantially different from those generated on parts made by other metalworking techniques. Although the characteristics of the fractured regions are the same, ductile dimples, cleavage, etc., the interior pore surfaces add complications to a fractographic analysis. The smooth surfaces of the inherent porosity are not only common with PM materials, they are expected. Part density, the effectiveness of sintering, and the history of the part in the green state have large effects on the microstructure and subsequently, the appearance of the broken surfaces. The fraction of pore edges, which appear as smooth free particle surfaces, varies with density, where low-density regions display large amounts of these free surfaces. Sintering causes the particles forced into intimate contact during compaction to form metallurgical bonds. Better sintering results in neck growth and a more homogeneous distribution of alloying elements. Inappropriate handling of green parts or compaction/ejection problems can create internal cracks within the parts prior to sintering. Techniques will be presented both to prepare surfaces for analysis and to quantify the appearance of fracture surfaces. Examples of PM materials broken using different types of loading will be used to demonstrate the use of these methods. In addition, an investigation using an FC-0205 material was conducted to investigate the effects of the three variables mentioned above. Test pieces were compacted to several densities and sintered using various temperatures to produce density and sinter quality variations. Further, cracks were introduced into green test pieces and examined after sintering. SEM analysis of the fractures was used in concert with light microscopy on prepared cross-sections to evaluate and quantify the resulting fracture surfaces. Introduction Materials fail when exposed to stresses greater than their characteristic strengths. Whether the stresses are tensile, compressive, impact, fatigue, etc., exceeding the material strength limits will produce failure, frequently accompanied by the creation of a fracture surface. The scale of the details on the fracture surface ranges in size from macroscopic to microscopic [1] and the microscopy techniques used to analyze these surface features are known as fractography and quantitative fractography. Quantitative fractography uses rigorously defined stereological methods to produce quantitative information on the geometric

characteristics and microstructural features contained on the exposed surfaces. [2-6] The terms in which this information is calculated include number, length, area, etc. Use of these techniques in evaluating failed parts and surfaces can be very beneficial in explaining both the response of a part or material in service to research programs where part design and material development are studied. Analyzing parts made using the PM process presents several complications. These include the inherent porosity, alloy distribution resulting from the methods used for alloying, and the microstructure, which can be heterogeneous and quite complex. In most PM parts, the vast majority of the porosity exists as a complicated interconnected network and the microstructure can be a collection of transformation products and phases, both stable and unstable at room temperature. The alloying method used in the manufacture of the powders, coupled with the part density and sintering conditions, determines the amount and location of the transformation products and how the part responds to loading. All of these variables contribute to the properties of the finished part and the appearance of a fracture surface when a part fails.

Figure 1. Epoxy skeleton of porosity showing the morphology of the pore network. Compaction is the initial manufacturing step in the production of PM parts. During compaction, particles are forced into intimate contact and the initial character of the pore network is formed. Upon sintering, the contact regions between particles form sinter necks, strengthening the compact into a coherent part. Characteristics of the base powders and compaction pressure also define the density. Figure 1 shows an epoxy skeleton of a pore network inside a PM part. The first step in forming the skeleton was to vacuum impregnate a porous iron sintered bar with epoxy. This was followed by removal of the iron from the composite using dilute acids. The epoxy shows the locations of the iron particles as the vacated areas within the pore structure and the gaps in the resin are the prior sites of metallic contact. It is clear from the shape in the central region of the figure that an iron particle occupied this space and the holes in the epoxy are contacts formed with neighboring iron particles. These contacts, or necks, can be the weakest areas in the cross-section and the eventual location of fracture. However, increasing the part density resulting in

an enlargement of the neck area and strengthening of the necks can transfer the fracture to other weaker locations in the structure as will be seen later in the paper. The alloying method also has a large effect on the response of a material under load due to the local chemical composition and hardenability. Distribution of the alloying elements is first controlled by the method used to introduce them to the iron or steel powder. Figure 2 shows a comparison of two microstructures from parts having the same overall chemical composition (4 w/o Ni, 1.5 w/o Cu, 0.5 w/o Mo, 0.6 w/o graphite), but produced from powders made using different manufacturing methods. In the image on the left, all alloying additives (except graphite) were added to the melt (prealloyed) producing a uniform chemical composition between and throughout each particle. In the right image, all metallic additives were partially alloyed during an annealing operation (diffusion-alloyed) resulting in a heterogeneous chemical composition within each particle. Because of the differences in local chemical composition and microstructure, the two materials will behave differently when loaded and will probably produce fracture surfaces very different in appearance.

Figure 2. Photomicrographs of two materials having the same chemical composition but different microstructures. This is a result of the methods used for alloying and variations in local alloy content. Experimental Procedures The combination of part density, microstructure, and applied load determine the characteristics present on the fracture surface. In this study, a mixture of a water atomized iron, copper, and graphite (FC-0205) was used as the primary example material to illustrate how some of these factors affect the appearance of the fracture surface and to demonstrate the effectiveness of the metallographic procedures. Transverse rupture strength (TRS) bars were pressed to three densities, 6.5, 6.8, and 7.1 g/cm³ and sintered at 1066 °C (1950 °F), 1093 °C (2000 °F), and 1120 °C (2050 °F) in a belt furnace at a constant belt speed of 7 cm/min under a 90 nitrogen/10 hydrogen atmosphere. This resulted in a sintering time of 15 minutes at temperature, with the time at temperature defined as a temperature within 5 °C of the intended sintering temperature. Physical and mechanical properties were measured on the bars and the broken bars were examined using both light and electron microscopy (SEM) to determine differences in the details on the fracture surface. For comparison, cracks were introduced into green bars prior to sintering and included in the visual study. Additionally, other material types were used to show the effects of several metallographic sample preparation techniques.

The main difficulty in quantifying the features on a fracture surface is very basic. In order to relate numbers to the surface and the various features on the surface, the area of the fracture surface must be known. Once the area has been measured or calculated, other quantities of interest can be determined. Many useful evaluations are performed using scanning electron microscopy; this viewed area does not, however represent the entire surface but is a projection of the surface. To overcome this problem, techniques have been devised to analyze planar, vertical cross-sections taken through the fracture surface. The profile measurements are then used to determine the actual fracture surface area. Making these measurements requires utilization of the best possible metallographic sample preparation techniques to assure accuracy in the representation of the profile. Metallographic Sample Preparation: Fracture surfaces are very delicate and require careful handling to preserve the microstructural details. [7] In some cases, this detail is the only evidence existing to explain the material behavior. Several techniques are used in the evaluation of the surfaces. They range from SEM inspection of the entire exposed surface, to the removal, preparation, and examination of vertical sections through the fracture surface. Preparation of the vertical sections requires careful handling, protection from deformation, and after preparation, clear delineation of the surface for measurement. Fortunately, once the surface has been protected from damage, normal metallographic preparation procedures can be used. Vertical sections can be removed using abrasive cut-off saws but extreme care must be exercised not to damage the surface. Wafering saws equipped with thin abrasive or diamond blades are recommended. Once sectioned, the surface can be mounted using some of the commercially available mounting powders, but with possible major drawbacks. Compression mounting materials, due to the pressure required to form the mount, introduce a risk of deformation to the fracture surface through transfer of the pressure to the fine details. In addition, the fillers in the mounting powders can agglomerate along the fine details on the fracture surface, preventing the mounting material from creating close contact with the surface. A castable epoxy is a possible alternative to the compression materials. Figure 3 shows the edge of a sponge iron based TRS bar fracture surface. This material was selected because of the larger pore-surface-to-material-volume ratio compared with a sample made with atomized iron powder. It also shows a large surface area at the fracture edge. The sample used for this image was impregnated with epoxy before sectioning with an abrasive wafering saw. It is important to note the pressure for sectioning should always be into the fracture surface rather than away from it. The impregnated epoxy appears to have filled the pore spaces and covered the fracture edge with a continuous coating. However, two possible problems are encountered using the epoxy. First, viewing the edge of the epoxy-impregnated surface could be problematic because of the transparency of the epoxy. The actual edge may be difficult to define because of the glassy appearance of the epoxy. Second, the epoxy is generally softer than the metallic samples, which may result in a rounding of the sample edge and distortion of the surface details. The best and most reliable method for protecting the sample edges appeared to be one of the electroless nickel-plating systems. [23] Obtaining correct results with these systems relies on paying close attention to procedural detail, especially in maintaining the correct plating temperature. The process involves a thorough cleaning of the sample, plating the fracture surface and the surrounding area, allowing the sample to dry, then removing the vertical section using a wafering saw. Again, the pressure for sectioning must be into the fracture surface rather than away from it. This will prevent any peeling of the plating from the surface. Figure 4 shows the results of the nickel-plating on a high temperature sintered stainless steel impact fracture. This sample was used to demonstrate the uniformity of the coating on a highly irregular surface. The protective plating is seen as the tan layer between the bright, porous metallic specimen and the dark mounting material. The coating appears to fill the details and valleys on the surface very well. Consequently, the details on the surface are maintained, while also providing a hard edge to maintain sample flatness.

Figure 3. Appearance of a sponge iron fracture surface profile impregnated using epoxy before sectioning.

Figure 4. Stainless steel impact bar fracture surface coated with the electroless nickel-plating. Quantitative Microscopy: A combination of SEM analysis on the area of the fracture and light microscopy on the prepared cross-sections provides a great deal of information into the behavior of materials. In many cases, if the fracture surface is handled with care, no additional surface preparation is

required prior to SEM examination. Determining the area fraction (AA) of the features present on the SEM image can be performed using manual or automated methods. [8-10] With the PM surfaces, a simple count using an overlay of an x/y grid can give a reliable assessment of the amount of fracture and pore structure present on the surface. It must be remembered that the SEM surface may not represent the entire, three-dimensional surface, but is a projection of the surface. Using projections for measurement requires a slight change in the symbols used for the measurement designation. The AA is changed to A′A with the prime character indicating the measurement was made on a projected image. The procedure for performing the count, a systematic manual point count, is well documented and easily accomplished. [8-10] Live images from the SEM or photomicrographs of individual fields are used as the subject material. An equally spaced x/y grid is overlaid on the image and counts are made when the x/y crossing points correspond with the features of interest. The counts are accumulated for multiple fields and this sum then divided by the total number of grid points applied to the images. The result is an estimate of the area fraction of the counted features over the total area. Multiple fields must be measured and the location of the fields chosen on a random basis or, if specific areas are in question, entire areas can be sampled and measured. The calculated value is dimensionless and is accurate for any image magnification. Figure 5 shows an example of an area from a fractured PM specimen with the overlaid x/y grid. Manipulation of the data is determined by the nature of the fracture and the literature discussing projected images should be consulted before designing a test procedure. [2,4,5,8-10] The SEM analysis also provides an opportunity to make qualitative assessment of the surfaces regarding the types of fracture present.

Figure 5. An SEM image of an FC-0205 fracture surface with an x/y grid overlay. The 48 points located at the intersections of the x and y lines are the sampling points. Analysis on the vertical sections is used to determine the roughness parameter of the profile, RL. [11] This is calculated by dividing the actual length of the fracture surface by its projected length. For the image presented in Figure 4, the length of the boundary between the nickel coating and the stainless steel sample is measured as the actual fracture length and the horizontal length of the image is the projected length. The roughness of the fracture profile is calculated using Equation 1:

RL = LA / LP (1)

where RL is the roughness parameter of the profile, LA is the actual length of the fracture, and LP is the projected length. This value is dimensionless, has a range from 1 for a flat surface, to a theoretical value approaching ∞ for an extremely irregular one, and can be used to compare surface profiles. Nevertheless, it is more frequently used in calculating the actual surface area of the fracture. Numerous methods using RL to estimate fracture surface area were developed in the 1980’s with varying degrees of success. [12-16] In 1990, an unbiased technique using a combination of the angular distribution of individual fracture profile segments with RL was devised. [17-19] In this technique, the profile of the fracture is separated from the image and divided into small segments, as seen in Figure 6b. The angle of each segment is measured and the distribution of the angles determined using 18, 10° classes. The measured value of each angle is determined with the vertical direction at 0°, as shown in Figure 6a. Each angular class is numbered, starting at class 1 (the lowest angular value) and proceeding to class 18. The fraction of each class within the total distribution is then calculated.

a. b.

Figure 6. Image ‘a’ shows the distribution of the angles used to determine the 18 class sizes and ‘b’ is a portion of a fracture edge separated into the line segments. The direction of each segment can be compared with image ‘a’ to locate the proper angular class. Equation 2 shows the expression used to calculate the roughness of the fracture surface area (RS). The value of Ψ is determined using Equation 3 and the coefficients (ai) found in Table 1. As was discussed with RL, RS is the surface of the actual fracture divided by the projected area of the fracture. The values of RS also range from 1 for a flat surface to approaching ∞ for a highly irregular surface.

RS = RL • Ψ (2)

(3) where Ψ is the profile structure factor, ai is the coefficient from Table 1, and hi is the fraction of each class in the total distribution.

Table 1. Coefficients for Calculation of the Profile Structure Factor (Ψ) as Determined by the Angular Distribution of the Fracture Segments

Class i Coefficient (a) Class i

1 1.565 18 2 1.5232 17 3 1.4508 16 4 1.3599 15 5 1.2655 14 6 1.1694 13 7 1.0906 12 8 1.0336 11 9 1.0037 10

The coefficient values are symmetrical with respect to 90°. With the minimum value for RS at 1, it follows the coefficient around the 90° angle (classes 9 and 10) is the lowest. Further, as the angles diverge from 90°, the coefficient values become larger and the RS will increase. Results Fracture surfaces from the FC-0205 TRS samples were examined using both the light and electron microscopy. Samples chosen for analysis of the cross-sectional profile were plated with the electroless nickel and sectioned perpendicular to the fracture surface, through the plane where the load was applied. Metallographic preparation was as discussed previously. No special preparation was needed for the surfaces examined using the SEM. A′A Measurements: The surfaces were analyzed using the x/y grid overlay for the amount of fracture present. [20,21] It should be noted the type of fracture was irrelevant and any fracture present was counted where appropriate. Figure 7 shows the results of the systematic point counts. As was expected, the lowest densities at each sintering temperature produced the least material fracture with the amount of fracture increasing at a relatively constant rate with the increase in density at each sintering temperature. It should be noted that, at the three densities used, the volume percent porosity for each density was approximately 17 v/o for 6.5 g/cm³, 13 v/o for the 6.8 g/cm³ samples, and 10 v/o at 7.1 g/cm³. Comparing these values with the amount of fracture measured on the projections of the fracture surfaces, the 6.5 g/cm³ values ranged from 17 to 24 area percent, the 6.8 g/cm³ from 24 to 30 area percent, and the 7.1 g/cm³ from 32 to 40 area percent. This correlates to pore area percentages on the fracture surfaces from 76 to 83 area percent at 6.5 g/cm³, 70 to 76 area percent at 6.8 g/cm³, and 60 to 68 area percent at

7.1 g/cm³. These percentages are vastly different from the volume percent porosity present in the test pieces. This discrepancy in the volume percent porosity compared with the amount of pore surface on the fracture is caused by the process used to create the fracture surface. A randomly selected 2-D plane from a sintered part shows approximately the same amount of porosity as is contained in the material volume. However, the fracture surface produced by overstressing a part is not a random surface, but one created by the relative strengths of localized areas. The areas of fracture on the projected surface represent specific weak regions in the load-bearing cross-section and not randomly selected sites in the microstructure.

0

5

10

15

20

25

30

35

40

45

6.4 6.5 6.6 6.7 6.8 6.9 7.0 7.1 7.2

Density (g/cm³)

Proj

ecte

d A

rea

Perc

ent F

ract

ure

1120 °C(2050 °F)1093 °C

(2000 °F)

1066 °C(1950 °F)

Figure 7. Results from the determination of projected area percent fracture on the fracture surfaces. The images in the left column of Figure 8 show the porosity variation in the unetched cross-sections of samples sintered at 1093 °C. The samples are shown increasing in density from top to bottom. The large pores in each image are a result of the copper particles melting and leaving behind oversize pores. Increase in densification is seen as a reduction in the small and intermediate size pores. The textured regions in the right column of Figure 8 are the fractured areas: the areas counted during the point count analyses. The increase in fracture area is apparent as the density increases, but the amount and size of the individual areas are often relatively small.

Relative porosity = 17% ρ = 6.5 g/cm³ A′A fracture = 21%

Relative porosity = 13% ρ = 6.8 g/cm³ A′A fracture = 27%

Relative porosity = 10% ρ = 7.1 g/cm³ A′A fracture = 36%

Figure 8. Unetched light microscopy cross-sections and SEM fracture surface images of the 1093 °C samples. Some of the results from the mechanical and physical testing are shown in Table 2. In addition, the relative sintered densities and projected amounts of fracture are compared. A slight reduction in density is seen between the green and sintered conditions due to melting of the copper at the intermediate and

highest sintering temperatures. This density decrease is a result of diffusion of copper into the iron matrix causing the ferrite to swell and increasing in the overall material volume. [22]

Table 2. Comparison of Mechanical Properties, Density, and Projected Area % Fracture

Sample ID

(Green Density)

Sintering Temp (°C)

Sintered Density (g/cm³)

TRS (psi x 10³)

Dim. Chg. (%)

Volume %

Density

Proj. Area %

Fracture

6.5 1066 6.45 69.9 0.21 82 17

6.5 1093 6.39 95.4 0.47 81 21

6.5 1120 6.40 101.6 0.42 81 24

6.8 1066 6.73 86.6 0.26 86 24

6.8 1093 6.65 119.5 0.54 84 27

6.8 1120 6.69 125.2 0.46 85 30

7.1 1066 7.04 109.9 0.33 89 32

7.1 1093 6.96 151.7 0.56 88 36

7.1 1120 6.97 153.3 0.63 89 40 Qualitative Analysis: SEM examination of the fractures allows for direct comparison of surfaces from samples processed under different conditions. In Figure 9, a surface from a sintered bar containing a green crack is shown. No fracture is visible after sintering at 1120 °C. Only the smooth particle surfaces are visible because all particle-to-particle contacts formed during compaction had been separated prior to sintering. In comparison, the characteristics of the fracture surfaces from bars without green cracks showed considerable change as the density and sintering temperature were varied. All samples sintered at the lowest temperature exhibited only ductile dimpling and failure through pearlite colonies. As the sintering temperature was increased, coupled with an increase in density, a substantial amount of brittle failure was observed. These changes can be seen in Figure 10 where images of two 7.1 g/cm³ bars are compared. The left image (Figure 10a) was sintered at 1066 °C and is free of any brittle fracture. Figure 10b was pressed to the same density as 10a but sintered at 1120 °C. The fracture regions in Figure 10a are entirely at the sinter necks while a large transparticle area is shown in image Figure 10b. The alloying of the copper at the higher sintering temperature has apparently strengthened the sinter neck regions and transferred the area of fracture to a weaker, transparticle area.

Figure 9. Surface of an internal green crack after sintering.

a. b.

Figure 10. 7.1 g/cm³ samples sintered at (a) 1066 °C and (b) 1120 °C showing different characteristics due to the melting and alloying of the copper, and the strengthened sinter necks.

RL and RS Determinations: Cross-sections were removed from the 7.1 g/cm³ density test pieces after nickel-plating and prepared for observation using light microscopy. An automated image analysis program was developed to perform the RL and eventually, the RS analysis by separating the fracture edge, measuring the total fracture length and the horizontal projection, and making the angular measurements of the segments of the fracture surface. Additionally, one of the samples containing the green cracks was processed and tested for comparison. Using the automated program, the entire fracture surface was sampled and measured. The results are displayed in Table 3.

Table 3. Results of the RL and RS Testing on the Sintered 7.1 g/cm³ TRS Bars and a Bar Containing Green Cracks

Sample ID RL RS

Green Crack 2.792 3.584 1066 °C (1950 °F) 2.729 3.583 1093 °C (2000 °F) 2.300 2.950 1120 °C (2050 °F) 2.727 3.451

It is interesting to note the similarity in the RS values of the green crack surface and the 1066 °C sample where, due to the low sintering temperature, the copper has not melted. Possibly, the fracture paths in both cases are similar. Fractures occur in the green condition at particle areas forced into contact through compaction, probably at the smallest contact areas. These regions would be the same or similar in the lowest temperature sintered sample because of the lack of additional alloying from the copper. Upon melting of the copper with the 1093 °C sinter, the RS value drops, indicating a flattening of the surface and less reliance of failure through the smallest neck regions. With the further increase in sintering temperature to 1120 °C, the RS value increases almost to the value of the green crack and 1066 °C samples. This may be due to the further alloying of the copper and a change in the fracture characteristics to include an increased amount of cleavage. Discussion The features seen on the fracture surfaces of PM materials are dependent on the density, microstructure, and application of the load. Techniques are available to quantify the surfaces and help explain the behavior of the materials. The amount of information can be maximized using a combination of light and electron microscopy (SEM). With the SEM analysis, the area fraction of the features on the fracture surface can be determined using proven methods and the features can be well defined with the available high magnifications. The total area of these features is the load-bearing area of the cross-section and this changes as the local composition varies with changes in processing conditions. As processing changes, local hardenability and strength may improve with a corresponding change in the eventual fracture site. The light microscopy techniques allow for measurements of profiles and quantification of profile and surface ratios. In this study, the SEM results showed an increase in the amount of fracture as a result of increases in both density and sintering temperature. The areas of fracture also become larger as the density is raised due to the enlarged size of the particle-to-particle contacts from the higher compaction pressures. Increasing the sintering temperature above the melting point of the added copper causes the liquid phase to aid sintering and strengthen the necks through diffusion of the copper into the ferrite. With the combination of higher density and sintering temperature, an increase in the amount of transparticle fracture was also observed. This indicates the transfer of the load from the previously weak neck regions to areas through the

particles. The change in the fracture characteristics is a clear indicator of a change in the load bearing areas of the cross-section due to localized alloying during sintering. The use of light microscopy added another beneficial means of looking at the fracture surfaces. Unlike SEM analysis where sample preparation may not be needed, the techniques used to prepare samples for analysis using light microscopy are extremely important. Protection of the surfaces and correct representation of the microstructure must be accomplished if the analysis is to be meaningful. With fracture surfaces, nickel-plating is a reliable and reproducible technique used to both protect the surface and provide a hard barrier to prevent edge rounding. Measurements of fracture profiles and angular distributions of profile segments can allow the calculation of the actual fracture surface area. The comparison of surface irregularity can be an indicator of how a material is strengthened and the combination of examining the cross-section in the unetched and etched condition can provide clues to how a material fails. The surface roughness values for the green-fractured sample and the low temperature sintered sample were almost identical. This may show a correlation where the small regions forced into contact during compaction are the only areas to experience sintering when the sintering temperature is below the melting point of the added copper. As the temperature is raised, the surface characteristics change along with the areas providing the strength of the compact. The areas prone to fracture prior to the introduction of the liquid copper now may not be the weakest areas and the crack profile becomes flatter, even though more fracture area is seen. This finding indicates a larger load bearing area with an increase in the overall material strength. With the further increase in sintering temperature, the surface changes again, probably attributable to the distribution of the copper and strengthening of the ferrite. These are characterized by increases in the material strength, the amount of brittle fracture, and in the total amount of load-bearing surface. Acknowledgements The authors would like to thank Mr. Paul Kremus for performing the material tests and supplying the transverse rupture bars for our metallographic evaluation. References

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