Microstructural Evolution of W-Ni-Fe During Liquid Phase Sintering A Quenching Study

Transcription

1 Microstructural Evolution of W-Ni-Fe During Liquid Phase Sintering A Quenching Study N.B. Erhardt*, P. Suri, R.M. German, Center for Innovative Sintered Products, 147 Research West, The Pennsylvania State University, University Park, PA Abstract Densification and subsequent distortion of tungsten heavy alloys is investigated through microstructural evolution. In this experiment, samples are quenched to identify and isolate the events leading to distortion during liquid phase sintering. Samples of - 8.4Ni-3.6Fe, and -4.9Ni-2.1Fe (maintaining a 7:3 Ni: Fe ratio), are quenched from different temperatures ranging from 1440 C (2624 F) to 1500 C (2732 F) and after hold times of up to 3utes at 1500 C (2732 F). The samples began to distort after hold times longer than two minutes at 1500 C (2732 F). The samples did not distort from the thermal processing. Introduction Liquid phase sintering (LPS) is frequently used as a net-shape processing method. [1] Tungsten heavy alloy (WHA) components can be manufactured by LPS, and such products are found in a variety of applications including heat sinks for electronics, kinetic energy penetrators, radiation shielding, and vibrating masses for cell phones. [2] LPS compacts solid state sinter before the liquid forms. During sintering, solid-solid bonds form between the grains, giving compact rigidity. Once the liquid forms, the necks between the higher melting phase particles partially dissolve. Distortion can occur if the solid is soluble in the newly formed liquid, although that is not the only condition. Distortion will depend on a number of factors including solid volume fraction, connectivity, contiguity, and porosity just to name a few. The degree of solid state sintering that occurs prior to liquid formation will also play a role in how much a sample does or does not distort. Liquid phase sintering is also influenced by gravity. This is most pronounced when there is a density difference between the solid and liquid phase. In the case of tungsten heavy alloys, the tungsten grains (density of 19.3 g/cm 3 ) settle through the Ni-Fe liquid matrix (density ~8.6 g/cm 3 ) towards the bottom of the compact under the force of gravity (just like rocks in a pond). Since the difference in density between the Ni-Fe matrix and the tungsten grains is about 10 g/cm³, the settling grains produce a change in connectivity and contiguity from the bottom to the top of the sample. The goal of this study is to measure and analyze the trends in density, grain size, contiguity, and connectivity during initial sintering of and samples. The

2 desired result is to capture a snapshot of the microstructure as the compacts sinter. Since the emphasis is on capturing the instantaneous microstructure, a quenching method was the employed. Experimental Procedures Elemental powders of tungsten, iron, and nickel were mixed to give compacts with 88 and 93 wt% W while maintaining a 7:3 Ni:Fe ratio. That ratio of nickel to iron was chosen to avoid forming intermetallics. The tungsten powder used in the mixture was rod milled dry for 1 hour in a 2000 cm 3 plastic jar at 90 rpm using >95% pure tungsten rods with a diameter of 10 mm and 180 mm in length. The weight ratio of rods to powder was 10:1. An argon atmosphere was utilized during milling to prevent oxidation of the tungsten. The elemental powders were combined by weight to give the desired compositions, along with 0.5wt.% ethylene bis-stearamid added as a lubricant. The elemental powders and ethylene bis-stearamid were subsequently blended in a Turbula mixer for 2utes to ensure homogeneity. The mixed powders were hand pressed on a Carver press to a pressure of 175 MPa. The cylindrical compacts measured 12.5 mm in diameter and varied in height from 10 to 12 mm. The green compacts were near 60% of the theoretical density. Debinding and presintering were accomplished in one step. The samples were heated in a CM retort furnace to 500 C using a heating rate of 5 C/min for 1 hour and subsequently heated to 1000 C at 10 C/min for 1 hour. The process gas was hydrogen (dew point of 67 C) for the debinding and presintering cycle. The samples were sintered in a vertical CM furnace. The geometry of the furnace allowed the sintered compacts to be quenched at the specified time. The samples were heated at 10 C/min up to the desired quench temperature. The samples were heated at 10 C/min up to 1400 C, and then the heating rate was reduced to 5 C/min to ensure temperature uniformity. The samples were sintered in a molybdenum crucible suspended in the hot zone of the furnace with a tungsten wire. Upon reaching the desired temperature and hold time, the entire crucible was quenched in water to capture the instantaneous microstructure. For this set of quenching experiments, the temperatures and isothermal hold times were as follows: Table 1: Quench temperatures and hold times for the samples. Temp( C) Time (min) To examine the microstructure, the cylindrical samples were cut in half along the vertical axis and mounted in a thermoset for ease of handling. The samples were then polished to achieve the necessary surface finish. The last polishing step used colloidal silica with an approximate grain size of 0.04µm. Optical micrographs were taken of the polished surface to obtain a minimum of 400 grains per sample. Using IMAGIST II image analysis software by Princeton Gamma-Tech, the micrographs were filtered to discriminate the grains from the non-tungsten phases and rendered as binary images.

3 This result is in an image divided into two parts: tungsten grains and the Ni-Fe (and dissolved tungsten) matrix. The grain size data was acquired by determining the area of a grain, then by assuming a spherical grain, the equivalent diameter was calculated. Density measurements were taken using Archimedes method. The connectivity was measured using the following method. Consider one grain from an optical micrograph, and count the number of grains that are in contact with it in the two dimensional cross-section of the micrograph (this is similar to counting the four white squares around a black one on a checkerboard). Repeat this process for each grain to be considered. A minimum of 400 grains were used for all microstructural measurements. The two-dimensional connectivity reported is an average of those values. To measure the contiguity, a square grid was overlaid onto an optical micrograph. Tracing one line (vertical or horizontal) of the grid, count the number of times it intersects solid liquid interfaces, N SL, and solid-solid interfaces, N SS. Repeat for the next gridline, until either all vertical or horizontal gridlines have been examined. The contiguity, C g, was calculated using Equation 1. C g 2N SS 2 N N SS SL (1) Results and Discussion Figure 1 displays the evolution of the average grain size with sintering temperature and isothermal hold time at 1500 C. It can be seen for and that the grain size increases throughout the sintering cycle; a change in grain growth can be seen once the samples reach 1500 C Grain Size (µm) C 1480 C 1490 C 1500 C 1500 C C C C Figure 1: Plot of average grain size with sintering temperature and isothermal hold time up to 1500 C.

4 The higher volume fraction of liquid phase in the during sintering contributes to the initiation of solution-reprecipitation grain growth kinetics earlier in the sintering cycle when compared to. This can be seen in Figure 1 as the average grain growth evolves to the increased rate associated with solution-reprecipitation after a hold time of two minutes at 1500 C, while the does not reach this higher rate until a hold time of ten minutes at 1500 C. A decrease in the average grain size is noted for the samples which were quenched after a two and five minute hold time at 1500 C. A possible explanation for this is the tungsten is quickly dissolving into the Ni-Fe matrix once the liquid forms. The curvature of the tungsten grains is the driving force for this event. Elemental powders were used to make the samples, and the nickel and iron powders began with zero tungsten content. Once the nickel and iron melt to form the liquid matrix, there is an increased solubility for the tungsten in the matrix. And the formation of the liquid phase provides a rapid transport path allowing the tungsten to diffuse more rapidly between the grains. The amount of tungsten we would expect to find in the Ni-Fe matrix at 1500 C under equilibrium conditions is about 19 wt%. [7] This results in the tungsten grains dissolving into the liquid phase, giving a decrease in the average grain size. It has been reported that the matrix phase becomes supersaturated with tungsten soon after liquid forms, and it can take several minutes for the amount of tungsten in the liquid phase to reach equilibrium. [7] Figure 2 shows the density evolution with temperature and time. Both the and sintered to full density in this set of experiments. It can be seen that the samples reached full density near 1465 C, which is the temperature liquid was observed to form. 100 Theoretical Density, % C C C C C C Figure 2: Plot of the fractional density with temperature and hold time.

5 The density began to decrease after a two minute hold at 1500 C, corresponding to sample distortion. Upon sectioning the distorted samples, large internal pores were discovered, which accounts for the decrease in density. The compacts, however, did not distort during this thermal processing cycle. The sample density did not decay once it was near fully dense, but gradually increased to 100% dense. By inspecting the contiguity (Figure 3), a distinct difference in the behavior between the and can be seen. As the liquid formed, the necks created during solid state sintering partially dissolve leading to a decline in the contiguity in both samples. [3-6] This event is especially pronounced in the sample after a two minute hold at 1500 C. Upon liquid formation, the tungsten will preferentially dissolve at the necks due to the driving force associated with the higher curvature. The necks have the highest curvature, and thus the highest driving force. For the same reason, small grains will dissolve faster than the larger ones. The partial dissolution of the necks will result in a decrease in the contiguity. This can be seen as a gradual effect in the samples from 1475 C to 1500 C. Note that the samples reach the equilibrium value for contiguity immediately after the decrease. Also, once liquid phase sintering had begun, the samples always had a higher contiguity Contiguity C 1480 C 1490 C 1500 C 1500 C C C C Figure 3: Plot of contiguity with sintering temperature and time. The next point of interest on the contiguity plot for occurs at 1500 C after a two minute hold and is an increase in the contiguity. This coincides with the onset of distortion. As the compacts distort into the elephant foot shape, the grains are able to shift and adjust their position. During distortion, the mobility of the grains causes them to slide past one another. As grains contact each other at 1500 C, bulk diffusion can occur, lending strength and increasing the contiguity. The increase in contiguity of the

6 is caused by grain settling which increases the grain coordination number in the settled region. The connectivity evolution of the samples shown in Figure 4 is similar to the contiguity evolution. The connectivity gradually decreases following liquid formation. The abrupt increase in connectivity for the sample quenched after a two minute hold at 1500 C can be understood in the context of the sample distorting, just like the contiguity. As the sample distorts, the connectivity is at a minimum. As the grains come to rest after settling, the number of contacts per grain is higher than at the time of distortion, resulting in a higher connectivity and rigidity that slows further distortion. The connectivity of the sample does not follow that of the contiguity; rather it progressively decreases with increased sintering Connectivity C 1480 C 1490 C 1500 C 1500 C C C C Conclusions Figure 4: Plot of connectivity with sintering temperature and time In this study -8.4Ni-3.6Fe, and -4.9Ni-2.1Fe were quenched at various points in the sintering cycle, and the microstructural parameters were compared. There is an optimal amount of sintering that will produce a fully dense compact without slumping. The distortion of a tungsten heavy alloy can be related to the progression of contiguity, connectivity and density. The evolution of the connectivity and contiguity with liquid formation and distortion was identified. A contiguity decrease was witnessed in both and once the liquid permeated the sample, which dissolved some of the existing solid skeleton. In the sample, a subsequent increase in the contiguity was seen corresponding to the distortion event. The density exhibited an anticipated decrease with the slumping event in the samples, while the samples achieved full

7 density with no degradation over time. The grain growth shifted to a higher rate upon liquid formation. Acknowledgements We gratefully acknowledge the funding provided by NASA-GEDS program, with Mike Purvey (Project Manager) and Witold Palosz (Project Scientist) of Marshall Space Flight Center. References 1. German, R. M. Sintering Theory and Practice, John Wiley and Sons, New York, 1996, pp Belhadjhamida, A., German, R. M., Tungsten and Tungsten Alloys by Powder Metallurgy. A status review, Tungsten and Tungsten Alloys, 1991, pp Fredricksson H., Eliasson A., Ekbom L., Penetration of Tungsten Grain Boundaries by a Liquid W-Ni-Fe Matrix, International Journal of Refractory Metals and Hard Materials, 13, 1995, pp German R. M., Microstructure of the Gravitationally Settled Region in a Liquid- Phase Sintered Dilute Tungsten Heavy Alloy, Metallurgical and Materials Transactions, 26A 1995, pp German R. M., Manipulation of Strength During Sintering as a Basis for Obtaining Rapid Densification Without Distortion, Materials Transactions 42, 2001, pp German R. M., Limitations in Net Shaping by Liquid Phase Sintering, Advances in Powder Metallurgy, 4, 1991, pp Chan T. Y., Lin S. T., Microstructural Evolution on the Sintered Properties of W-8 pct Mo-7 pct Ni-3 pct Fe Alloy, Journal of Materials Science, Vol. 35, No. 15, 2000, pp Upadhyaya A., German R. M., Gravitational Effects During Liquid Phase Sintering, Materials Chemistry and Physics, Vol. 67, No. 1-3, 2001, pp Kipphut C. M., Bose A., Farooq S., German R. M., Gravity and Configurational Energy Induced Microstructural Changes in Liquid Phase Sintering, Metallurgical Transactions Vol. 19A, 1988, pp Upadhyaya A., German R. M., Shape Distortion in Liquid-Phase-Sintered Tungsten Heavy Alloys, Metallurgical and Materials Transactions A 1998, pp