4.1 MICROSTRUCTURE

In the alloy microwave sintered at 1783 K (1510°C), pores observed between the tungsten grains (shown in Figure 2(a) ) indicate that the matrix has not completely penetrated between the tungsten grains. Lower sintering temperature of 1783 K (1510°C) could possibly be the reason behind incomplete matrix penetration and consequently the presence of porosity. On the contrary, at higher temperature of 1793 K (1520°C), pores are completely eliminated, tungsten grains and matrix are more evenly distributed and matrix has penetrated between most of the tungsten grains (shown in Figure 2(b) ). With marginal increase in the temperature, matrix (liquid) fluidity is expected to improve and the rearrangement of tungsten grains, dissolution-reprecipitation are enhanced resulting in uniform distribution of grains and matrix. ^{Liu, Ma, & Cai (2011)} have made similar observations in 93W-4.9Ni-2.1Fe alloy microwave sintered at 1773 K (1500°C). It is reported that at 1773 K (1500°C), liquid flow and particle rearrangement are insufficient resulting in non-uniform distribution of angular shaped tungsten grains in the matrix. However, if the sintering temperature is increased up to 1823 K (1550°C), matrix (liquid phase) flow, migration and rearrangement of tungsten grains, dissolution and reprecipitation are quite significant resulting in complete densification and near-spherical tungsten grains. Very similar to the observations made by ^{Liu et al. (2011)} in terms of grain shape, it is also observed in the present study that though average grain size is similar in both the alloys, most of the tungsten grains areangular shaped at 1783 K (1510°C) compared to the presence of a number of near-spherical grains at 1793 K (1520°C).

^{Park, Baik, Kang, and Yoon (1996)} have reported that if the volume fraction of the matrix is sufficiently high, the grains would be nearly spherical in shape, but with low matrix volume fraction the grains accommodate their shape to their neighbours and become angular. Similar observations are made in the present study; the alloy microwave sintered at 1783 K (1510°C) has substantially lower matrix volume fraction (listed in Table 2) and the rearrangement of tungsten grains within this low matrix volume fraction has resulted in angular shaped tungsten grains (shown in Figure 2(a)) whereas at 1793 K (1520°C), the matrix volume fraction is sufficiently high for the tungsten grains to distribute more evenly and transform to near-spherical shape by Ostwald ripening mechanism.

In terms of other key microstructural parameters such as dihedral angle and contiguity, the alloy microwave sintered at 1783 K (1510°C) has higher dihedral angle and contiguity (listed in Table 2) compared to the alloy microwave sintered at 1793 K (1520°C). ^{Rabin & Germán (1988)} have reported that dihedral angle; the ratio between the solid-solid and solid-liquid interfacial energies, is dependent on sintering temperature. With increasing temperature, the solid-liquid interfacial energy decreases more rapidly than the solid-solid interfacial energy and henee dihedral angle decreases with increase in sintering temperature . ^{Liu & Germán (2001)} reported that a low dihedral angle system has low solid-liquid interfacial energy and henee favours liquid penetration between the solid-solid contacts. In the present study, among the microwave sintered alloys, the dihedral angle (listed in Table 2) is found to decrease with increase in temperature from 1783 K (1510 ° C) to 1793 K (1520 ° C). Low dihedral angle obtained at 1793 K (1520°C) has favoured matrix (liquid) penetration between the tungsten grains and the grains are separated by the matrix resulting in low contiguity.

Contiguity, as reported by ^{Germán (1985),} is the quantitative measurement of interphase contact. In the case of tungsten heavy alloys, the number of contact points between tungsten-tungsten grains are measured and defined as contiguity. has also reported that contiguity has strong dependence on dihedral angle and it increases with increase in dihedral angle. In the present study, a similar trend is observed; the alloy microwave sintered at 1783 K (1510°C) has high dihedral angle between the grains and henee has higher contiguity whereas the alloy microwave sintered at 1793 K (1520°C) has lower dihedral angle and henee lower contiguity.

4.2 MECHANICAL PROPERTIES

Tungsten-Tungsten contiguity is a major factor influencing both tensile and impact properties of heavy alloys^{. Humail, Akhtar, Askari, Tufail, & Qu (2007)} have reported that these properties show a decreasing trend with increasing contiguity. ^{Churn & Germán (1984)} have reported the relationship between elongation to failure and contiguity as £f = K₂ (1 - C_w) where £f is the elongation to failure, K₂ isa material constant and C_w is the contiguity. In the present study, between the microwave sintered alloys, elongation and contiguity follow a similar trend; the alloy sintered at 1783 K (1510°C) exhibits higher contiguity and consequently lower elongation (listed in Table 2) whereas the alloy sintered at 1793 K (1520°C) has higher elongation owing to its lower contiguity. In terms of the ultimate tensile strength (listed in Table 2), in the alloy sintered at 1783 K (1510°C), presence of contiguous tungsten grains and porosity result in premature failure in tensile loading and henee lower tensile strength is observed. Thus, a combination of superior tensile strength and elongation is achieved in the alloy sintered at 1793 K (1520°C) compared to the alloy sintered at 1783 K (1510°C). Authors, in their earlier work ^{(Prabhu et al., 2014)} have also reported similar observations on the effect of contiguity on the tensile properties of 90W-7Ni-3Fe and 90W-6Ni-2Fe-2Co alloys. A comparative plot of the tensile properties obtained in the present study and in the authors' previous work ^{(Prabhu et al., 2014)} is shown in Figure 5.

Fig. 5 Effect of contiguity on tensile properties of heavy alloys. Present study and literatura ^{(Prabhu et al., 2014)} valúes are plotted. 

In terms of tensile fractography, fracture modes of tungsten heavy alloys can be classified as: (i) debonding along tungsten-tungsten interface, (ii) decohesion along tungsten-matrix interface, (iii) tungsten grain cleavage and (iv) matrix failure ^{(Churn & Germán, 1984)}. Tungsten-tungsten interface, being the weakest interface is the crack initiation site whereas crack propagation can oceur in one of the following two modes; (i) intergranular failure, if weak tungsten-tungsten interfaces are abundant and the tungsten-matrix interface is also not strong enough to stop crack propagation, then the crack propagates along these interfaces, leaving the tungsten grains intact and (ii) transgranular failure, if weak tungsten-tungsten interfaces are less frequent and tungsten-matrix interface is strong enough to stop interfacial crack propagation, then the crack moves within the grain resulting in grain cleavage ^{(Churn & Germán 1984)}. In terms of the properties obtained in these two modes of failure, intergranular failure results in inferior properties compared to transgranular failure.

Authors, in their earlier work ^{(Prabhu et al., 2014)} have reported transgranular failure and superior tensile properties in the case of microwave sintered 90W7Ni-3Fe and 90W-6Ni-2Fe-2Co alloys compared to intergranular failure and inferior tensile properties in conventionally sintered alloys.

In the present study, between the microwave sintered alloys, the alloy sintered at 1783 K (1510°C) does not exhibit a single dominant failure mode, instead, the fractograph has features of both intergranular and transgranular failure (shown in Figure 3(a) ). Features of intergranular failure such as tungsten-tungsten debonding, tungsten-matrix decohesion and isolated matrix rupture are observed along with tungsten grain cleavage and secondary crack propagation which indícate transgranular failure (shown in Figure 3(a) ). Contrarily, in the case of the alloy sintered at 1793 K (1520°C), tungsten grain cleavage and crack propagation through the grain are dominant compared to limited occurrences of tungsten-tungsten debonding (shown in Figure 3(b) ).The tensile properties (listed in Table 2) obtained are in agreement with the tensile fractographs; the alloy sintered at 1793K (1520°C) has predominant transgranular failure and superior properties compared to the alloy sintered at 1783 K (1510°C) which has both intergranular, transgranular failure and henee relatively inferior properties.

Contiguity, apart from affecting the tensile properties, has also been reported to be a major factor influencing the impact toughness of a tungsten heavy alloy ^{(Das, Kiran, Chakraborty, & Prasad, 2009)}. ^{Joonwoong et al. (1993)} have reported that with increasing matrix penetration (or decreasing tungsten-tungsten interfacial área or low contiguity) the fraction of ductile-dimple failure of the matrix increases, implying that the plástic deformation of the additional matrix phase along the crack propagation increases the impact energy. Under high strain rate loading such as impact loading, the matrix phase undergoes ductile - dimple fracture whereas tungsten grains fail by cleavage ^{(Joonwoong et al., 1993)}. Impact toughness valúes of the microwave sintered alloys are listed in Table 2. The alloy sintered at 1783 K (1510°C) has higher contiguity, lower matrix volume fraction (listed in Table 2) and henee lower impact toughness compared to the alloy sintered at 1793 K (1520°C).

Impact fractographs of both the alloys are shown in Figure 4 (a) and 4(b) . Though both the alloys exhibit the following features; (i) tungsten grain cleavage, (ii) ductile-dimple failure of the matrix and (iii) tungsten-tungsten debonding, the alloy sintered at 1783 K (1510°C) has more frequent instances of tungsten-tungsten debonding compared to the alloy sintered at 1793 K (1520°C).This observation indicates that the dominant mode of failure is intergranular in the alloy sintered at 1783 K (1510°C), thereby explaining the marginally lower impact toughness (listed in Table 2) obtained in the alloy.

In case of the alloy sintered at 1793 K (1520°C), tungsten grain cleavage and ductile-dimple failure of the matrix are the major features observed compared to tungsten-tungsten debonding. Increase in the impact toughness of the alloy sintered at 1793 K (1520°C) can be attributed to lower contiguity and higher matrix volume fraction. Authors, in their earlier work ^{(Prabhu et al., 2014)} have also reported similar observations on the effect of contiguity on the impact toughness of 90W-7Ni-3Fe and 90W-6Ni-2Fe-2Co alloys. A comparative plot of impact toughness of microwave sintered alloys obtained in the present study and in the authors' previous work is shown in Figure 6.

Fig. 6 Effect of contiguity on impact toughness of heavy alloys. Present study and literature ^{(Prabhu et al., 2014)} valúes are plotted. 

4.3 COMPARISON WITH CONVENTIONAL SINTERING

In order to compare microwave and conventional sintering, particularly in the case of high tungsten content heavy alloys, 93W-4.9Ni-2.1Fe alloy is sintered in conventional sintering mode using the same sintering parameters of 1793 K (1520°C) and holding time of 60 min. The heating rate used in microwave sintering process is 20 K min¹ (20°C min¹) and in the conventional sintering process is 3 to 5 K min^-1 (3 to 5°C min¹). Authors in their earlier work have studied the effect of similar heating rate by using 20 K min ¹ (20°C min¹) in these two modes of sintering using 90W-7Ni-3Fe, 90W-6Ni-2Fe-2Co alloys and have reported that higher heating rate of 20 K min¹ (20°C min¹) in conventional sintering results in highly contiguous, irregular tungsten grains and inferior properties ^{(Prabhu et al., 2014)}. Henee heating rate of 3 to 5 K min^-1 (3 to 5°C min¹) is being used in the present study in the case of conventional sintering.

Contrary to the full densification and desirable microstructure obtained at 1793 K (1520°C) in the case of microwave sintering, conventional sintering has resulted in only 99.6% densification and partially developed microstructural features. Porosity is observed between the tungsten grains (shown in Figure 2(c) ).Though the sintering parameters of 1793 K (1520°C)/60 min are the same for both the alloys, the conventionally sintered alloy has coarse tungsten grains, higher dihedral angle, higher contiguity and lower matrix volume fraction (listed in Table 2) compared to the microwave sintered alloy.

Possible reasons for the dissimilar microstructural features obtained between the two modes of sintering could be the heating mechanism and the sintering time. Unlike microwave sintering which involves volumetric heating, conventional sintering involves heat transfer from the surface to the core of the sample. With the absence of volumetric heating, sufficient time has to be provided in conventional sintering for heat transfer to oceur over the entire sample and result in full densification and desirable microstructural changes. In the present study, though the heating rate is 3 to 5 K min¹ (3 to 5°C min¹), the holding time of 60 min proves to be inadequate for complete densification and microstructure development. Henee, the alloy conventionally sintered at 1793 K (1520°C) does not have a fully developed microstructure (shown in Figure 2(c) ) primarily because of the reduced processing time.

Another factor that contributes towards microstructure development is the dissolution of tungsten in the matrix. ^{Liu et al. (2012)} have reported that microwave energy accelerates dissolution of tungsten in the matrix. Though the mechanism is not clearly understood, it has also been observed by the authors in their previous work ^{(Prabhu et al., 2014)} that dissolution of tungsten in the matrix is higher in microwave sintered alloys compared to conventionally sintered alloys. Enhanced tungsten dissolution in the matrix (or higher solid solubility in the liquid) facilitates matrix penetration between the grains thereby decreasing the dihedral angle. Tungsten grains are thus separated by the liquid phase resulting in decrease of tungsten-tungsten contiguity. Thus microwave sintered alloys have lower contiguity compared to conventionally sintered alloys.

Influence of contiguity on tensile properties, which has been discussed earlier, is also observed in the conventionally sintered alloy. Conventionally sintered alloy has substantially inferior tensile strength and elongation (listed in Table 2) owing to the presence of presence of porosity, contiguous tungsten grain network and low matrix volume fraction. Fracture surfaces of tensile tested samples of both microwave and conventionally sintered alloy are shown in Figure 3(a) and 3(c). Microwave sintered alloy, as mentioned earlier, has predominant transgranular failure and henee superior mechanical properties whereas conventionally sintered alloy exhibits predominant intergranular failure and as expected inferior tensile properties. Thus, the fractographic features (as shown in Figure 3(a) and of both the microwave sintered and conventionally sintered alloys are found to be consistent with their tensile properties.

In terms of impact toughness, microwave sintered alloy has substantially higher impact valué (176 J/cm²) compared to the conventionally sintered alloy (14 J/cm²). Matrix penetration between the tungsten grains (indicated by contiguity and dihedral angle) can be attributed to the varying impact toughness valúes obtained between the microwave and conventional alloys. Microwave sintered alloy, with lower contiguity and dihedral angle has sufficient matrix penetration between the tungsten grains and henee higher impact toughness whereas higher contiguity and higher dihedral angle result in lower impact toughness in conventionally sintered alloy. Similar observations on impact toughness reported by the authors ^{(Prabhu et al., 2014)} is compared with the present study in Figure 6.

Impact fractographs are shown in Figure 4(a) and 4(c). The observations made in impact fractographs are very similar to those made using tensile fractographs. Microwave sintered alloy exhibits transgranular failure of the tungsten grains and ductile-dimple failure of the matrix (shown in Figure 4(a) ). Conventionally sintered alloy does not show any evidence of grain cleavage and matrix failure; instead the alloy exhibits intergranular failure where the grains are separated along the weak tungsten-tungsten and tungsten-matrix interfaces leaving the tungsten grains intact (shown in Figure 4(c) ).

Though the sintering temperature, holding time are exactly the same, the conventionally sintered alloy shows neither improved microstructure ñor superior properties as compared to the microwave sintered alloy. Superior microstructure and mechanical properties in microwave sintered alloy are possibly because of the lesser sintering time involved owing to the volumetric heating mechanism.

Thus it is apparent that microwave sintered alloys will exhibit favourable microstructural features if sintering is carried out at appropriate temperatures. This in turn will have extremely favourable implications with regards to mechanical properties. Thus, there is a need to develop microwave furnaces in the lines of conventional sintering furnaces so that large number of microwave sintered alloys can be produced in a short time for the process to be economically viable.