
1. Introduction
Since molybdenum is one of the high melting temperature materials and has superior ductility and toughness compared to tungsten, it is often used for ultra-high temperature components, such as rotating anodes of X-ray tube, electrodes for melting glass and hot metal forming dies [1]. To improve high temperature strength and recrystallization temperature, the molybdenum alloy TZM has been developed with containing 0.5%Ti and 0.07%Zr. There have been various reports on effect of composition, hot forming, grain size, etc. on microstructure, high temperature strength, and so on [2–8]. High temperature fracture toughness is one of the most important properties for high temperature structural components, such as the rotating anodes of X-ray tube. However, the reports on high temperature fracture toughness have been hardly available, except some reports on high temperature fracture toughness of tungsten [9]. Cockeram [10] tried to evaluate high temperature fracture toughness JIC of LCAC, TZM and ODS molybdenum alloys by the compliance method according to ASTM E1820. However, it was not successful due to large deformation during stable crack growth.The main reason for lack of reports on high temperature fracture toughness will be the difficulty of fracture toughness test at high temperature.
In the present study, high temperature fracture toughness JIC was evaluated based on the convenient fracture toughness JIC test method [11] for two kind of TZM alloys, one with Ti and Zr carbides and the other with Ti and Zr oxides. Two different forging rates were also applied to the two kinds of TZM alloys to investigate the effect of forging rate on high temperature fracture toughness.
2. Experimental procedure
2.1. Materials
Two kinds of TZM alloys, TZM-A and TZM-B, were prepared: For TZM-A, Mo powders were mixed with TiC and ZrC powders, followed by cold isostatic pressed, sintered at temperatures above 1800 °C, and then hot-forged at 1600 °C with post-heat-treatment at 1500 °C for 1h, detail processes of which have been presented in Ref. [12]. Two kinds of forging rates, 25% and 36%, were applied to TZM-A for investigating the effect of forging rate on fracture toughness (hereafter noted as TZM-A25 and TZM-A36, respectively). Forging rate is defined as the percentage of reduction in height. On the other hand, TZM-B was prepared from Mo, TiH2 and ZrH2 powders with the same manufacturing processes as TZM-A and the same two forging rates were also applied to TZM-B (noted as TZM-B25 and TZM-B36). The chemical compositions of TZM-A and TZM-B are shown in Table 1. The room temperature mechanical properties of the present four kinds of TZM alloys are shown in Table 2. The Vickers hardness test was conducted with applied load of 196 N for 10s and the result is also listed in Table 2. Microstructures of the TZM alloys were observed by using the optical microscope. Fig. 1shows the microstructural observations of as-received materials before test. As seen from the figure, the grains for the present four materials are almost equiaxed with similar average grain sizes ranged from 42 to 47 μm. No elongated grains due to forging could be observed and it is also known that the recrystallization temperature of TZM is around 1400–1500 °C [5,13]. Therefore, as-received microstructure before high temperature JIC test would be the recrystallized one. The Ti and Zr carbide particles mainly along the grain boundary for TZM-A and the Ti and Zr oxide particles mainly along the grain boundary for TZMB were confirmed by the EPMA analysis, as shown in Fig. 2.
2. 2. Fracture toughness test
The convenient fracture toughness JIC test method [11] has been proposed, where the enough depth of side-groove is introduced to make the stable crack growth starting point coincided with the maximum load point. Therefore, the JIC value can be evaluated at the maximum load point by using only one specimen without obtaining J-R curve.Therefore it is useful to investigate effects of parameters, such as loading rate and temperature on JIC, where numbers of JIC tests are needed according to the standard JIC test method. In the present study, single edge notch beam (SENB) specimens were machined from the forged blocks in the longitudinal direction, where the notch direction was parallel to the forging direction. Shape and dimensions of the SENB specimen are shown in Fig. 3.
A fatigue pre-crack was introduced at room temperature using a servo-hydraulic testing machine. The specimen was set on a fourpoint bending fixture with a loading span of 10 mm and a supporting span of 30 mm and then sinusoidal wave form of cyclic loading was applied with a stress ratio of R = 0.1 and a frequency of 15 Hz. Since the TZM alloy was too brittle to avoid the unstable fracture under tensile mode of pre-cracking like ceramic materials, the following pre-cracking method was adopted in the present study [14]: (1) As the first step, a compressive-compressive mode of loading at notch root region was applied to nucleate a small fatigue crack, and then (2) tension-tension mode of loading was applied by changing the specimen direction upside down to propagate the fatigue crack up to the necessary length for a pre-crack. The reason for successfully introduction of a small fatigue crack at notch tip under compressive loading is that due to the local plastic deformation formed at notch tip in the first cycle of compressive loading, the local tensile stress at the notch tip is repeated in the following loading cycles, which can nucleate a fatigue crack. In the present method, the unstable fracture cannot be occurred because the tensile stress region is limited only near notch tip region with the surrounding compressive stress. During the fatigue pre-cracking process, the crack length was monitored by a travelling microscope. The introduced fatigue pre-crack lengths were controlled to be within 4.0–4.5 mm. The pre-crack length for calculating JIC value was determined as the mean value of nine points, which were measured on the fracture specimen at equally divided locations in the direction of the specimen thickness.
The U-shaped side-groove was machined with various side-groove depth ratios (B − BN)/B after introducing a fatigue pre-crack for determining the suitable side-groove depth, where B and BN are the nominal and the net thickness of the specimen, respectively.
The JIC tests were conducted on an Instron-type universal testing machine with a capacity of 10 kN under three-point bending with the supporting span length of 30 mm at a crosshead speed of 0.5 mm/min. The JIC tests at room temperature were conducted in an open air atmosphere, while the JIC tests at high temperatures ranged from 800 °C and 1200 °C were performed in a vacuum electric furnace equipped with the testing machine. The vacuum in the furnace was maintained below 9 mPa during the test. The high temperature tests were started after holding for 15 min at the test temperature in order to achieve the uniform temperature distribution in the specimen.The J value was evaluated by the following equations:
where Amax is the area under the load-displacement curve up to the maximum load point, b is the ligament length, Be is the effective thickness of side-grooved specimen [11].
The fracture surfaces were observed in detail on a scanning electron microscope (SEM). The Vickers hardness tests of specimens after high temperature fracture toughness tests were conducted with applied load of 196N for 10s.
3. Results and discussion
3.1. Determination of side-groove depth
In the convenient fracture toughness JIC test method [11], it is essential to determine the suitable side-groove depth for evaluating the JIC value at the maximum load point. In the present study, the suitable side-groove depth for the TZM alloy at high temperatures has been investigated. As the first step, the JIC value of TZM-B36 at 800 °C was evaluated according to the standard ASTM E1820 using multiple specimens without side-groove. The relationship between J and crack extension Δa for TZM-B36 at 800 °C is shown in Fig. 4. The intersection point between the regression line (J-R curve) and 0.2 mm offset line could give the valid JIC value of 26 kJ/m2.
As the next step, the convenientt JIC tests with various side-groove depths were carried out at 800 °C for the same material. From the results, the relationship between JC and side-groove depth ratio is shown in Fig. 5. It is found from the figure that the JC value decreases to reach a constant value at the side-groove ratio of 0.3. The constant JC value was about 26 kJ/m2 and equal to the JIC value determined according to the standard JIC test method. Therefore, it can be concluded that the JIC value can be evaluated by the convenient test method with the side-groove depth ratio of 0.3.
3.2. High temperature fracture toughness
Fig. 6 shows the JIC values for TZM-A25, TZM-A36, TZM-B25 and TZM-B36 at high temperatures ranged from 800 °C to 1200 °C. It is found from the figure that the JIC values for TZM-A25 and TZM-A36 with Ti and Zr carbide particles dispersed along grain boundaries are higher than those for TZM-B25 and TZM-B36 with Ti and Zr oxide particles dispersed along grain boundaries. In Fig. 1, examples of microstructural observations for the four materials after the high temperature fracture toughness tests are also shown. As seen from the figure, before the tests, the average grain sizes are not much different between two materials; 44.5 μm for TZM-A and 45.5 μm for TZM-B. However, after the tests, the grain size of TZM-A (53 μm) and that of TZM-B (52 μm) are also not much different but larger than those before the tests. The high temperature JIC test was started after holding for 15 min at the test temperature and loading time was about 3 min. The larger grain sizes after the high temperature tests might be due to the testing time of about 18 min at the test temperature. It has been known that Ti and Zr carbides along the grain boundary are effective to suppress the grain growth as well as to improve the recrystallization resistance [7]. However, in the present TZM-A with carbides, significant suppression of grain growth could not be observed. The hardness of TZM-B measured after the high temperature tests is low compared to that of TZM-A, as seen in Fig. 7. The similar slight decrease of hardness at high temperatures in the range of 800 °C–1200 °C compared to the hardness at room temperature is also found in Ref. [5]. It has been also reported that carbide particles contribute to increase of recrystallization temperature and to strength [6]. Therefore, it is speculated that lower fracture toughness at high temperatures in TZM-B compared to TZMA would result from higher resistance to softening at high temperatures in TZM-A compared to TZM-B.
The JIC values are almost identical for both TZM-A25 and TZM-A36 as well as for both TZM-B25 and TZM-B36 regardless of forging rate, as found from Fig. 6. As seen from Fig. 1, no recrystallization was found at the present high temperatures lower than 1200 °C but some amount of grain growth was observed, which indicated the existence of annealing effect at high temperatures. Therefore, the forming effect due to forging would be mostly eliminated at high temperatures. Since the annealing treatment at 1500 °C was conducted after the forging process,fracture toughnesses KIC of TZM-A and TZM-B at room temperature were almost identical regardless of forging rate, as shown in Table 2.
It is also found from Fig. 6 that in the temperature range from 800 °C to 1200 °C, the fracture toughness JIC was almost constant or gradually decreased. Although no other high temperature fracture toughness data for TZM alloys have been reported, the mechanical properties measured after high temperature annealing have been available. The hardness [5,13] and yield stress [15,16] were almost constant at the temperatures ranged from 600 °C to 1300 °C. The tensile strength also showed similar temperature dependency to the yield stress and gradually decreased or almost constant at temperatures between 700 °C and 1200 °C [16]. These temperature dependencies of yield stress and tensile strength in the temperature range from 800 °C and 1200 °C are almost similar to that of fracture toughness JIC obtained in the present study.
Examples of fracture surface observations are shown in Fig. 8. As found from the figure, the cleavage fracture was dominant on the fracture surface of the specimen tested at room temperature, while the dimple fracture was dominant at the high temperatures for all four TZM alloys used. No significant difference of dimple fracture morphology was identified among the present four materials.
4. Conclusions
The convenient fracture toughness JIC test method was applied to evaluate high temperature fracture toughness JIC values for two kinds of TZM alloys, one with Ti and Zr carbides and other with Ti and Zr oxides. The main conclusions are summarized as follows.
(1) The convenient JIC test could be successfully applied to evaluate high temperature JIC values for the TZM alloy by selecting a suitable side-groove depth.
(2) The JIC values were almost constant or gradually decreased with increasing temperature in the temperature range from 800 °C to 1200 °C for the present four kinds of TZM alloys.
(3) The JIC values for TZM-A with carbide particles were high compared to those for TZM-B with oxide particles at high temperatures ranged from 800 °C to 1200 °C.
(4) The JIC values for both TZM-A25 and TZM-A36 and for both TZMB25 and TZM-B36 were almost identical, respectively. Therefore,influence of forging rate on the high temperature fracture toughness seemed not to be significant.
Paper Citatioin Information:
Int. Journal of Refractory Metals and Hard Materials 66 (2017) 52–56
TZM alloy powder (molybdenum-titanium-zirconium alloy powder) is used in aerospace high-temperature components, nuclear industry structural parts, high-temperature molds, electronic devices and sputtering targets. It adapts to extreme environmental requirements with its high melting point, excellent high-temperature strength and stability.
Welcome to contact with Vicky for more powder details at +86-13318326185