Pressureless two-step sintering of ultrafine-grained refractory metals: Tungsten-rhenium and molybdenum

doi:10.1016/j.jmst.2022.01.033

Abstract

Abstract:

The challenge of sintering ultrafine-grained refractory metals and alloys to full density is hereby addressed by pressureless two-step sintering in tungsten-rhenium alloy and pure molybdenum. Using properly processed nano powders (∼50 nm average particle size), we are able to sinter W-10Re alloy to 98.4% density below 1200 °C while maintaining a fine grain size of 260 nm, and sinter molybdenum to 98.3% density below 1120 °C while maintaining a fine grain size of 290 nm. Compared to normal sintering, two-step sintering offers record-fine grain sizes and better microstructural uniformity, which translates to better mechanical properties with higher hardness (6.3 GPa for tungsten-rhenium and 4.0 GPa for molybdenum, both being the highest in all pressurelessly sintered samples of the respective material system) and larger Weibull modulus. Together with our previous demonstration in tungsten, we believe that two-step sintering is a general effective method to produce high-quality fine-grained refractory metals and alloys, and the lessons learned here are transferable to other materials for powder metallurgy.

Key words: Powder metallurgy, Refractory metals, Porosity, Two-step sintering, Grain growth

Zhongyou Que, Zichen Wei, Xingyu Li, Lin Zhang, Yanhao Dong, Mingli Qin, Junjun Yang, Xuanhui Qu, Ju Li. Pressureless two-step sintering of ultrafine-grained refractory metals: Tungsten-rhenium and molybdenum[J]. J. Mater. Sci. Technol., 2022, 126: 203-214.

Figures/Tables 14

Fig. 1. SEM images showing the morphology of (a) the raw and (b) sieved W-10Re powders, and (c) W-10Re powders’ particle size distributions, and SEM images of (d) the raw and (e) sieved Mo powders, and (f) Mo powders’ particle size distributions. Note y axis in (c) and (f) shows the accumulative volume fraction in the unit of vol%.

Fig. 2. Microstructures (fracture surfaces) of the raw-powder derived W-10Re samples sintered at (a) 900 °C, (b) 1300 °C, and (c) 1500 °C without holding, and the sieved-powder derived W-10Re samples sintered at (d) 900 °C, (e) 1300 °C, and (f) 1500 °C without holding.

Fig. 3. (a) Relative density, (b) average grain size, and (c) sintering rate dρ/dt of the raw-powder (in red) and sieved-powder (in blue) derived W-10Re samples in constant-sintering-rate experiments up to 1500 °C.

Table 1. Sintering data of W-10Re from constant-heat-rate sintering and two-step sintering.

Sintering conditions	Raw powders		Sieved powders
Sintering conditions	ρ (%)	G_avg (nm)	ρ (%)	G_avg (nm)
900 °C for 0 h	54	98	54	73
1000 °C for 0 h	56	120	55	82
1050 °C for 0 h	59	150	57	95
1100 °C for 0 h	61	180	59	120
1150 °C for 0 h	66	190	71	160
1200 °C for 0 h	72	230	78	210
1250 °C for 0 h	77	300	84	310
1300 °C for 0 h	82	350	88	340
1350 °C for 0 h	85	680	93	510
1400 °C for 0 h	92	750	95.0	560
1500 °C for 0 h	95.2	1290	98.1	1160
1200 °C for 0 h, 1100 °C for 10 h	80	250	82	240
1200 °C for 0 h, 1150 °C for 10 h	92	300	98.4	260
1230 °C for 0 h, 1150 °C for 10 h	94	350	98.6	310
1250 °C for 0 h, 1150 °C for 10 h	97.4	470	99.6	400
1300 °C for 0 h, 1200 °C for 10 h	99.0	550	99.5	530

Fig. 4. Microstructures (fracture surfaces) of the raw-powder derived Mo samples sintered at (a) 900 °C, (b) 1100 °C, and (c) 1300 °C without holding, and the sieved-powder derived W-10Re samples sintered at (d) 900 °C, (e) 1100 °C, and (f) 1300 °C without holding.

Fig. 5. (a) Relative density, (b) average grain size, and (c) sintering rate dρ/dt of the raw-powder (in red) and sieved-powder (in blue) derived Mo samples in constant-sintering-rate experiments up to 1400 °C.

Table 2. Sintering data of Mo from constant-heat-rate sintering and two-step sintering.

Sintering conditions	Raw powders		Sieved powders
Sintering conditions	ρ (%)	G_avg (nm)	ρ (%)	G_avg (nm)
900 °C for 0 h	61	67	62	58
1000 °C for 0 h	68	80	65	68
1050 °C for 0 h	72	96	67	79
1100 °C for 0 h	75	140	73	110
1150 °C for 0 h	78	180	76	130
1200 °C for 0 h	84	460	85	220
1250 °C for 0 h	88	1130	90	340
1300 °C for 0 h	91	2250	94	580
1350 °C for 0 h	93	3050	95.4	950
1400 °C for 0 h	95.2	4170	97.3	1750
1100 °C for 0.5 h, 1050 °C for 10 h	/	/	85	160
1130 °C for 0.5 h, 1030 °C for 10 h	/	/	89	180
1100 °C for 0.5h, 1050 °C for 10 h	/	/	95.2	250
1120 °C for 0.5 h, 1070 °C for 10 h	92	490	98.3	290
1150 °C for 0 h, 1100 °C for 10 h	/	/	98.5	850

Fig. 6. Arrhenius plot of DGB and calculated Ea using (a) Johnson's method and (b) Herring's method. Dash lines are linear fitting results of the corresponding data points. Linear fitting parameter R2 is 0.940 for W, 0.950 for W-10Re, and 0.973 for Mo in (a) and 1.000 for W, 0.812 for W-10Re, and 0.987 for Mo in (b).

Fig. 7. Fracture surfaces of (a, b) the sieved-powder derived W-10Re sample two-step sintered at T1=1200 °C for 0 h and T2=1150 °C for 10 h, and (c, d) the raw-powder derived W-10Re samples two-step sintered at T1=1300 °C for 0 h and T2=1200 °C for 10 h.

Fig. 8. Fracture surfaces of (a, b) the sieved-powder and (c, d) raw-powder derived Mo samples two-step sintered at T1=1120 °C for 0.5 h and T2=1070 °C for 10 h.

Fig. 9. EBSD results of (a-d) the sieved-powder derived W-10Re sample two-step sintered at T1=1200 °C for 0 h and T2=1150 °C for 10 h, (e-h) the raw-powder derived W-10Re sample two-step sintered at T1=1300 °C for 0 h and T2=1200 °C for 10 h, and (i-l) the raw-powder derived W-10Re sample sintered at 1500 °C for 0 h. (a, e, i) Inverse pole figure map, (b, f, j) normalized grain size distribution, (c, g, k) distributions of grain boundary misorientation angles (dash lines: Mackenzie's distribution for randomly oriented cubic grains [56]), and (d, h, l) pore figures.

Fig. 10. EBSD results of (a-d) the sieved-powder derived Mo sample two-step sintered at T1=1120 °C for 0.5 h and T2=1070 °C for 10 h, (e-h) the raw-powder derived Mo sample two-step sintered at T1=1120 °C for 0.5 h and T2=1070 °C for 10 h, and (i-l) the raw-powder derived Mo sample sintered at 1300 °C for 0.5 h. (a, e, i) Inverse pole figure map, (b, f, j) normalized grain size distribution, (c, g, k) distributions of grain boundary misorientation angles (dash lines: Mackenzie's distribution for randomly oriented cubic grains [56]), and (d, h, l) pore figures.

Fig. 11. (a) Weibull distribution of Vickers hardness for the sieved-powder derived W-10Re sample two-step sintered at T1=1200 °C for 0 h and T2=1150 °C for 10 h (abbreviated as “sieved W-10 Re, TSS” in blue), the raw-powder derived W-10Re sample two-step sintered at T1=1300 °C for 0 h and T2=1200 °C for 10 h (abbreviated as “raw W-10Re, TSS” in red), and the raw-powder derived W-10Re sample sintered at 1500 °C for 0 h (abbreviated as “raw W-10Re, NS” in black). (b) Comparison of our hardness data with the literature reports [[57], [58], [59], [60], [61]].

Fig. 12. (a) Weibull distribution of Vickers hardness for the sieved-powder derived Mo sample two-step sintered at T1=1120 °C for 0.5 h and T2=1070 °C for 10 h (abbreviated as “sieved Mo, TSS” in blue), the raw-powder derived Mo sample two-step sintered at T1=1120 °C for 0.5 h and T2=1070 °C for 10 h (abbreviated as “raw Mo, TSS” in red), and the raw-powder derived Mo sample sintered at 1300 °C for 0.5 h (abbreviated as “raw Mo, NS” in black). (b) Comparison of our hardness data with the literature reports [30,32,33,45,62].

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