Influence of Fe on Microstructure and Mechanical Properties in P-doped Ni-Cr-Fe Alloys

doi:10.1007/s40195-018-0815-4

Abstract

Abstract:

The influence of Fe on the microstructure and mechanical properties of P-doped Ni-Cr-Fe alloys has been investigated. Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe. In addition, the precipitation of M₂₃C₆ was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.

Key words: Fe, P, Segregation, Precipitate, Mechanical, properties

Xin-Tong Lian, Wen-Ru Sun, Xin Xin, Fang Liu, Dan-Dan Zheng. Influence of Fe on Microstructure and Mechanical Properties in P-doped Ni-Cr-Fe Alloys[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(6): 771-779.

Figures/Tables 11

Table 1 Chemical compositions of different Ni-Cr-Fe alloys (wt%)

Alloy no.	Fe	P	Cr	C	Ni
1	15.2	0.025	17.17	0.039	Bal.
2	30.5	0.025	17.11	0.040	Bal.
3	45.1	0.024	17.14	0.044	Bal.
4	60.7	0.026	17.17	0.041	Bal.

Fig. 1 Dendrite structure of as-cast Ni-Cr-Fe alloys: a alloy 1; b alloy 2; c alloy 3; d alloy 4

Table 2 Distribution and partition coefficients of elements in different alloys by EPMA (wt%)

Alloy no.	Region	Ni	Fe	Cr	P
1	Dendrite core	67.94	15.27	16.79	-
	Interdendritic area	66.46	15.10	18.36	0.079
		1.02	1.01	0.91
2	Dendrite core	51.51	31.82	16.67	-
	Interdendritic area	51.30	30.78	17.84	0.080
		1.00	1.03	0.93
3	Dendrite core	37.80	44.68	17.49	0.035
	Interdendritic area	36.71	46.43	16.86	-
		1.03	0.96	1.04
4	Dendrite core	21.67	60.27	18.02	0.044
	Interdendritic area	20.23	62.73	17.04	-
		1.07	0.96	1.06

Fig. 2 Vickers microhardness of different alloys in dendrite and interdendrite regions

Fig. 3 Microstructures of as-rolled alloys after heat treatment: a alloy 1; b alloy 2; c alloy 3; d alloy 4

Fig. 4 Effects of Fe content on tensile properties of alloys

Fig. 5 Morphologies of fractures surfaces after tensile tests for alloy 1 a, e, alloy 2 b, f, alloy 3 c, g, alloy 4 d, h at low a-d, high e-h magnification

Fig. 6 Effects of Fe content on stress rupture properties of alloys

Fig. 7 Morphologies of fractures surfaces after stress test for alloy 1 a, e, alloy 2 b, f, alloy 3 c, g, alloy 4 d, h at low a-d, high e-h magnification

Fig. 8 Longitudinal microstructures of stress-ruptured alloys at 650 °C and 150 MPa: a alloy 1; b alloy 2; c alloy 3; d alloy 4

Fig. 9 Morphologies of grain boundaries in stress rupture of a, c alloy 1, b, d alloy 3 at a, b low, c, d high magnification, e TEM image, f EDS analysis of M23C6 in alloy 1

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