Adequate amount of B element can improve the endurance (creep) life of high temperature alloys to varying degrees, on the strengthening mechanism, early research is believed to be the B bias in the grain boundaries, filling the grain boundary vacancies, slowing down the diffusion of the element in the grain boundaries caused by. It has also been reported that B retarded the grain boundary damage by delaying the segregation of S into the grain boundary vacancies and hindering the growth of grain boundary carbides. Recently, the grain boundary behavior of B in several high-temperature alloys has been investigated by atomic probes, and a large amount of B was found to be polarized at the grain boundaries. The so-called “pinning” mechanism, in which B may directly impede the dislocation motion, and the mechanism of “mechanically strengthening of chemically disordered thin layers on both sides of the grain boundaries” have been proposed . mechanism. The Gibbs interfacial excess concentrations of B and C at the grain boundaries of Ni-CrMo-based solid solution-strengthened high-temperature alloys containing 0.018% B and 0.027% C (both atomic fractions) have been measured to be 6.5×1018 and 1.3×1017 atoms/m2, respectively, by using a three-dimensional atom probe, which indicates that the amount of grain-boundary agglomeration of B is much larger than that of C when the total contents are similar. The different precipitation characteristics of secondary borides on the grain boundary surfaces of three high-temperature alloys were investigated by the extractive carbon replica technique, and the inhibitory effect of B on the secondary lamellar Ti2CS and MC flakes at the grain boundary was reported. Using the concept of solute atom mismatch, it is discussed that solute atoms with large mismatch can prevent solute atoms with small mismatch from polarizing to the grain boundary.
The above mechanism studies are generally based on a single experimental method and heat treatment conditions, and the total content of the interstitial elements in the alloy is generally used for the discussion, while the solid solution amount of the interstitial elements prior to aging should be decisive for the actual polarization at the grain boundaries and the behavior of secondary precipitation. In addition, due to the diversity of high-temperature alloy compositions, the tendency of grain-boundary agglomeration and secondary precipitation behaviors of various alloys are very different and need to be further investigated. In this work, the influence of B content on the high-temperature properties of a complex alloyed nickel-based high-temperature alloy is investigated. By using the grain boundary extraction carbon replica technique, chemical phase analysis and SEM fracture analysis, as well as changing the heat treatment conditions, we investigated the relationship between the grain boundary secondary precipitation behavior and the grain boundary bias effect of the interstitial atoms B and C, the relative solid solution and the bias conditions, as well as their influence on the high-temperature performance mechanism.
1 Experimental method.
The materials used in the experiments were hot rolled bars of Nimonic 90 alloy with different B contents and diameters of 28 mm, and their chemical compositions are shown in Table 1. The smelting process was vacuum induction + electroslag remelting, and the ingot weight was about 170 kg.
The two heat treatment processes are as follows.
Process Ⅰ (normal heat treatment): 1200 ℃, 2 h air-cooled +1050 ℃, 4 h air-cooled +850 ℃, 8h air-cooled.
Process III: 1200 ℃, 2 h cooling to 1050 ℃ at a rate of 5 ℃ / min, 4 h air cooling +850 ℃, 8 h air cooling.
The tensile and durability properties of the experimental alloys were tested at 900℃. The average grain diameter was determined by the truncation method, fracture analysis was carried out by scanning electron microscope (SEM) type S-4200, and the contents of C and B in the solid solution primary MC and M3B2 were determined by chemical phase analysis. The chemical phase analysis specimens were quenched rapidly into ice brine after solid solution holding to maintain the distribution of solid solution B in the alloy.
In order to observe the characteristics of the precipitated phases on the grain boundary surfaces, the low-temperature fracture technique was used to obtain along-crystalline sections of the specimens, and extracted carbon complexes were prepared on these along-crystalline surfaces. The extracted carbon complexes were characterized by JEM-200 CX transmission electron microscope (TEM) and analyzed by selective electron diffraction (SED) and EDAX.
2 Experimental results
2.1 Organization and properties of normal heat treatment (process Ⅰ) state
The experimental alloys were analyzed and tested for their mechanical properties after normal heat treatment and machining in the same batch. The results show that: with the B content (mass fraction (%), the same below) rise, grain refinement; 900 ℃ tensile plasticity and endurance life with the B content of the law of change is similar, in the range of 0.003%-0.008%0 (alloy Nos. 1-3) is almost unchanged in the 0.018%-0.022% (alloy Nos. 6,7) near the peak.
900 ℃ tensile, persistent fracture are along the crystal fracture, but the two along the crystal section of the plasticity characteristics with the B content of the law of change is very obvious and similar. Such as B content is very low alloy Nos. 1-3, 900 ℃ tensile fracture for a typical brittle fracture along the crystal, the secondary crack is very serious (Figure 2a); alloy No. 4 local grain boundary surface plastic fracture characteristics; B content continues to rise, the plastic region increases, the toughness of the fossa becomes larger; B content of 0.018% and 0.022% of the alloy Nos. 6, 7 The grain boundaries of alloys Nos.6,7 with B contents of 0.018% and 0.022% show the best plasticity characteristics; with a further increase in the B content to 0.027%, the toughness fossa becomes shallower and the grain boundaries become brittle.
Under normal heat treatment, the primary MC and M3B2 phases in the form of inclusions and the γ phase precipitated by aging exist in the alloy matrix. Primary M3B2 increases with increasing B content, but no significant effect of B on the amount and morphology of primary MC and γ phases was observed. The study of grain boundary extracted carbon complexes shows that secondary large-size MC films (Fig. 3a), M6C dendrites and fine thin films of M23C6 (Fig. 3b) are mainly precipitated on the grain boundary surface of alloy Nos. 1-3. The chemical formulas of such MC films and M6C dendrites are approximated as (Ti0.81Mo0.12W0.07)C and (MO0.45W0.13Cr0.38Ni000.02CO0.02)6C, respectively. In alloy No.4, the size of such MC films becomes smaller, the number of M6C and M23C6 decreases, and a part of secondary M3B2 particles are precipitated. In alloys No.6 and No.7, all the above secondary carbides disappeared from the grain boundary surface and were replaced by uniformly dispersed secondary M3B2 particles with the chemical formula of (Mo0.22W0.12Ti0.03Cr0.25Ni0.19CO0.18)3B2.The B content increased to 0.027% (alloy No.9), and M3B2 precipitated densely at the grain boundary.
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Post time: Aug-12-2023
