2020 Volume 61 Issue 2 Pages 326-329
A 35Mn–60Cu–3Al–2Fe (at%) alloy was prepared by vacuum induction melting followed by hot forging at 1073 K, and then heat treated at 1113 K for 5 h. Further, the alloy was semisolid heat treated at 1153, 1163, 1173 and 1183 K for 20 min. The microstructure and damping capacity of the alloys at different temperatures have been investigated comparatively. The results show that the microstructure of the alloy at 1113 K comprises single phase while that at 1153–1183 K consists of Mn-rich and Mn-poor phases. The area fraction of Mn-rich phase raises with increase in semisolid treatment temperature. The damping capacity of the alloy is lowest at 1113 K and raises with increasing the semisolid treatment temperature (1153–1183 K) due to gradually increased amount of Mn-rich phase.
Mn–Cu based alloys bear broad application prospects in many fields for vibration and noise reduction.1) The damping mechanism of these alloys have primarily been achieved by relaxation of boundaries of {101} twins induced by fcc-fct martensitic transformation.2) Moreover, the starting phase transformation temperature linearly depends on Mn content in the alloy and can be raised by aging treatment due to the formation of spinodal decomposition microstructure containing Mn-rich and Cu-rich zones in nanoscale.3)
Generally, Mn–Cu based alloys are prepared by vacuum induction melting, then hot forging or rolling and finally heat treatment (solid solution and aging) to obtain homogeneous equiaxial grains.4) Recently, Zhong et al.5) and Zhang et al.6) have reported that the damping capacity of the alloy with dendrite is higher than that with equiaxial grain due to Mn dendritic segregation upon appropriate aging. However, they could have not presented relevant results regarding the effects of various dendrite microstructures on damping capacity of the alloys, since the area fractions of dendritic and interdendritic regions and their compositions especially Mn concentrations are not easily dominated by vacuum induction melting technology.
In the present work, therefore the semisolid heat treatment has been employed to obtain the duplex microstructure composed of Mn-rich and Mn-poor phases similar to that of dendritic and interdendritic regions. And such duplex structure has been controlled by changing the semisolid treatment temperature. The microstructure and damping capacity of Mn–Cu–Al–Fe system alloy with single and duplex structures, especially the effects of duplex structure on damping capacity of the alloy have been studied comparatively. Al and Fe are added to the alloy in order to improve its casting performance, corrosion resistance and mechanical properties.7,8)
A Mn–Cu based alloy with a nominal composition of 35Mn–60Cu–3Al–2Fe (at%) was prepared by vacuum induction melting technology. The alloy ingot was hot forged to thin plate with a thickness of about 10 mm at 1073 K. The samples with a size of 10 × 10 × 100 mm3 were cut from the alloy plate by electro-discharge machining (EDM), and then heat treated at 1113 K for 5 h. Further, some of samples were semisolid heat treated at 1153, 1163, 1173 and 1183 K for 20 min (the selection of temperature is based on differential scanning calorimetry (DSC) analysis). Both the heat treated and semisolid heat treated samples were aged at 703 K for 1 h.
The constituent phases of the samples with a size of 8 × 8 × 8 mm3 by EDM were identified by X-ray diffraction analysis (XRD, Panalytical X’Pert PRO, Cu Kα). The microstructure of the samples was observed by scanning electron microscopy (SEM, Zeiss Supra-55) and transformation electron microscopy (TEM, Tecnai G2 F30 operating at 300 kV). The chemical compositions of constituent phases in the samples were determined by energy dispersive X-ray spectroscopy (EDS, Inca X-sight). Moreover, the quantitative analysis of the constituent phases was performed using Image-Pro Plus 6.0 high resolution imaging analysis software.
The damping capacity (Q−1, tangent value of phase angle difference between stress and strain) of the samples with a size of 1.5 × 1.5 × 50 mm3 by EDM was examined in an internal friction instrument using inverted torsion pendulum mode. The measurement had been carried out at room temperature with a strain amplitude range of 0–10−3 and a frequency of 1 Hz.
As shown in Fig. 1, the (111), (200), (220) and (311) diffraction peaks belong to fcc structure exist in the XRD patterns of Mn–Cu based alloys regardless of heat treatment temperature. It should be noted that the (220) peak is relatively symmetric at 1113 K and gradually becomes more and more asymmetric with increase in semisolid treatment temperature (1153–1183 K). Particularly, the relatively obvious inflection is observed in the right sides of (220) peaks at 1173 and 1183 K (Fig. 1), implying fcc-fct phase transformation although there is no evident separation of this peak into both (220) and (202) ones.9)
XRD patterns of Mn–Cu based alloys.
Figure 2 shows the SEM images of the alloys and Table 1 gives the chemical compositions of their constituent phases. Combined with XRD, SEM and EDS results, the microstructure of the alloy at 1113 K consists of single solid solution phase (indicated by arrow ‘1’) while that at 1153–1183 K comprises Mn-rich and Mn-poor phases (i.e. duplex microstructure composed of light and dark phases, indicated by arrows ‘2’, ‘4’, ‘6’, ‘8’ and ‘3’, ‘5’, ‘7’, ‘9’ respectively). Moreover, the area fraction of Mn-rich phase raises with increase in semisolid treatment temperature (about 39.1, 40.0, 49.0 and 57.6% at 1153, 1163, 1173 and 1183 K respectively, Fig. 2). The EDS analysis results indicate that the Mn concentrations of Mn-rich and Mn-poor phases vary in the ranges of 42.66–45.43 at% and 29.82–32.91 at% respectively. And the former phase has higher Fe concentrations than the latter one (1.96–2.72 at% and 0.58–0.86 at% respectively), however, they possess relatively similar Al concentrations (1.98–2.70 at%), as presented in Table 1.
SEM images of Mn–Cu based alloys.
Figure 3 shows the TEM images and corresponding selected area diffraction pattern (SADP) from the alloy semisolid treated at 1183 K. Many fringe regions (fct structure, indicated by white rectangle box) which turn out to be from Mn-rich domain of tweed microstructure (i.e. spinodal decomposition microstructure with sizes of 101 nm order of magnitude, present in Mn-rich or Mn-poor phase with sizes of 101 µm order of magnitude) and contribute to damping mechanism of the alloys in this work by Yin et al.10) and Adachi et al.,11) are observed in the alloy in Figs. 3(a) and (b). Also, the SADP result shows the existence of diffraction ring (Fig. 3(c)), indicating the different orientations among a maximum of four fct variants induced by fcc-fct transformation.12)
Typical TEM image (a, b) and SADP (c) from the alloy semisolid heat treated at 1183 K: (c) shows SADP of the region indicated by the circle in (a).
Figure 4 shows the variation of damping capacity of the alloys with strain amplitude at room temperature. It can be seen that the damping capacity of the alloy although which is still relatively low overall due to its low Mn content, significantly raises with increase in semisolid treatment temperature over the whole strain amplitude (the lowest damping capacity is exhibited by the alloy heat treated at 1113 K compared to those semisolid treated at 1153–1183 K).
Variation of damping capacity of the alloys with strain amplitude at room temperature.
As given in Table 1, the Mn concentration of single solid solution phase at 1113 K is 34.56 at% which is lower than those of Mn-rich phases (42.66–45.43 at%) at 1153–1183 K. According to the following two empirical equations:3,10)
| \begin{equation} M_{\text{s}} = 1264.8c_{\text{Mn}} - 731.8 \end{equation} | (1) |
| \begin{equation} \text{c/a} = 2.638 - 3.317c_{\text{Mn}} + 1.618c_{\text{Mn}}{}^{2} \end{equation} | (2) |
The microstructure of the alloy heat treated at 1113 K is composed of single solid solution phase while that semisolid heat treated at 1153–1183 K consists of Mn-rich and Mn-poor phases (i.e. duplex microstructure). And the area fraction of Mn-rich phase raises with increase in semisolid treatment temperature. The damping capacity of the alloy with single phase is evidently lower than that with duplex phases. Furthermore, with increasing the semisolid treatment temperature, the damping capacity of the alloy gradually increases due to the increasing amount of Mn-rich phase at room temperature over the whole strain amplitude. This paper will contribute to guiding the design and development of novel duplex Mn–Cu alloys with high damping capacity.
This work was supported by the National Natural Science Foundation of China (No. 51701167) and Fundamental Research Funds for the Central Universities (No. 2682017CX073).
