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Tensile property and deformation failure of Ti-5Mo-5V-2Cr-3Al alloy
Zhu Qifang, Sun Zeming, Wang Fusheng, Jia Housheng, Wang Shihong1, and Shen Guiqin1
General Research Institute for Nonferrous Metals, Beijing, 100088, China 1 Beijing University of Aeronautics and Astronautics, Beijing, 100083, China
Dynamic tensile test of Ti-5Mo-5V-2Cr-3Al under the condition of solution treatment was studied, and the martensitic phase transformation induced by the stress during the tensile load, strain-hardening, grain rotation deformation and crack formation, meeting and failure were also studied. The results show that the stress induces the martensitic phase transformation, and the martensite pieces rotate orientedly and are swallowed each other under the external stress to form the macroplastic deformation. When loading stress continuously, the slips will arise in these grains. The slip bands and martensite pieces cross to form slip steps. With the slip intensifying, the grains are stretched and rotate, which results to the stress concentration and crack occurs gradually.
1. Introduction
Ti-5Mo-5V-2Cr-3Al alloy is a new type of high strength near P-titanium alloy. Compared with the same kind of alloy, it has unique strength and comprehensive mechanical properties, so it has broad prospects of development and application. In this paper, the microstructure, tensile property, deformation failure and rotation character of the alloy after solution treatment were studied [1-3], which will contribute to understanding the alloy and provide technical basis for engineering application.
ded and mechanical polished to mirror according to the metallurgical sampling process, then the sample was etched to expose the metallurgical structure. Dynamic tensile test was carried out on a JSM-5800 scanning electron microscope with tensile table. The conventional tensile test was carried out on an MTS-8800 tester.
Because the stable P-element content of this Ti-5523 alloy biases to lower limit, the transition temperature Tp measured by metallography is 825 ±5 °C, which is higher than usual Tp = 800 °C.
2. Experimental
The experimental Ti-5Mo-5V-2Cr-3Al alloy used for preparing dynamic tensile samples is hot-rolled band with 4 mm thick. The process of preparing dynamic tensile sample is that the band blank (after heat treatment) was linear cut to the sample with required size, the sample surface was grin-
Fig. 1. The microstructure of Ti-5523 alloy is single metastable P-phase
3. Results and discussion
Solution treatment of Ti-5523 alloy is water quenched at 830 °C for 1 h, which is beta area quenching, and the microstructure of Ti-5523 alloy is single metastable beta phase [4], as shown in Fig. 1. The tensile test was carried out on this sample, and Fig. 2 is the relative curves of tensile
- T = 830 °C
11 II III I IV I i
0 1 2 ЗА/, mm
Fig. 2. The relative curves of tensile load (p) vs. displacement (A/ )
© Zhu Qifang, Sun Zeming, Wang Fusheng, Jia Housheng, Wang Shihong, Shen Guiqin, 2004
load (p) vs. displacement (A/). As seen in Fig. 2, the curves can be divided to four stages, region I is the stage of elastic deformation, region II is the stage of plastic deformation with low hardening rate, region III is the stage of high hardening rate, and the last region IV is the stage of necking failure. XRD analysis and dynamic tensile test on alloy show
Table 1
Tensile properties of Ti-5523 alloy with solution treatment
Heat treatment Qb, MPa Q0.2’ MPa cN W, %
830 °C, 1 h WQ 778 217 21 60
that martensitic phase transformation induced by stress occurs basically at stage II [5, 6], which is P ^ a". This transformation shows fully during solution treatment of beta area at 830 °C. The metastable beta phase was induced to cause martensitic phase transformation of P ^ a" by the lower tensile stress, so this stress is also called excitation stress, the value of which roughly equals to that of the yield stress in the tensile curve (a 02 = 217 MPa). Because the formed martensite pieces will rotate orientedly and be swallowed each other under the external stress, the macroplastic defor-
Fig. 3. Photos of dynamic tensile sample solution treated. P = 636 (a); 649 (b); 810 (c); 850 (d); 880 (e) 945 (f); 953 N (g)
mation is formed. That is the yield phenomenon occurs at stage II in the P-A/ curves. With the development of transformation induced by stress, the quantities of martensites increase constantly, and the corresponding deformation quantities increase. As which belongs to phase transformation plasticity, not plastic deformation made by slip mechanism, the hardening rate is low. When transformation of P ^ a" is over, continuously loading the stress, the marten-site pieces themselves and residual beta substrate create slip deformation and the hardening rate is high, which corresponds to the stage III in the P-A/ curves. Table 1 lists tensile properties data of the alloy.
The transformation of P ^ a" during drawing, the deformation and failure of the dynamic tensile sample after solution treatment were observed with SEM. Figures 3 are the SEM photos of the samples with solution treatment at different tensile stages. Figure 2 is the original state of the alloy, a few of acicular martensites are observed in microstructure, which were formed during sample preparation. Figure 3(a) is the large number of martensite pieces formed after loading 503 N on the alloy, where part of martensite pieces combine and expand after loading 636 N. Figure 3(b) show that martensite pieces increase continuously, and part of grains begin to slide.
The slip band and martensite pieces cross to form slip steps (Fig. 3(c)). With the slip intensifying, the grains are
stretched and rotate (Fig. 3(d, e)), which results to the stress concentration at grain boundary [7] and crack occurs gradually (Fig. 3(/")). The crack expands from boundary to transgranular, and turns away when crossing with the mar-tensite pieces, but the main expanding direction is vertical with that of the external stress. Transgranular slip is also liable to create crack (Fig. 3(g)). When drawing to some extent, the necking is formed and the above cracks expand and join together to cause transgranular fracture.
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