anodization replica of precipitates in age-hardening ti alloys...anodization replica of precipitates...
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Anodization Replica of Precipitates in Age-Hardening Ti Alloys
.D::mgyan Ding1, Zhaohui Li1
, Yi Yang2, Congqin Ning3, Hegang Liu1, Peng Ge2
, Liang Feng2
I> School of Materials Science and Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai J iao Tong University ,Shanghai, 200240 ,China
2> Northwest Institute for Nonferrous Metal Research, Xi' an Shaanxi, 710016 ,China 3 > State Key Laboratory of High Performance Ceramics and Super fine Microstructure ,Shanghai Institute of Ceramics ,Chi
nese Academy of Sciences, Shanghai, 200050, China
Precipitation-hardening is one of the important strengthening mechanisms which governs the mechanical properties of various kinds of alloys. To
characterize the precipitates in age-hardening Ti alloys, traditional metallography and transmission electron microscopy as well as metallurgical
replica have been employed for either destructive or non-destructive testing. However, these characterization methods usually depend on skilled
sample preparation and analysis, which lacks a good repeatability and sufficient imaging contrast. In the present work.our recent progress is reported in developing a novel replica method to characterize the precipitates in age-hardening Ti alloys. Based on a weak anodization and ultrasonic treatment, anodization replicas of various kinds of precipitates and grain boundaries are successfully fabricated in BT22 and TilOCr alloys. The
phase-dependent anodization at different phase areas resulted in the formation of diversified replicas. The in-situ geometric replicas of the precipi
tates enable us to conduct a multi-scale and multi-dimensional characterization of both the primary and secondary precipitates, which shows great
advantages over traditional characterization methods.
Keywords: Titanium alloy ,age-hardening, precipitation, metallography.replica
1. Introduction
Precipitation-hardening or age-hardening is one of the important strengthening mechanisms which governs the mechanical properties of various kinds of alloys1·3>. To characterize the precipitates in age-harden
ing Ti alloys, traditional metallography using grinding/ polishing/ etching, transmission electron microscopy, and metallurgical replica method such as extraction replica have been employed for either destructive or non-destructive testing4
•5>. However, these character
ization methods usually depend on skilled sample preparation and analysis, which are usually time-consuming and lack a good repeatability and sufficient imaging contrast.
For example, if we want to characterize the ultrafine secondary precipitates formed after an ageing process, the predominant characterization method at present is either SEM observation of well polished and well etched Ti alloy sample or TEM observation of an ion-thinned alloy sample. One of the main drawbacks of these characterization methods is that the imaging quality may be not good. The SEM observation could be greatly affected by the polishing and etching process especially for hard-to-etch alloys. And the TEM observation will be limited to very small areas of the sample and in many cases we could only obtain poor contrast images of ultrafine precipitates. In addition, rare work has been reported on a metallurgical replica characterization of ultrafine or even large precipitates in agehardening Ti alloys, although traditional metallurgical replica based on carbon thin films or polymer films could be employed in non-destructive testing of inaccessible surface, surface particles and defected areas.
Thus, developing an efficient characterization method rather than employing traditional methods
seems to be highly desirable for rapid evaluation of the precipitates and ageing processes as well as mechanical properties for many age-hardening Ti alloys.
In the present work, our recent progress is reported in developing a novel replica method to characterize various kinds of precipitates in several age-hardening Ti alloys.
2. Experimental
Bulk materials of BT22 ( Ti-5Al-5Mo-5V-1Cr-
1Fe) and TilOCr alloys were solutioned at elevated temperatures (860°C for BT22 and 900°C for TilOCr) and aged at lower temperatures ( 500°C - 600°C for BT22, 400°C - 600°C for TilOCr) to obtain various kinds of age-hardening materials. The age-hardening BT22 and TilOCr alloy samples with a size of 10 X 10 mm were cut from bulk materials. The plate samples were grinded with emery papers and anodized at a OC voltage of 15 V or 20 Vin IM NaH2P04 solution con
taining 0. 5 wt% HF to obtain anodically oxidized surfaces. After 1. 5 hours of anodization, the anodized samples were taken out of the electrolyte, dried for direct observations or further ultrasonic treatment. The ultrasonic treatment in water lasted for about 10 minutes, which could remove some or all of the anodic oxides. Surface morphologies of the as-anodized samples and ultrasonically treated samples were investigated with field-emission scanning electron microscope ( SEM, FEI SIR ION 200).
3. Results and Discussion
BT22 alloy is one of the high-performance Ti al
loys widely used in industry. And TilOCr alloy is a typ
ical binary alloy. Both alloys could be strengthened through age-hardening treatment. A solution-treatment
8. Environmental Behavior • 1925 •
and age-hardening at elevated temperatures will lead to
the formation of al~ two-phase microstructures. Anodization of pure Ti or Ti alloys to form oxide films at the top surfaces has been widely reported in literatures1
-9>.
These works usually focused on the oxides rather than
the alloy substrate. Our experimental results indicate that, to approach a phase precipitates, a weak anodization of the age-hardening alloy and lateral ultrasonic treatment could realize in-situ metallography replicas
of the precipitates.
3. I Replica of Precipitates in Age-hardening BTI2 Alloys Surface morphology of an as-anodized BT22 alloy
is shown in Figure l(a). Large quantity of oxide particles can be seen at the top surface of the as-anodized
sample. There is no trace of precipitates. After an ultrasonic treatment of the above anodized sample, the
top oxide layer was removed and the replicas of various kinds of precipitates can be found (Figure l(b) ) . It can be inferred from Figure l(b) that the lamellar or plate
let a phase precipitates, i. e. , ultrafine secondary precipitates, had been in-situ oxidized to form the platelet
replicas. And relatively larger precipitates, i. e. , primary precipitates, had been in-situ oxidized to form parti
cle replicas consisting of oxide nanotubes. The platelet replicas and the particle replicas offer us a clear image
of the a/~ two-phase structures. With these anodization replicas,it is easy to tell the large primary precipitates from the lamellar secondary precipitates, easy to meas
ure the precipitate size/ volume fraction and finally correlate to mechanical properties such as yield strength.
1''igure I. Anodized BT22 sample and precipitate replicas. ( a) Sur
face morphology of the as-anodized BT22 sample showing the forma
tion of oxide particles at top, ( b) Multi-dimensional replicas of the a
phase precipitations after ultrasonic removal of the particle layer
The anodization replicas also render us great freedom to investigate the influence of solution-treatment
temperature on the age-hardening process. Figure 2 shows SEM images of the precipitate replicas of the BT22 alloys solutioned at 810°C and aged at either 550°C or 600°C for 5 hours. Quite different precipitate
replicas of the age-hardening alloys could be observed. The alloy aged at 550°C had relatively finer precipitates (whether the primary or the secondary precipitates). Whereas, the alloy aged at 600°C had much larger precipitates. Such a difference directly proves that ageing
at the higher temperature will result in the formation of large precipitates. In addition to the formation of precipitate replica, replica of the grain boundary could be also obtained. As shown in Figure 1 ( b) and Figure 2.
Figure 2. SEM images of the precipitate replicas of the BT22 alloys
solutioned at 810'C and aged at (a) 550'C and (b) 600'C for 5 hours
Figure 3. OM image of the replicas of both the grain bounda
ries and quite large precipitates for the BT22 alloys solutioned
at 900'C and aged at 550'C for 5 hours
• 1926 • Proceedings of the 12'h World Conference on Titanium
The replicas of the grain boundaries could be directly observed. An optical microscopy of the grain boundary or grain replicas is therefore available. Figure 3 presents OM image of the repl icas of both the grain boundaries and the precipitates in the BT22 alloys solutioned at 900°C and aged at 550°C for 5 hours. These SEM images and OM images permit us to measure the grain size and thus indirect ly evaluate the precipitation behavior related to both the aging process and the solution-trea tment process.
3. 2 Replica of Precipitates in Age-hardening TilOCr Alloys Our experimental works further proved that the
above replica method could be used for many other agehardening Ti alloys like the binary Til OCr alloy. Figure 4 shows SEM images of the precipitate replicas of the TilOCr alloys aged at 500°C and 600°C for 4 hours. Compared with the ultrafine replicas of the precipitates in the alloy aged at 500°C , the replicas of the precipitates in the alloy aged at 600°C are much larger, which indicates that ageing temperature has great influence on the precipitation. Figure 5 demonstrates the influence of ageing time on the precipitation of the secondary precipitates and some grain- boundary precipitation. The average size of the secondary precipitates increases with the ageing time and grain- boundary precipitation could be quite different. Similar to the replicas of the BT22 system, the grain boundary replicas could be
Figure 4. SEM images of the precipitate replicas of
the Ti 1 OCr alloys aged at (a) soo·c and ( b ) 600"C
for 4 hours
clearly observed under the optical microscope ( as shown in Figure 6) . T his once again proves that the anodization replicas here for multi-scale characterization of ul trafine precipitates and grains of age-hardening T i
a lloys. Based on the above microstructura l observations and the great freedom and simplicity of fabricating the anodization replicas, we believe that this anodization replica method could be a useful tool for rapid characterization of age-hardening Ti alloys in both academic and in
dustrial labs.
Figure 5. SEM images of the replicas showing the influence of
aging time on the precipitation of the Til OCr alloys aged at
600"C for (a) 30 minutes , (b) 2 hours, (c) 8 hours, (d) 24 hours
and ( e) 48 hours
4. Conclusions
In summary, representative alloys of BT22 (Ti-5 Al-5 Mo-5 V-1 Cr-1 Fe) and Til OCr, which had been so-1 utioned and aged at different stages, were selected as the model systems to demonstrate how anodization rep
li cas of precipitates could be rea lized. With the anodization replicas, a multi-scale and multi-dimensional char
acterization of both the primary and secondary precipitates could be realized. uch a novel replica method offers great freedom for rapid characterization of precipi
tates in both industrial and academic labs.
Acknowledgements This work was fina ncia l supported by Shanghai
Pujiang Program ( o. 07 pj14047) , 863 Plan of China (No. 2006AA02Al), National Key Project of China
( o. 2007CB613805) and National Science Foundation of China (No. 51001088) .
8. Environmental Behavior • 1927 •
Figure 6. OM images of the replicas of both the grain
boundaries and the precipitates in the TilOCr alloys aged at 600'C for (a) 30 minutes, ( b) 8 hours, (c) 24
hours and (d) 48 hours
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