The Role of Lengthscale in the Creep of Sn-3Ag-0.5Cu Solder Microstructures

Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near-<110> orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing (λ2), eutectic intermetallic spacing (λe) and intermetallic compound (IMC) size, indicating a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature ($$ \frac{T}{{T_{M} }} $$ T T M > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction and strain heterogeneity is identified between β-Sn in dendrites and β-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings; however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest λ2 and λe) is only localised next to IMCs without growth in size.

Early warning signals before paroxysmal activity at Stromboli volcano, Italy

&lt;p&gt;Mt. Stromboli is an active volcano, located near the coasts of Sicily (Italy), in the Mediterranean Sea. Its volcanic activity is characterized by mild and frequent explosions, sometimes interrupted by occasional episodes of more vigorous activity, which can be accompanied by lava flows and more energetic eruptions, known as &amp;#8220;major&amp;#8221; or &amp;#8220;paroxysmal&amp;#8221; eruptions, according to the energy dissipated during the event.&lt;/p&gt;&lt;p&gt;Stromboli produced vulcanian eruptions in 2003, 2007 and July-August 2019, which were well recorded by the INGV monitoring network. In particular the last three events are studied through records from borehole strainmeters, which allow us to infer many details of source dynamics. These events are clearly preceded by a slow strain buildup, starting several minutes before the paroxysms, which can be used in future for civil protection purposes. The eruptions then consist of two or more pulses, with oscillations ranging from several seconds, as in 2007, to some minutes, such as in 2019 and lasting from several minutes to one hour after the explosions.&lt;/p&gt;&lt;p&gt;Mechanisms involved in the triggering process of the vulcanian explosions include an increase of magma flux ascending from sources located from 2 to 5-7 km depths and morphological complexity in the upper feeding system.&lt;/p&gt;&lt;p&gt;A preliminary early-warning algorithm, based on an evaluation of strain rate change, has been defined: it has shown itself capable of ascertain the occurring eruptions minutes before their summit onset.&lt;/p&gt;&lt;p&gt;Valuable information are embedded in the data used in the current work, which could be used not only for scientific purposes but also from civil protection for monitoring reasons. Such a variety of possible usage needs the setting of principles and legal arrangements to be implemented in order to ensure that data will be properly and ethically managed and in turn can be used and accessed from the scientific community.&lt;/p&gt;&lt;p&gt;Particular care is needed in order to harmonize the different rules regarding use of data/information, to identify any potential legal issues related to Intellectual Property (IP) and to set up clear and consistent principles related to IP Rights.&lt;/p&gt;

Strain Rate Sensitivity of Tensile Properties in Ti-6.6Al-3.3Mo-1.8Zr-0.29Si Alloy: Experiments and Constitutive Modeling

The complex deformation usually involves wide strain-rate change. However, few efforts have been devoted to investigate the effect of strain rate history on the tensile behavior of α + β titanium alloy. In present paper, tensile tests of Ti-6.6Al-3.3Mo-1.8Zr-0.29Si alloy were carried out under both constant and variable strain-rate conditions within the region from 10−3~500 s−1. A single stress pulse experimental technique was utilized to conduct the recovery tests. The strain-rate history effect was examined. It is found that the flow stress is independent on the strain rate history, though the alloy exhibits obvious positive strain rate sensitivity. The Taylor-Quinney coefficient of the plastic work converted to heat is proved as 0.9 at high strain rates. The cavitation fracture mechanism is revealed by microstructural observation over the full range explored. In basis of the experimental results and other pulished literatures, empirical Khan-Huang-Liang constitutive model was suitably modified to account for the strain-rate dependent behavior. Good agreement is achieved between the modeling prediction results and experimental data.

Extrapolation of Imaginal Minimum Creep Rate in Compression by a Concept of SATO-Index

Creep characteristics of alloys and compounds have been evaluated mainly by the minimum creep rate or the steady-state creep rate, and by its stress and temperature dependences. In some cases, however, direct comparison of the minimum creep rate or the steady-state creep rate are not practically easy due to difficulties of experiment, i.e., a long duration of primary stage of creep deformation. The minimum creep rates are not always precise representative value, which is directly evaluated from experiments. It should be valuable, if one could estimate the minimum creep rate from creep curve in primary stage. I have proposed a method of quantitative evaluation of creep curve based on the evaluation of strain rate change and its strain dependence during creep [1-3]. The value that reflects a shape of creep curve is named “Strain Acceleration and Transition Objective-Index (SATO-Index)” [4]. SATO-Index and related differential equation show a strain dependence of strain rate and lead entre creep curve by numerical integration. This concept provides quantitative information of shape of each creep curve, and information of the entire creep curve. In this paper, examples of evaluation and extrapolation of creep rate from primary stage in compression are presented. It is concluded that the extrapolation with the concept of SATO-Index reasonably provides imaginal minimum creep rate. Usability of extrapolation of creep curve by the concept is presented.

Investigation on Hardening and Softening Behavior of Steel after Rapid Strain Rate Changes

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Study on Effects of Non-Isothermal Condition and Strain Holding on Environmentally Assisted Fatigue in PWR Primary Water Environment

To understand the fatigue behavior of austenitic stainless steels in a simulated PWR primary water environment, the patterns were studied. Austenitic stainless steel Type 316 plate was used as the test material. Regarding non-isothermal testing: isothermal and non-isothermal fatigue tests were carried out for several patterns of temperature change and strain rate change. Typically, fatigue lives for non-isothermal tests with an out-of-phase strain change pattern were longer than those for isothermal tests. Regarding strain holding testing: multiple groups of strain range cycles were separated by a long hold time and several test cases were carried out. Testing shows there is little difference in fatigue life for strain holding tests with high strain amplitude.