针对Zr-4合金高温扩散连接过程中晶粒粗化和界面第二相析出导致接头性能下降的问题，本工作采用超声冲击在Zr表面制备约70 μm厚的梯度纳米结构(GNS)，以降低扩散连接温度并提升接头强度，并在740~800 ℃范围内开展扩散连接实验(保温时间30 min、压力10 MPa)。结果表明，梯度纳米结构由纳米晶、纳米层片及变形晶粒组成，富含高密度晶界、位错及孪晶。表面纳米晶可促进界面孔洞闭合，并有效抑制第二相的生长和聚集，使其在界面分布更均匀。同时，在连接过程中距接头界面15~100 μm的区域形成异常粗大晶粒，其最大尺寸可达基体晶粒的7.2倍。断裂行为分析表明，异常粗大晶粒并未成为裂纹源，反而通过诱导非均匀塑性变形和形成大量孪晶结构，促发额外的加工硬化效应，强化接头局部区域。GNS-Zr/GNS-Zr接头的剪切强度随着连接温度升高而增大，在800 ℃时最高达376.9 MPa，在相同连接条件下相较Zr/Zr接头提升1.2~1.6倍，且在低温下提升幅度更为显著。

Zr alloys are widely used as cladding materials in light-water reactors because of their low neutron absorption and excellent corrosion resistance. However, achieving high-quality diffusion bonding of Zr alloys at conventional high temperatures is challenging, where grain coarsening and formation of interfacial secondary-phase particles (SPPs) degrade joint performance and may compromise the dimensional accuracy of precision components. This study develops a low-temperature, high-strength diffusion-bonding technique for Zr-4 alloy via surface nanocrystallization, and elucidates the associated microstructural evolution and strengthening mechanisms. A gradient nanostructure (GNS) with a thickness of approximately 70 μm was fabricated on the Zr-4 alloy surface via ultrasonic impact treatment (UIT). The GNS comprised nanograins, nanolamellae, and deformed grains, with high densities of grain boundaries, dislocations, and twins. This surface nanostructure was designed to enhance atomic diffusion, reduce bonding temperature, and improve joint properties. Diffusion-bonding experiments were performed for 30 min at temperatures ranging from 740 ^oC to 800 ^oC under a pressure of 10 MPa. The results revealed that the surface nanograins significantly accelerated interfacial void closure and suppressed SPPs overgrowth and aggregation, resulting in a more dispersed distribution of SPPs along the bonding interface. Abnormal grain growth appeared at 15-100 μm from the bonded interface, with the largest grains reaching up to 7.2 times the size of the matrix grains. This abnormal grain growth is attributed to the uneven distribution of strain energy within the GNS, which enables some grains with energy, orientation, or size advantages to grow preferentially by continuously consuming surrounding finer grains. Fracture behavior analysis revealed that cracks initiated neither at the bonded interface nor within the abnormally large grains, but in the Zr matrix region approximately 130 μm from the interface. These grains exhibited numerous deformation twins and acted as crack propagation barriers by coordinating deformation with the surrounding finer grains. Despite their lower yield strength, the abnormally large grains positively contributed to joint strength through a strengthening mechanism induced by hetero-deformation. The shear strength of the Zr/Zr and GNS-Zr/GNS-Zr joints improved as the bonding temperature increased. The GNS-Zr/GNS-Zr joint achieved the highest shear strength of 376.9 MPa at 800 ^oC. Under the same bonding conditions, GNS-Zr/GNS-Zr joints exhibited 1.2-1.6 times higher shear strength than the Zr/Zr joints, with greater improvements at lower temperatures.